Humboldt State University ® Department of Chemistry

Robert A. Paselk Scientific Instrument Museum

 
From: Snell, Foster Dee. Colorimetric Analysis. D. Van Nostrand Company, New York (1921) pp i-viii, 1-31.
 
© Copyright 2000 R. Paselk


 

COLORIMETRIC
ANALYSIS

F. D. SNELL

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ILLUSTRATED
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NEW YORK
D. VAN N0STRAND COMPANY
Eight Warren Street
1921



Copyright, 1921
By D. Van Nostrand Company

Printed in the United States of America



PREFACE


It has been the endeavor of the author so far as is possible to avoid needless repetition of material in this work. Therefore all the fine points as to theory, appa-ratus and its use and the figuring of results which will apply to practically every test given, have been gathered together into three chapters at the beginning of the book. From this it will be seen that a knowledge of these chapters, particularly Chapters Two and Three, is necessary to the success of anyone except an experienced operator, in the use of the methods of determination given on the succeeding pages. It has been the endeavor of the author to mention that a particular type of test was not applicable to those cases where the choice of methods is more limited than usual.


This book is, in essence, an attempt to combine in one volume for ready reference, all the colorimetric tests which experience has shown to be at all practical. Much material will be found here which has never been in print outside of the scientific magazines as well as the standard tests which may be found by reference to any complete work on chemical analysis. In going over the material, it has been evident to the author that little has been done with regard to the determination of the range of greatest accuracy of many of the tests. Whenever such information is available it has been incorporated, so that the operator may know what degree of accuracy he may expect from a sample after he knows the approximate amount of test substance contained in the sample.

iii

 

 

iv

COLORIMETRIC ANALYSIS

It is hoped that this book will furnish a source to which students may refer for a knowledge of the colorimetric methods practical for use and a source to which practical workers may refer for information as to the methods of performing the particular tests with which they are occupied. In all cases, weights of substances to be dissolved as standard have been given to four places, although that by no means implies that absolute accuracy is necessary in the last two places, since the dilution will minimize any error made there. The giving of these accurate weights will, however, show the operator the direction in which the weight should tend in these last two places and thus can assist to greater accuracy without increasing the labor of weighing to any great extent.

The author wishes to particularly acknowledge the. splendid assistance rendered him by the Library of the University of the State of New York in placing at his disposal their files of magazines and reference works. Acknowledgement is also made of the courtesy of Eimer & Amend, C. J. Tagliabue Co. and Arthur H. Thomas Co. in the furnishing of cuts. Acknowledgment is made of the services of C. A. Tyler and W. H. Pearce in the reading of proof.

All factors have been figured by the 1918 table of atomic weights.

F. D. S.



CONTENTS


 CHAPTER

SUBJECT

PAGE

 I.

 CONDITIONS OF USE OF COLORIMETRIC METHODS

1

II.

APPARATUS USED AND METHODS OF USING IT

5

III.

FIGURING OF RESULTS

27

The remainder of the table of contnets is provided for interest - these chapters are NOT included.

IV. DETERMINATION OF IRON 31
A. By Potassium Sulfocyanate . .. .. . . . . .31
B. As the Chloride in Concentrated Acid 35
C. By Potassium Ferrocyanide 36
D. By Salicylic Acid . . . . . . . . .. . . ... . . . . . 37
F As the Sulfid ...39
F. By Acetylacetone 41
G. By Dimethylglyoxime .42

V. DETERMINATION OF COPPER . . . . . . . . . . . . . . . . ..44
A. By Ammonia 44
B. As the Chloride in Concentrated Acid .46
C. By Salicylic Acid 47
D. By Potassium Ferrocyanide . . . . . . . . .48
E. As the Sulfid .. . . . . . . . .. . .. .. . . . . . . .50
F. By Potassium Ethyl Xanthate 51
G. As the Bromide . . . . . . . . . . . . . . . . . . 53

VI. CARBON IN STEEL 55

VII. LEAD, BISMUTH AND ARSENIC 60
Lead as the Sulfid 60
Bismuth as the Iodide . . . . . . . . . . . . . . ...62
A. Arsenic by the Stain Produced on Mercuric Bromide Paper by Arsin. ..64
B. Arsenic by Silver Nitrate 67

CHAPTER SUBJECT PAGE

VIII. ALUMINUM AND CIIROMIUM .68
Aluminum by Alizarin-S 68
A. Chromium as the Chromate 70
B. Chromium by Disodium 1.8 dihydroxy- naphthalene 3.6 disulfonate .71

IX. NICKEL, COBALT, MANGANESE AND ZINC . . .73
A. Nickel by Potassium Thio-carbonate . . 73
B. Nickel as the Chloride in Concentrated Acid 74
A. Cobalt as the Chloride in Concentrated Acid .76
B. Cobalt by a-Nitroso-b-Naphthol 76
A. Manganese as Permanganate, Oxida-tion by Persulfate 78
B. Manganese as Permanganate, Oxidation by Periodate . . . . . . . . . . . . . . . . ..80
Zinc by Resorcinol .81

X. POTASSIUM AND MAGNESIUM .83
A. Potassium by Determination of the Potassium
Platino Chloride by Re-duction with Stannous Chloride .83
B. Potassium as the Chlorplatinate byPotassium Iodide . . . . . . . . . . . . . . . . ..86
Magnesium by Determination of the Phosphate as Phosphomolybdate . . .86

XI. Gold .89
A. Method of Dowsett .......... .89
B. Method of Prister . . . . . . . . . . . . . . . . . . ..90
C. Method of Doring . . . . . . . . . . . . . . . . . . ..91
D. Method of Rose . . . . . . . . . . . . . . . . . . . . .92
E. Method of Cassal .93
F. By Decomposition of the Cyanide by Potassium Bromide and Sodium Peroxide
G. By Decomposition of the Cyanide by Ammonia .94
H. By Metaphenylenediamme .94
CHAPTER SUBJECT PAGE

X11. TITANIUM, VANADIUM AND TUNGSTEN .... ..95
A. Titanium by Hydrogen Peroxide . . . . ..95
B. Titanium by Thymol 97
A. Vanadium by Hydrogen Peroxide . . . . .97
B. Vanadium by Strychnine . . . . . . . . . . . . 100
Tungsten as the Oxide in Colloidal Suspension .101

XIII. FLUORINE, CHLORINE AND PERCHLORATES .102
Fluorine by Estimation of Its Bleach-ing Action on an Oxidised Tita-nium Solution ..102
Chlorine by o-Tolidine . . . . . . . . . . . . . . 103
Perchlorates by Methylene Blue . . . . . ..106

XIV. NITRIC AND NITROUS ACIDS, AND AMMONIA . . 107
A. Nitric Acid by Brucine Reaction . . . . 108
B. Nitric Acid by Diphenylbenzidene .. 110
C. Nitric Acid by Phenolsulfonic Acid . . .111
D. Nitric Acid by Pyrogallol . 113
A. Nitrous Acid by Sulfanilic Acid and a-Naphthylamine .114
B. Nitrous Acid by Metaphenylenedi amine Reaction 116
C. Nitrous Acid by Zinc Iodide Starch Solution 116
D. Nitrous Acid by a-Naphthylamine Hy drochloride 117
A. Ammonia by Nessler's Reagent . . . . . . .118
B. Ammonia by Phenol . . . . . . . . . . . . . . . . ...119

XV. PHOSPHORUS, SILICA AND BORON . ... . . . .. . 121
A. Phosphorus as the Phosphomolybdate. ..121
B. Phosphorus, Separated by Precipita-tion as Magnesium Phosphate . . . . 122
Silica by Ammonium Molybdate in the Presence of Phosphorus . . . . . . ...124
A. Boric Acid by Curcumin . . . . . . . . . . . . ..126
B. Boric Acid by Turmeric Paper . . . . . . ..127
CHAPTER SUBJECT PAGE

XVI. OXYGEN AND HYDROGEN PEROXIDE .129
A. Oxygen by Cuprous Chloride . . . . . . . . .129
B. Oxygen by Adurol 132
Hydrogen Peroxide by Its Oxidising Action on Ferrous Iron ..133

XVII. SULFUR, HYDROGEN SULFID AND SELENIOUS ACID ..135
A. Sulfur by Paraphenylenedimethyldi-amine ..135
B. Sulfur by the Action of Hydrogen Sulfid on Arsenious Oxide Paper . . .136
Hydrogen Sulfid by Methylene Blue 137
Selenium as Selenious Acid by Potas-sium Iodide ..138

XVIII. SALICYLIC ACID AND CYANIDES . . . . . . . ... . . ..140
Salicylic Acid by Fehling's Solution.. .140
Cyanides by Changing to Sulfocyanates and Coloring with Iron . . . . . .141

XIX. COLOR OF WATER, OILS AND DYES 144
Color of Water 144
Color of Oils . . . . . . . . . . . . . . . . . . . . . . . .145
Color of Dyestuffs . . . . . . . . . . . . . . . . . . ..145

XX. NEPHELOMETRY ........................147


 

COLORIMETRIC ANALYSIS


CHAPTER ONE.

CONDITIONS OF USE OF COLORIMETRIC METHODS.

Colorimetric methods of analysis consist of treating a solution of the substance to be tested with a reagent, in such a way as to produce a color which is proportional in intensity to the amount of the substance being tested for, present in the solution. The methods are not only applicable to the determination of many metals but also to acid radicals and compound radicals to a limited extent. The unknown is therefore spoken of in this general discussion as the test substance. The color having been produced, the solution containing an unknown amount of test substance is then estimated by comparison with a standard solution by one of the following four methods:

1. The sample diluted to a definite volume may be compared with a series of standards of the same volume, previously prepared, the amount of test substance in which is known and the value of the unknown then taken to be that of the standard to which it conforms most nearly. In this way the amount of test substance present is obtained without figuring since, if the volume and color of the unknown and the standard are the same, their contents of test substance will also be identical.

2. The standard and sample may be placed in similar graduated tubes and the darker diluted until its color when observed horizontally thru the tube is the same as that of the other. When this point is reached then each

1

 

 

2

COLORIMETRIC ANALYSIS
of one solution must contain exactly the same amount of test substance as each of the other and the amount in the unknown is to the amount in the known directly as their volumes. This is the method of dilution.

3. A definite sample of unknown may be placed in one flat bottom graduated tube and amounts of the standard poured into another similar tube until their colors, when observed vertically thru the length of the columns of liquid, are identical. The amount of test substance in each tube will then be the same and since the amount per c.c. in the standard is known the total amount in the standard may be computed, which amount is identical with the total amount in the sample. To state it more briefly, their concentrations are inversely proportional to their depths. This is the method of balancing.

4. The sample may be made up to a definite volume and nearly that volume of water in a similar container be treated with the same reagents for bringing out the color of the solution as were used with the sample. A concentrated solution of standard is then run into the blank from a burette, adding it drop by drop when the end point is near. The volume of the blank may be brought up by the addition of more water until the two colors and volumes are identical. The amount of standard used in the making of the duplicate is then a measure of the amount of test substance in the sample. This is the method of duplication.

In order for the colorimetric test to be accurate the color produced by the action of the given reagent on the test substance must be the only color present in the solu-tion. Therefore a colorimetric test is not possible if the original solution is colored before the test is begun, unless that. color is produced by the test substance or will be removed by the reagent used. In some particular in-

 

 

 

CONDITIONS OF USE.

 3

stances a slight contamination of one color may be allowed and equalized by contaminating the sample and standard equally. Neither is a colorimetric test possible if the solution to be tested contains anything other than the test substance which will give a precipitate or color with the given reagent. It is essential for the success of colorimetric methods that the color produced be reasonably permanent and that the conditions under which it was produced be such that they can be duplicated without great difficulty. Under certain conditions tests are made with colors which fade after short exposure to the light.

Colorimetric tests may be classified into two classes according to the reason for their use. Some find their popularity because they are fast, and the accuracy of the test is sacrificed for speed in obtaining the final result. The second class find use because they furnish a method of determination of small amounts of substances with greater accuracy than is possible by gravimetric or volumetric methods. The first class may all be made in a short time but it may often happen that the method of preparation of the second class of tests may take hours to insure the accuracy desired.

In spite of the limitations placed on its use by the previous requirements the colorimeter is coming into use more and more every day because of its answer to the demand in nearly every laboratory for speed. Colorimetric methods, used because of their speed, give results in five minutes to one hour from the time the test is begun which is in all cases less than half the time similar tests could be made by other methods. A leading brass manufactory of the country obtains an analysis of its brass from the laboratory within forty-five minutes after the delivery of the sample. Of the five constituents determined, four are determined by colorimetric methods.

 

 

4

COLORIMETRIC ANALYSIS

Thus the main thing to be said for colorimetry is that its methods are rapid and reasonably accurate.

A broad field of usefulness of colorimetry is the analysis of salts or substances easily soluble in water or acid for the amounts of various impurities present. The methods are very delicate and accurate, some being so delicate as to detect one part in one hundred million parts of water, but they are seldom such as to permit of the determination of quantities greater than one per cent without resorting to aliquot parts and using a portion of the solution of the sample instead of the whole. The methods of accurate dilution are discussed in the next chapter; the only drawback to their use is that the use of a factor for determining the final result multiplies the error made in the comparison, often as much as a thousand times.


 

CHAPTER TWO.

APPARATUS USED AND METHODS OF USING IT.

As previously outlined there are four types of determinations made by colorimetric methods, each of which uses a more or less specialized form of apparatus.

In cases where a series of permanent standards are made up these are usually placed in either round or square glass bottles. Such a series of standards should be placed in a row on a shelf, with sufficient space between each two for setting in a similar bottle. The sample should be treated in a similar bottle and, after making up to the same volume as that of the standards, it may be set into various of the gaps between the standards until the place is found where one standard is higher than the sample and the one on the other side lower, as estimated from the intensities of their colors. The position of the sample relative to these two known quantities can then be estimated. In some cases when permanent standards are used one bottle may be made to serve as two standards. This is accomplished by the use of a rectangular bottle, twice as long as it is broad. The sample is then compared in a similar bottle and results are as follows: If the long way of the sample bottle compares with the long way of the standard the amounts of test substance in the two are the same. If the narrow way of the sample compares with the long way of the standard the sample contains twice as much test substance as the standard. If the long way of the sample compares with the narrow way of the standard the sample contains half as much test substance as the standard. Altho no fur-

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6

COLORIMETRIC ANALYSIS

ther specific reference will be made to their use, this use of these special bottles is applicable in all cases where permanent standards are to be made up, except those where the actual color rather than the depth of color changes with the concentration. Care should be taken to see that the bottles used in all cases are clear in color and free from flaws.
In many cases the color produced by a reaction fades in a short time in the light. In such a case either a standard must be prepared at the same time as the sample and test made by one of the other methods or. permanent artificial standards must be prepared. These are conveniently prepared from solutions of inorganic salts, combining the colors in such a way as to give the desired color. The solutions which find the greatest favor for this purpose are half normal solutions of the nitrates or chlorides of cobalt, iron and copper.
1 These three solutions may be combined in such a way as to form any color desired except the deep blues and reds. To remedy this it has been found that some of the missing colors can be obtained by using a solution of potassium dichromate instead of the iron solution and the remaining colors are obtained by combinations of potassium dichromate and potassium permanganate. As an example it is found that a mixture of the first series suggested, cobalt three parts, to iron nine parts, diluted with water, corresponds to the color given by Nessler's reagent in reaction with ammonia. Varied dilutions of this may be used for a series of standards, making each standard to correspond to the result obtained by the use of that known amount of ammonia treated with the reagents necessary to bring out the color. In a similar manner permanent standards may be prepared for any

1 J. Franklin Institute, 180, 200

 

 

APPARATUS USED

 7

test, keeping in mind the fact that every standard must be tested against a known amount of the test substance to make sure of its accuracy.

For the production of nonpermanent standards plain test tubes are ordinarily used. The solution may be quickly and conveniently emptied from these and the loss in case of breakage is not so great as from the use of graduated tubes such as Nessler tubes or from the use of bottles.


The apparatus for tests by the dilution method consists essentially of a pair of graduated tubes. There is, however, a very convenient device (Fig. 14) manufactured for use with this method known as the colorimetric camera. This is a light-proof box, painted black inside, with holders for the two tubes near one end. The end near the tubes is fitted with a ground glass screen; the other end is fitted to the face of the observer so that no side light may enter. By the use of this a more accurate judgment as to the colors of the two tubes may be made than if they are compared out in the open. One advan-

 

8

COLORIMETRIC ANALYSIS

tage of this apparatus is that it does not tire the eyes of the operator so quickly as do some of the types of colorimeter used for the balancing method. In all cases possible a colorimeter should allow the use of both eyes for making the comparison so as to eliminate fatigue. Sometimes the comparison of the two tubes is made by holding them in the hand and looking at them against a sheet of white paper.


Whether or not the camera is used for the comparison the operation of the dilution method is always the same. The standard and sample are in two graduated tubes or in two Nessler tubes. The former are used when the camera is used for the comparison, the latter are useful when no further apparatus is used. The colors of the two solutions should be nearly alike to begin with. The experienced operator soon learns to choose his sample so that the. resultant color will be nearly the same as that of the standard which he is using. Water is then added to the solution of the darker, carefully mixing after each addition, until the colors of the two solutions when observed horizontally thru the tubes appear to be identical. When this point is reached the concentration of the test substance in each solution must be the same and their contents are then to each other as the volumes. Since the amount of test substance in the amount of standard used is definitely known, it is easy to figure the test substance in the sample by a direct proportion. Care should be taken that the tubes used are clear and free from flaws and that their thickness of glass, internal diameter and graduations are identical. A special type of tube manufactured for carbon determinations is applicable to all dilution tests. This has a bend about two inches from the top so that the tube may be lightly shaken to mix the water added without danger of the contents being

 

 

APPARATUS USED

 9

spilled. Note that the standard may as well be diluted to match the sample as the sample to match the standard, provided only that the original volume of the standard is recorded and that the final colors are not too deep or too light to allow of accurate measurement.


Determinations by the method of duplication are usually carried out in graduated Nessler tubes. The sample is first diluted to some convenient, definite volume. The same reagents as were added to the sample are then added to a volume of water amounting to about half the volume of the sample, this will usually be specified for the particular experiment since the amount of water used for the blank varies according to the concentration of the standard to be used. To the blank containing the same reagents as those used for the sample there is now added a standard solution, carefully mixing after each addition, until the color of the sample is duplicated by that produced in the blank by the addition of the standard. The color of the blank having been made to balance that of the sample they will still differ in volume. It is simple enough to figure at this point by proportion the amount of standard that would be necessary if the volumes were equal but, to eliminate any chance of error in that, it is best to duplicate the volume as well as the color of the standard. This is accomplished by the addition of water and standard alternately until the two solutions are identical in both color and volume. As said, a little simple mathematics will tell an operator of comparatively little experience the exact amounts of standard and water to be added after the colors have once been duplicated. The same cautions apply to this test as to the previous one insofar as apparatus is concerned; the tubes used must be of the same size, thickness of glass and internal diameter and the graduations on the outside

 

10

COLORIMETRIC ANALYSIS

must correspond in height. That is, the ten c.c. mark on each must be at the same distance from the bottom, otherwise the contents are not the same or the tubes are not accurately marked.


The apparatus for the balancing method is the most elaborate of all and the use of it, the simplest. The various instruments used include Hehner cylinders, Lovibond Tintometer and various types of colorimeters. The Campbell-Hurley, Duboscq, Schreiner, Stammer, Saybolt, and White types are described. Other specialized forms are used in some special applications but their use is mostly in physiological chemistry.


The Hehner cylinders are the simplest of the instruments for this use. They consist of two glass tubes with flat bottoms and each has a side tube with stop-cock, about three inches from the bottom. For the carrying out of a determination the solution of the sample is placed in one tube and the other tube is partially filled with the standard so that the depth of color seen by looking vertically downward thru the length of the column of liquid is deeper than that seen by similarly looking down thru the sample. An amount of standard is then drawn off, such that the colors of the two tubes when observed in this way are the same. The tubes are then said to be balanced and the readings of the volumes taken. The amount of standard used and its content of the test substance per c.c. is known. The test substance in the sample is then inversely proportional to the volume if the content per c.c. is desired, or the total amount of test

 

 

APPARATUS USED

 11

 

12

COLORIMETRIC ANALYSIS

substance in the two solutions is identical. For the convenient balancing of the two liquids several mechanical appliances have been devised for the purpose of increasing the speed of the determination or of increasing its accuracy.


The Campbell-Hurley type of colorimeter illustrated is the common form of such an instrument. This instrument
1 is a modification of the Kennicott-Sargent type of apparatus and is sometimes known as the Kennicott colorimeter. The following is the method of operation as illustrated by the diagram. The unknown solution to be tested is placed in the tube A and, since the volume can be readily governed so as to come to some even graduation, these are only placed at five c.c. intervals. The standard solution is placed in the right-hand tube B which, because of the fact that the accuracy of the test depends upon the careful reading of the volume in this tube, is graduated to single c.c. The readings of tubes A and B have been given as in c.c. but, for such readings to be accurate, tubes A and B must be of the same identical bore. The same effect is obtained in less accurately made instruments by having the graduations to cm. rather than c.c. The proportion may then be obtained between the heights of the two columns, the same as tho the units used were c.c., since the columns will have colors proportional to the depths regardless of their amplitude. The method used for graduation must be taken into account in figuring the results of the test and will be discussed under that head. Tube B is permanently connected by a glass tube with the reservoir C in which the glass plunger D works, so that the level of the liquid in B may be readily controlled by raising or lowering the plunger. As the tube B and the reservoir

1J. A. C. S., 33, 1112.

 

 

APPARATUS USED

 13

C are made in one piece the standard comes in contact with glass only, thus preventing the possibility of chem-

ical change due to coming into contact with the container. The plunger D is sometimes provided with a rubber collar E to prevent it from coming into contact with the

 

14

COLORIMETRIC ANALYSIS

bottom of the reservoir and the resultant breakage possible. The tubes A and B and reservoir C rest on wooden supports with holes under A and B for the passage of light. All glass parts are held in place by spring clips which allow for the easy removal of the parts for cleaning.

For operation the colorimeter is turned with the back toward a window, preferably a north one, and the mirror G is so adjusted as to reflect skylight upward thru tubes and B. By this arrangement the back of the colorimeter serves as a screen to cut off all light except that reflected upward from G. The light, passing upward thru the tubes A and B, impinges on the two mirrors H and I cemented to brass plates sliding in grooves cut at. an angle of 45° in the sides of the wooden box J. This box has a loosely fitted cover so that it may be removed for the cleaning of the mirrors. The mirror H is cut vertically and cemented in such a position as to reflect one half of the circular field of light coming thru the tube A. The light, passing upward thru B, is re-flected horizontally by the mirror I thru a hole in the brass plate supporting the mirror H. One half of the circular field of light from the tube B is cut off by the mirror H, the vertical edge of which acts as a dividing line between the two halves of the circular field. The image of one half of the tube B is then observed in juxtaposition to the opposite half of the image of the tube A.

The juxtaposed images are observed thru a tube K, 2.5 cm. in diameter and 16 cm. long, lined with black felt and provided with an eyepiece having a hole 1.5 mm. in diameter. At the point H in the tube K is placed a diaphragm having an aperture 8 mm. in diameter. All parts inside the box J except the mirrors are painted black so that no light except that coming thru the tubes A and B passes thru the tube K. By having the aper-

 

 

 

APPARATUS USED

 15

tures in the eyepiece and the diaphragm properly proportioned only the images of the bottoms of tubes A and B can be seen, thus preventing the interference of side light, from the vertical sides of the tubes.

A person looking thru the eyepiece observes a single circular field divided vertically by an almost imperceptible line when the two solutions are of the same intensity. By manipulating the plunger D the level of the liquid in B can be easily raised or lowered, thus causing the right half to assume a darker or lighter shade at will. In matching colors with an ascending column in B, that is, gradually deepening the color of the right half of the field, the usual tendency is to stop a little below the true reading, while in comparison with a descending column the opposite is the case. At first the operator should take a reading in each direction until after a little practice this tendency to error has been overcome.

In tests on a large number of titanium solutions by oxidation with hydrogen peroxide, using all concentrations from a very dilute (light yellow) to a fairly concentrated (deep orange) solution, the average percent of error was found to be less than one percent and the median error less than one half percent.

In some cases the depth of the column of liquid observed cannot be used as a measure of the test substance present as the concentration changes not only the depth of the color but the color itself. In cases where that is so, it will be mentioned in connection with the directions for the particular test.

A modified form of balancing instrument has a mirror directly over the tubes A and B reflecting forward the two circles of light which come to it. The mirror is encased in a lightproof box painted black on the inside

 

 

16

COLORIMETRIC ANALYSIS

 

 

 

APPARATUS USED

 17

 

 

18

COLORIMETRIC ANALYSIS

and about eight inches long. This is fitted to the eyes of the operator at the forward extremity so that side light cannot enter. The arrangement of the tubes A, B and C and plunger D is the same as in the previous instrument. By making the holes under the tubes A and B so small that they do not extend to the side walls of the tubes side light is largely eliminated and accurate results secured. In this case the alteration is made in the color of the right hand circle of light as compared with a circle of similar size appearing beside it.

The Lovibond Tintometer is another type of instrument, one which we shall not describe here at any length on account of the limited use that it finds. The instrument is expensive and so is comparatively rare. The comparison is made by graduating a liquid to match a glass of a certain color, thus doing away with the use of a liquid standard. Since a permanent liquid standard may be prepared for this use and so eliminate large expense this is not recommended. Glasses may be obtained for determining carbon in steel and color of water, petroleum, etc., in addition to which by combinations of glasses the color of any solution may be matched.

In the Duboscq type of instrument the same result as in the Campbell-Hurley type is obtained by a different method. The two independent tubes, A and B, are of the same size and are for holding the solutions of the unknown and the. standard. Each is mounted in a wooden holder, M, N, which slides up and down in a slit cut in the backboard of the instrument and which is fastened at the chosen place by a thumbscrew. Light is reflected upward thru the tubes by a mirror G. Directly over tubes A and B which contain the solutions to be tested are two glass plungers, O, P, of a diameter equal to about half that of A and B. The bottoms of these plungers

 

 

 

APPARATUS USED

 19

are finely ground and in the best instruments are fused on. In the cheaper instruments the bottoms are fastened with some sort of adhesive.

The telescope, K, for observation of the colors is perpendicular to the base so that the operator looks downward into the instrument. The light reflected upward thru the solutions in A and B is so reflected by the prisms, I, I, in the box J that two fields appear side by side, one front A and one from B. The arrangement of the prisms is such that the images observed in the field of the telescope are those of the bottoms of the plungers, O, P, rather than of the entire depth of liquid in A and B. By suitable cutting down of the aperture by screens all reflection from the sides of the tubes is cut off.

For use, the instrument is set to face a source of light and mirror C is set at the proper angle to reflect skylight upward thru A and B. The cylinder having the lighter color is then moved upward by sliding its holder in the slit in the backboard until the plunger just touches the surface of the liquid. The other container is then moved upward observing its movement thru the telescope until the image of the base of the plunger in that liquid appears to be of the same intensity as that observed from the other field. The instrument is then said to be balanced and the depths of liquid underneath the plungers have the same relation to each other as the total depths of liquid in A and B when the Campbell-Hurley instrument is balanced. The slits cut in the backboard in which the holders of A and B move are graduated so that the depths of liquid may be read off directly from these and possible errors in reading depths of liquid in glass due to the meniscus are eliminated.

The laws relating to the depths of liquid when the instrument is balanced are as follows. The concentration

 

 

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COLORIMETRIC ANALYSIS

per c.c. of the sample is to that of the standard inversely as the depths of their liquids. Diameter of the containing vessels being the same, the sample solution of the

depth indicated by A contains identically the same amount of test substance as a standard column of the depth indicated by B. The figuring of results from this data is taken up in the next chapter. The nephelometer is a modification of this type of instrument. For farther information regarding that use see Chapter Twenty.

 

 

 

APPARATUS USED

 21

The Schreiner colorimeter is a simple modification of the Duboseq instrument, eliminating many of the more expensive features. The prisms are omitted and two round fields near each other are observed. The holders for A and B are two brass clips and the reading is taken by graduations on the glass of these tubes. The application of this instrument is recommended by the manufacturers for soil analysis.

The Stammer colorimeter is also a modified form of the Duboscq, and is particularly used for determination of the color of sugar solutions and of oils. The colors of the fields are transmitted to the telescope, K, by prisms, I, I, as in the Duboseq type. The alteration is in the character of the fields. Instead of two movable containers and two fixed pistons as in the other instrument the containers are fixed and one movable piston, P, is provided for the variation of the column in B. A false piston, O, is provided so that the light thru A will have to pass thru a similar thickness of glass. The column in A remains absolutely permanent, no variation in that being possible. Light is reflected upward by a mirror at G as in previous instruments.

Recently a new type of instrument has been evolved from the combination of the Lovibond Tintometer and the Stammer Colorimeter, known as the Saybolt Chromometer. This instrument is intended solely for the evaluation of the colors of lighting oils. This is similar to the Stammer instrument in that two tubes are provided, but the left-hand tube, A, is modified to an empty tube

 

 

 

22

COLORIMETRIC ANALYSIS

in which the color is obtained by the addition of one or more carefully prepared glass disks. The sample in B is drawn off until only such an amount is left as will duplicate the color of the disk used. From the graduations on the glass of B and the directions of the makers the oil may then be graded as so many degrees above or

 

 

 

 

APPARATUS USED

 23

below the standard colors for lighting oils (see Chapter Nineteen), depending on the disk used

One other type, White's colorimeter,1 shows promise of great importance because of its ease of operation. In this the thickness of two glass prisms observed is the variable. The apparatus consists essentially of two wedge-shaped hollow glass prisms of exactly equal dimensions, open at the large end for the introduction of the solutions to be tested. The wedges are held in vertical position side by side in a camera and may be raised or lowered by rack and pinion actuated by thumb-screws. The wedges are screened from view on the side of the operator, except for a narrow horizontal slit across the middle of the camera thru which the solutions are observed when a test is being made. The carriers are graduated to correspond to the length of the wedges, the zero of the scale being opposite the indicator when the sharp edge of the wedge is opposite the narrow opening in the screen thru which the color is observed. The screens are adjustable so that the opening may be altered to suit the operator. The ground glass shutter at the forward end of the camera., for diffusing the light, is hinged in the manner of a door to facilitate the transfer of the wedges to and from the camera. To carry out a determination by this method equal quantities of the standard and of the material to be tested are diluted to equal volumes and convenient amounts of the solutions transferred to the wedges. The wedge containing the solution of unknown strength is set at the graduation representing the percentage, or a multiple of it, of the coloring matter in the standard. The wedge containing the standard is then adjusted so that the two fields seen thru the camera appear identical. The percentage of

1J. A. C. S., 34, 659.

 

 

24

COLORIMETRIC ANALYSIS

coloring matter in the unknown is then indicated by the reading of the scale on the carrier containing the standard. If the depths of color compared at first are too deep for accurate comparison the results may be checked without changing the solutions by setting the wedge containing the unknown at a new point on the scale and

again adjusting the other wedge until equality is reached. The maximum error reported on a long series of experiments with this instrument was .6 percent.

The figuring of results obtained by these balancing methods is discussed in the next chapter.

In some cases it may be impossible to choose a sample

 

 

 

APPARATUS USED

 25

having as small a content of the test substance as specified in the method, particularly if the substance being analyzed is an ore or alloy containing a fairly large percentage of the test substance. In that case the sample may be taken containing somewhat more of the test substance than specified for the test and dissolved as directed. This solution may then be accurately diluted and an aliquot part taken. If, for example, the method specifies that a sample must be under .001 grain of the test substance and the smallest sample of the substance being analyzed which can conveniently be weighed out will contain nearly one tenth gram of the test substance, this may be dissolved as specified and diluted to one liter in a volumetric. Ten c.c. of this may then be analyzed as directed and the weight of test substance found in that amount of solution multiplied by 100 will give the amount in the original sample. In case it is necessary to use such a method the final result cannot be expected to be as accurate in regard to the amount of test substance by weight as shown in the result but the results are identically as accurate so far as the percentage of error is concerned. To illustrate, while accuracy to .6 percent in the case of a weight of substance showing .001 grain test substance would allow an error of only .000006 grain test substance, if the sample showed .5 gram test substance (an amount greater than would ordinarily be present in a weight of sample used), the maximum error then possible would be .003 gram of the test substance.

The rapidity with which colorimetric tests may be made often admits of the attainment of considerably greater accuracy than the theoretical by rapidly doing several tests and then averaging the results obtained.

The intensity of the color of the two solutions is an important factor, so results may be expected to be more

 

 

26

COLORIMETRIC ANALYSIS

accurate if the standard and sample have nearly the same color before a test is carried out by dilution or balancing methods than if the colors differ greatly. The intensity should also not be too great or too dilute; in either case errors will be greater than if the color produced was intermediate between the two extremes.


 

 

CHAPTER THREE.

 

FIGURING OF RESULTS.

 

In case a series of standards is used the weight of test substance in the unknown may be told at once by looking at the amount in the standard to which it corresponds. The percentage is then readily figured from this by dividing by the weight of sample used, thus:

Weight of sample, 5 grams.
Weight of test substance present, .0004 gram.
Sample therefore contains .0004÷5 = .00008 gram test substance per gram sample = .008 percent.

The figuring of results when the dilution method is used is somewhat more complex. In that case the darker of the two solutions is diluted until the colors of the two are identical when observed horizontally thru the tubes, at which point the content of test substance per c.c. is the same and the content of test substance of one is to that of the other directly as their volumes. To illustrate:

Weight of standard used, .2 gram.
Weight of unknown used, .2 gram.
Standard contains .32 percent test substance.
Readings, standard 38 c.c., unknown 45 c.c.

The weights of the sample and standard being the same in this problem we may eliminate the figuring of the weight of test substance in the sample and proceed directly to estimate by proportion the percentage. This is estimated thus:

Percent in standard: Percent in unknown = Volume of standard: Volume of unknown:

27

 

 

28

COLORIMETRIC ANALYSIS

.32 : X = 38:45
X = .379

Therefore we find that the sample being tested contains .38 percent test substance.

The figuring of dilution method results does not always resolve itself into a mere comparison of percents as usually a definite volume of a standard solution whose content of test substance per c.c. is known is used as the standard. In that case the weight of test substance in the standard must be figured and from that the similar weight in the sample and then the percentage present in the sample. The method follows

Weight of sample used, 2 grams.
Standard used, 20 c.c. of a solution containing .00002 gram test substance per c.c.
Readings, standard 20 c.c., unknown 48 c.c.

From the readings above it will be seen that in this case the color of the sample was darker than that of the standard and therefore the sample was diluted. There would be no change in the method had the standard been diluted.

Weight of T. S. in standard: Weight of T. S. in
unknown =Volume of standard: Volume
of unknown.

Weight of test substance in standard is 20 X .00002= .0004 gram.

.0004: X = 20 : 48 ;
X = .00096.

Therefore the two grams of sample used contained .00096 grain of test substance and the percentage in the sample was .048 percent.

Results by the method of duplication are easily obtained. A standard solution is added to a blank con-

 

 

 

FIGURING THE RESULTS.

29

taining the same reagents as the sample until the color of the sample is matched. Water and standard are then cautiously added, alternately, until the volume as well as the color of the two solutions is identical. The result as to how much test substance is present in the sample is given by the amount necessary to form an identical standard. The percentage in the sample is then figured, thus:

Weight of sample, 5 grams.
Standard used contains .0002 gram test substance per c.c.
Standard used for duplication 2.3 c.c.
Then the total test substance used was 2.3 X .0002=.00046 gram.

Therefore the sample contained .00046 --t-5 gram test substance per gram of sample or .000092, which is .0092 percent.

Results obtained by the balancing method are some-what more complicated to obtain, especially if the apparatus used is graduated to cm. instead of cc.

To take, first, a case when the apparatus is graduated to c.c. and tubes A and B are identical, whether they be the Hehner cylinders or Campbell-Hurley apparatus or some other type of instrument,

Standard used contains .00002 gram test substance per c.c.
Volume of standard used to balance colors, 44 c.c.
Sample used, 4 grams.

The amounts of test substance in sample and standard are the same when the colors are balanced, that is the volumes differ but the total content is the same. Therefore the amount of test substance in the sample being the same as in the standard is

44 x .00002 = .00088 gram.

 

 

30

COLORIMETRIC ANALYSIS

Since the sample used was 4 grams it was therefore .022 percent, test substance.

To take the other case of cylinders graduated to centi-meters, the results obtained with a cheaper instrument are accurate but more calculation is involved, thus:

Weight of sample 2 grams.
Volume of sample after solution before addition to tube A=50 c.c.
Height of sample in tube A=7.3 cm.
Height of standard in B to balance-8.4 cm.
Content of standard per c.c.=.00001 gram test sub-stance.

When the colors of A and B are balanced the concen-trations of the contained solutions are inversely proportional to the volumes used, thus:

T. S. per c.c. in A: T. S. per c.c. in B=Depth
of liquid in B : Depth of liquid in A ;
X : .00001= 8.4 : 7.3.

X =.0000115 gram test substance per c.c. in sample solution.

Then the total test substance in the sample is

50 X .0000115 = .000575 gram.

This is .0002875 gram per gram of sample and the sample is therefore .02875 percent test substance.

 



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Last modified 3 August 2000