ANALYTICAL USES of COLOR M. G. MELLON, G. W. FERNER,
AND
J. P. MEHLIG
Purdue University, Lafayette, Indiana
Of the warions physical properties utilized i n the detection and determination of constituents, color is one of the most ~aluable. In general, its numerous applications may be considered as a means of controlling general operations i n analytical work and as a bask for methods of determining substances, both gualitatiwely and guantitatiwely. Examples of such uses are given.
meaning. It is in this general sense that the former term is used in the present outline, white and black being accepted as valuable colorimetric terms even though they do not imply hue. Probably, however, a more detailed study of colorimetric phenomena of analytical value will reveal the desirability of utilizing all three characteristics of color for the fullest development of its application in chemical analysis.
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N MAKING quantitative chemical analyses use is A . The control of ccrlain operations. The appearance or disapfrequently made of certain methods which have pearance of color, or a change of color, is used in the course of various operations as an indication of different items or been classified by some authors as physico-chemical conditions, as outlined below. procedures. In general, the determination by such a method of the desired constituent in anv . given svstem 1. PREPARATION O*. REAGENTS is based upon the magnitude of some physical property, a. Stability Of the system Fading or disappearance of the original hue. (Dilute. the value of which bears a definite and known relation- a. aqueous solution of ship to the amount of the constituent present. Ex- b. A~~~~~~~~~of a color amples of such properties are specific gravity, optical a'. I n the solution. (Alcoholic solution of potassium hyrotation, and color. droxide.) Color has long been as a distinguishing b'. I n a residue. (Aqueous solution of silver nitrate.)
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Property of kinds in the various a ~ ~ l cations of materials in art, industry, and science. Recognition of certain metals and natural dyes by their hues must date far back in chemical histoni. This early beginning of the qualitative application-of color has been greatly extended until now it is used as one of of identification of many 'Iements the and compounds. In addition to this use of color in qualitative analysis there has been a constantly increasinp development of colorimetric methods for determining-constitLents quantitatively, striking evidence of which is furnished by Yoe's recent book on colorimetry where procedures are given for the determination of some thirty-five elements in addition to many compounds. In view of the extensive reliance upon this property by the chemical analyst the object of the present paper has been to collect and classify these applications, giving specific examples wherever possible. Details of the methods of application are not mentioned on account of limitations of space and because experienced analysts are familiar with them. Occasionally there was some uncertaintv whether a &en annlication should be classified under "A" or With the evception of a few recent quantitative studies, analytical chemists have used the term color in its composite sense without attempting to designate the significance of the separate attributes, hue, brilliance, and saturation (colorimetric purity). Most chemists ord~narilyuse the general term color in situations where the term hue would more precisely indicate the intended
"B."
A.
b.
Correctness of procedure
b.
Characteristic color desired. (Amrnouiacal cupric sulfate.)
ia.- No color desired. (Phenoldisulfonic acid.) 2. SELECTIO~OP SAM^^^ a. Location to take material for a samvle. (Black out-croooinz -- of a vein of coal.) h. Type of material to take. (In heterogeneous material, such as many minerals, colored portions are either selected or rejected, or a representative mixture of both selected.) 3, PREPARATION OP - - ~ ~ ~ ~ - - a. undesirable reactions. (Oxidation of certain foodstuffs.) b. Progress of preparation. (Grinding cupric sulfate pentahydrate to a h e powder.) 4. SOLUTION O F SAWLE a. Presence of certain constituents. (Green from manganese in rocks fused with sodium carbonate.) b. Completion of certain reactions. (Disappearance of black particles in fusing residue from iron ore.) c . Adjustment of the composition of solutions a. Addition of an excess of some reaeent. (Reduction of a solution of ferric chloride with stannous chloride.) b. Attainment of desired p H value. (Neutralization indicators.) c. Runoval of undesired constituent. (Boiling out oxides of nitrogen.) 5. SEPARATION OP DESIRED CONSTITUENT (OR INTERPERIND ~
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SUBSTANCES)
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a.
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Formation. (Appearance of characteristically colored precipitates, such as nickel dimethylglyoxime.) b. Digestion a'. Attainment of desired physical state. (Change from yellow to orange of freshly precipitated lead chromate.) b'. Change in D H value of solution. (Loss of ammonia chane& color of an indicator.) " c. Filtration and washing. (Disappearance of yellow chloroplatinic acid when washing potassium chloroplatinate.)
d.
Ignition
a'. Comdetion of Drocess.
e.
(chance from hrown t o v d a w in
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annealina aold in assavine.) . b'. State of purity of the precipitate. (Brown barium sulfate from adsorbed iron.) b. Extraction a. Exhaustion of colored material from sample. (Alcoholic potash extract of Para rubber compound.) b. Presence of desired material in solvent. (Separation of eapper salt of proline with dry methanol.) c. Volatilization a. Attainment of conditions suitable for evolution. (Appearance of vapors of bromine and iodine on chlorination of mixed silver halides when proper temperature is reached.) 6. Completion of evolution of constituent. (Cupellation of allays in assaying, such as the disappearance of iridescence when lead is removed from gold.) c. Contamination by impurities. (Yellow ferric chloride in the residue when making canstant-boiling hydrochloric acid.) d. Electro-deposition a. By means of the electric current a'. Formstion of colored products in the solution. (Anodic color in a cyanide solution of silver.) 5'. Nature of the deposit during formation. (Bright us. brown, spongy deposits of copper.) c'. Nature of process of forming deposit. (Iridescent rings of lead peroxide under certain conditions.) d'. Completion of the process of deposition. (Disappearance of the color of an ammoniacal solution containing nickel.) b. By displacement a'. Progress of reaction. (Separation of copper on aluminum.) 6 . MeASUREMENI OF CONSTITUENTS IN SAMPLE a. By gravimetric methods a. Gas-evolution a'. Exhaustion of absorbing reagent. (Indicator solution in train in determining carbon in steel.) b. Gravimetric precipitation. (Colorless, fused silver chloride indicates pure precipitate in determining atomic weights.) c. Solution and extraction. (See 5, b.) d. Partition. (Colorless ethereal layer indicates progress of Rothe separation of iron and nickel.) b. By titrametric methods a . Indication of end-point in titrations a'. Standard solution serves as its own indicator ' Oxidation-reduction reactions-potassium penuanganate b'. Inside indicators a". Formation of a colored solution a"'. One color process Neutralization reactionsphenolphthalein Oxidation-reduction reactions-starch Precipitation reactions-ferric alum 5"'. Two color process Neutralization reactions-methyl orange Oxidation-redaction reactions--starch (standardizing sodium thiosulfate against potassium dichromate) Precipitation reaction~dichlorofluorrscein b". Formation of colored precipitate Precipitation reactionssilver chromate in determining chlorides c'. Outside indicators a". Formation of a colored solution b". Formation of a colored precipitate Precipitation reaction-potassium ferricyanide in determining iron c. By electrolytic methods. (Condition of dried deposit, such as colored film of oxide of cooner.) d. l % ygnsomrfric rnrthods o. I'xhau-tion of reagents. (Change of color of Awariru.) b. Progress of reaction. (Green color in ahsorption of rarlmn monoxide by Hoolamite.) ~~~
.. .
By physico-chemical methods
a. Optical a'. Colorimetric (See C, 1, a) b'.
Refractometric Lieht of definite hue (wave-length) " " , 6 ' . Polarimetric Same as b':. d'. Spectrophotometric (See C, 1, b)
B . T k qualitative detection of conditunts (nacro- end microscopic) 1. WITHOUT THE Am OF SPECIAL TESTS. (The general appearance and color of the material indicates its compositiongold, copper, and gems.) 2. WITH THE AID OF SPECIAL TESTS a. Action of solids on certain white surfaces. (Some minerals make colored streaks.) b. Reaction an heating a . Heating alone a'. Change in color a". Solutions. (Aqueous cobalt chloride from red to . blue.) 6". Solids a"'. Change in residue. (Zinc oxide from white t o yellow.) b"'. Evolution of colored vapor Iodine, nitrogen tetroxide c"'. Formation of tarry deposit Organic matter d"'. Formation of sublimate Arsenic b'. Production of colored flames. (Sodium.. strontium: use of cobalt glass in test for potassium.) 6'. Production of distinguishing spectral lines. (Lithium, thallium.) b. Heating in the presence of other substances a'. With concentrated sulfuric acid a". Blackeninr. (Oreanic matter.) b". Formation of colored deposit. (Sulfur, iodine.) c". Formation of colored vapors. (Bromine, nitrogen tetroxide.) b'. On charcoal a". Alone. (White residue of calcium oxide.) b". With sodium carbonate. (Colored beads or residues, as with gold or manganese.) c'. On plaster tablets. (Different reactions. with and without reagents.) d'. With beads. (Different colors with certain elements in borate and phosphate beads.) e'. With other reagents. (Different colors with certain residues, such as the blue with one containing aluminum when heated with cobalt nitrate.) c. Reaction in solution with reagents a. Reagent in test papers. (Litmus, silver nitrate for Gutzeit test.) b. Reaeent in solution a'. Formation of colored orechitate. (Autimonv trisulfide.) b'. Formation of colored calla~daldispersion. (Gold.) c'. Formation of colored solution. (Tetrammino cupric sulfate.) d. Reaction to light (including ultra-violet radiation) a. Darkening of precipitates. (Salts of silver.) b. Fading of color in solutions. (Alcoholic solution of potassium dichromate decolorized by ultra-violet radiation.) c. Fluorescence. (Some varieties of Willemite assume characteristic hue under ultra-violet irradiation.) d . Type of extinction. (When examined microscopically certain crystals exhibit different types of extinction by their different colorimetric phenomena.) e. Phosphorescence. (Certain substances phosphoresce after proper exposure to light.) f. Selective absorption. (Some substances, such as carbon
monoxide in the blood, show characteristic absorption spectra.) C. The puntitalive estimtion of conrlifuents 1. COLOXMETRIC METHODS a. Methods of comparison (visual and photoelectric) a. By standard sets a'. With transmission a". Solutions a"'. Single series type 1. Containing same material as that being determined. (Buffer solutions and indicators for p H values.) Containing permanent standards (usually inorganic). 2. (Amy's "Cu-Fe-Co" solutions.) b"'. Double series type. (Gillespie method for pH values.) 6"'. Wedge type. (Hellige method for various constituents.) b". Glasses. (Hellige discs.)
b'.
With reflection. (Color cards or plates, such as those of Munsell.) b. Bv dilution c. By duplication d. By balancing b. Methods of color-analysis (visual and photoelectric) a'. Withspectrophotometer a". Monochromatic analysis. (Dominant wave-length, brilliance, and colorimetric purity.) b". Trichromatic analysis. (Elementary excitation valuesred, green, and blue.) c". Monochromatic transmission 6'. With trichromatic matchinginstruments. (Lovihondtintometer, Guild colorimeter, Eastman colorimeter.) c'. With instrument using selected portions of spectrum. (Pulfrich photometer.) 2. MIcnoscoPIc M E ~ O D S . (Thickness by polarization colors of anisotropic substance.)
