ANALYTICAL EDITION
174
Vol. 3, No. 2
Hydrogen-Ion Concentration and the Color of Lead Chromate Pigments’ R. C. Ernst, E. Pragoff, Jr., and E. E. Litkenhous UNIVERSITY OF LOUISVILLE, LOUISVILLE, KY.
EAD c h r o m a t e pigThe hydrogen-ion concentration in lead acetate the measured electromotive ments are prepared by solutions is determined by potentiometric methods. force and the pH (according The saturated calomel electrode is used as the standard the addition of sodium to Sorensen, 6) is half-cell with the quinhydrone electrode as the indicaor potassium chromates or diE = 0.4526 - 0.0591 log pH a t 25’ C. (1) chromatcs to a solution of a tor electrode. The method is applied to the measuresoluble lead salt. The colors ment of hydrogen-ion concehtrations of lead acetate The electromotive force solutions in the preparation of lead chromate pigments. of the resulting chromate pigwas determined by the use of ments are regulated by the It is shown that the hydrogen-ion concentration is one the potentiometer using the addition of an acid or base of the determining factors in the colors of the resulting circuit as shown in Figure 1. before precipitation to conpigments and can be used to regulate these colors. The quinhydrone e l e c t r o d e A comparison is made of the colors obtained at the trol the acidity of the lead was used because preliminary solution. Considerable diffidifferent hydrogen-ion concentrations at various teminvestigation showed that the culty has been experienced peratures, concentrations, and times of precipitation. h y d r o g e n electrode would in determining the aciditv of give inaccurate results in solulead solution; The resuiting colors have therefore been diffi- tions containing lead, while the quinhydrone was satisfactory cult to reproduce. below a pH of 9. The changes in the hydrogen-ion concentration of lead Determination of pH in Lead Acetate Solutions acetate solutions was determined by carrying out the neutralization of an acid by a base in the presence of lead acetate. The literature reveals no electrochemical method for the The hydrogen-ion concentration of the solution was varied determination of the hydrogen-ion concentration in lead by the use of 0.1 normal solutions of acetic, nitric, hydrosolutions. Several methods have been suggested for the chloric, and sulfuric acids, and potassium hydroxide. Poanalysis of lead solutions. Langnecker’s (6) direct titration tentiometer readings of electromotive force were taken after method gives a measure of the hydroxyl radical, Kolthoff’s stirring the solutions and allowing the readings to become (4) sodium sulfate method determines the total lead and constant in each case. The quinhydrone electrode used dipped directly into the solution during a titration. The calomel electrode was connected to the solution by means of a salt bridge (potassium chloride and agar-agar) which prevented diffusion of potassium chloride into the lead solution. Results of the titrations of the acids and base alone and of the acids, base, and lead acetate are shown in Table I. These results show that acids and bases can be titrated with the quinhydrone electrode and that. a maximum inflection occurs a t the neutralization point. I n the case of the neutralization of the acid with the base in the presence of the lead salt, the inflection also occurs at the neutralization point.
L
hI+ Figure 1-Diagram B-Dry cell battery R-Resistance box S-D.P.D.T. switch C-Reversing switch SP-Student potentiometer
of Potentiometer K-Tapping keys G-D’ Arsonval galvanometer P-Protecting resistance W-Weston standard cell [E-To electrodes
acidity. Bergh’s ( I ) oxalic acid method gives only the total lead. The Fresenius and Cohn (3) sulfuric acid method gives the total lead and acidity. A colorimetric method by Westberg (7) gives, indirectly, a method for acidity of lead solutions. But none of these methods give any measure of the hydrogen-ion concentration of lead solutions. The hydrogen-ion concentration of the lead acetate solutions was determined by measuring the electromotive force of a cell consisting of the solution with the saturated calomel electrode as a half-cell and the quinhydrone electrode of Biilmann (2)as the indicator electrode. The relation between 1 Received September 19,1930. Presented before the Division of Paint and Varnish Chemistry at thc 80th Meeting of the American Chemical Society, Cincinnati, Ohio, September 8 to 12, 1930.
Effect of pH on Color of Lead Chromate Pigments
The effect of the hydrogen-ion concentration on the color of lead chromate pigments was determined by precipitation of the lead chromate from solutions having different hydrogenion concentrations. I n these experiments 0.5 normal solutions of both lead acetate and potassium dichromate were used. The lead chromates were prepared by treating 300 CC. of 0.5 N lead acetate with acid and base (potassium hydroxide) until the potentiometer read the voltage corresponding to the required pH, and this solution was precipitated using 300 cc. of 0.5 N potassium dichromate solution. Agitation was used throughout. All precipitations were performed in 34 2 seconds. The temperature was 25 * 2 O C. The value for the potentiometer reading with the corresponding pH is given. The pH’s of the dichromate solutions, corresponding to the value for the electromotive force read, are probably not true pH values. All values above a pH of 9 are doubtful. The colors were determined by rubbing the pigments in clear varnish, the mass tones by rubbing the pigments in
April 15, 1931
INDUXTRIAL A N D ENGINEERING CHEMIXTRY
175
raw linseed oil, and all tints and strengths by mulling 2 x 50 strokes, 50 mg. of color, 1 gram of zinc oxide, and 12 drops of linseed oil. The results of these experiments are given in Tables I1 to X, inclusive. Table 11-Effect of Variation of Acetic Acid a n d Potassium Hydroxide on H-Ion Concentration of Lead Acetate Solution (Potasqium dichromate added to the lead acetate solution) SAMPLE
I
LEADACETATE
I
POTASSIUX DICHROMATE
I
1 2
E
3.5
-245.0 218.0
4.0 5.0
3
4
6.0
6.5 7.0
6
7
a
8.0 9.0
9
10.0
PH
2.5
l67,O
2.5 2.5
98.0
2.5
67.0
2.5 2.5 2.5 2.5 2.5
0.0 +- 420.0 80.0 +140.0
E -305.0
305.0 30q.O 305.0 305.0 305.0 305.0 305.0 305.0
__
FIKAL D U
~
PH
3.95
4.25 4.45 4.55 4.80 5.08 7.30 10.75 11.30
The color of the pigments changes from a light yellow at a pH of 3 to a bright orange a t a pH of 9, where a maximum brilliance and darkness of shade are obtained. Above a pH of 9 a lighter brownish offshade appears. The masstones of samples 1, 2, 3, and 4 were light clean-looking chromate yellows, sample 1 not quite so clean-looking as 2, 3, or 4. Sample 5 was a trifle dirty; 6, a little red; 7, a light orange; 8, orange; and 9, a dirty brown. Samples 2, 3, and 4 were very strong yellows, 1 was a trifle weak, 5 was weak and dirty, and 6 was a trace weaker and redder than 2 to 4. Sample 7 had a good reddish tint and strength, 8 had a purple tint, and 9 was very weak. Table 111-Effect of Variation of Acetic Acid a n d Potassium Hydroxide on H-Ion Concentration of Potassium Dichromate Solution (Lead acetate added to the potassium dichromate solution) POTASSIUM DICHROMATE
LEADACETATE
SAMPLE E
E
FINAL PH
4.40 4.52 4.58 4.75 4.90 5.04
5.10 5.20 5.75 5.98 6.10
When the pH of the dichromate solution is changed rather than the lead acetate solution, the resulting colors change from a light yellow a t a pH of 2.5 to an orange a t a pH of 9. Above a pH 9 of a lighter offshade brownish yellow is formed. The masstones of samples 11, 12, 13, and 14 are a little more yellow than that of 2, 3, or 4, or else 2, 3, and 4 are a little greener. Sample 15 is a little deeper in shade and a trace dirty. Samples 16 and 17 are about as 2 and 3, 18 and 19 are a dirty brown and offshade, and 20 is almost as bad. Samples 11, 12, and 13 are as strong as 1, 2, or 3 but a deeper red tint, and 14, 15, 16 are a trace weaker. Sample 17 is the same strength as 2 , 3 , or 4; and 18,19, and 20 are very weak, 19 being best. Table IV-Effect of Variation of Acetic Acid a n d Potassium Hydroxide on H-Ion Concentration of Solutions Containing Precipitated Color (Potassium dichromate added to the lead acetate solution)
1
SAMPLE
LEADACETATE PH
E
I
POTASSIUM DICHROMATE
PH
E
ANALYTICAL EDITION
176
If the pH of the solution containing the precipitated color is varied with acetic acid and potassium hydroxide after precipitation, the color changes from a light yellow a t a pH of 4 to a brilliant orange a t a pH of 9. Above 9 an offshade color appears which is lighter than the shade of the pigment of a pH of 9. In masstone, sample 21 is slightly green and dirty, 22 is a good yellow shade, 23 is dirty, 24 and 25 are badly offshade, 26 and 28 a fair orange, and 27 is brown. Sample 21 is a little yellow in tint, of good strength, and a little dirty. Sample 22 is best. Sample 23 is reddish, 24 and 25 are weak, 26 and 28 are of fair strength with reddish tints, and 27 is a trace weaker than either 26 or 28. Table V-Effect of Variation of Hydrochloric Acid a n d Potassium Hydroxide on H-Ion Concentration of Lead Acetate Solution (Potassium dichromate added to the lead acetate solution)
1
LEADACETATE
SAMPLE
I
POTASSIUM DICHROMATE
1
29 30 31 32
E
PH
E
3.0 4.0 5.0 6.0
-270.0 215.0 157.0 98.0
2.5 2.5 2.5 2.5
-305.0 305.0 305.0 305.0
LEADACETATE
35 36
5.0 6.0
I
-270.0 215.0 157.0 98.0
POTASSIUM DICHROMATE
2.5 2.5
2.80 4.25 4.45 4.55
FINAL
~
PH
-305.0 305.0 305.0 305.0
2.70
Table VII-Effect of Variation of Sulfuric Acid a n d Potassium Hydroxide on H-Ion Concentration of Lead Acetate Solution (Potassium dichromate added to the lead acetate solution)
1
L
I1
% 39 40
POTASSIUM DICHROMATE
LEADACETATE
SAMPLE
PH
E
3.0 4.0 5.0
-270.0 215.0 157.0 98.0
6.0
E 2.5 2.5
-305.0 305.0 305.0 305.0
I1
FINAL pH
2.75 4.18 4.45 4.55
The colors were similar to those of 1, 2, 3, and 4. Sample 39 is the only good color for masstone. Sample 38 is dirty and green, and 37 is dirty and red, while 39 is of fair strength and yellow tint, 38 is weaker, and 37 has no tinting strength atla11. of Variation of Temperature of Lead Acetate a n d Potassium Dichromate (Potassium dichromate added t o the lead acetate solution)
Table VIII-Effect
TgM~ERA'
TURE
1
LEADACETATE PH
25
6.5
80 100
6.5 6 5
E
-67.0
67.0 67 0
I
E
2.5
-305.0 305.0
2.5 2.5
305.0
1
LEADACETATE
E
PH
I
-67.0
11
POTASSIUM DICHROMATE PH
E
2.5
-305.0 305.0 305.0 305.0 305.0
45 60
6.5 6 5
67.0 67.0
I
2 5 2 5
II
FINAL pH
4.80 4.80 4.80
I
No appreciable change in color was noticed in the pigments. The tints and strengths were close to those of sample 5 with the 45-minute color a trifle weak.
NORMALITY
0.5 1.0
LEADACETATE
PH 6 5 6 5
E
-
67 0 67 0
POTASSIUM DICHROMATE PH
E
2.5
-305.0 -305.0
FINAL PH 4.80 4.80
The color, masstone, and strength of these colors are identical with sample 5. Conclusions
Using the quinhydrone electrode to determine the hydrogen-ion concentration in the lead acetate and potassium dichromate solutions, lead chromates were precipitated. The acidity was regulated by means of acetic, hydrochloric, nitric, and sulfuric acids. I n each case the color changed from a yellow a t a pH of about 3.5 to an orange a t a pH of 9, which was a maximum. Above a pH of 9 the colors became offshades. When the temperature of the 0.5 normal solutions was varied, the color changed from a yellow at low temperatures to a maximum at 80" C. Above this temperature the color again became lighter. Change of time of striking and of concentration of metal ions seemed to have no appreciable effect on the color of the chromate pigmehts. Acknowledgment
The authors wish to express their appreciation to the Oil, Paint, and Varnish Club of Louisville for their financial contribution in this research, and to Andrew Snyder for his valuable suggestions and aid. Literature Cited
POTASSIUM DICHROMATE PH
TIME
Mix.
The colors were similar to those of 1, 2, 3, and 4, and the masstone similar to 1, 2, 3, and 4. The strength and tints were like 11, 12, and 13. Sample 34 is weakest.
~~
of Change of T i m e of Precipitation of Lead Chromate (Potassium dichromate added to the lead acetate solution)
--
Table VI-Effect of Variation of NitriO Acid a n d Potassium Hydroxide on H-Ion Concentration of Lead Acetate (Potassium dichromate added to the lead acetate solution)
1
Table IX-Effect
PU
The color of these pigments is a brilliant yellow. A slight change of shade in darkness is noticed in the masstones. Sample 29 is brightest and 32 is darkest, 32 showing a reddish tint. Samples 29, 30, and 31 are similar to 11, 12, and 13. Sample 29 is not quite so strong as 11 and the tint is the same. Samples 30, 31, and 32 gradually decrease in strength and the tint turns a slight purple. Sample 32 is quite weak.
SAMPLE
At 25" C., the color is light yellow and at 80" C., a maximum darkness is obtained. Above 80' C. , the color lightens and has a reddish hue. At 40" C., the masstone is dirty; a t 60" C., a fair yellow; a t 80" C., offshade greenish color; and a t 100" C., very light orange. The tints a t 40" C. and a t 60" C. are the same, and both have good strength and a reddish tinge. The color a t 80' C. is weak and dirty, and a t 100" C., slightly orange.
FINAL ~
PH
Vol. 3, No. 2
4.80 4.45
(1) Bergh, Afioth. Zlg., 24, 291 (1909). (2) Btllmann, Ann. chim. p h y s , 9, 15, 109 (1921), 16, 321 (1921) (3) Fresenius and Cohn, "Quantitative Chemical Analysis," Vol. 11, pp. 599-600, Wiley, 1915. (4) Kolthoff, Pharm. Weekblad, 67, 252-65 (1920). (5) Langnecker, Biochem. Z.,122, 34-8 (1921). (6) Sbrensen, Ibid., 21, 131 (1909). (7) Westberg, 2. physik. Chem., 4, 150 (1889)
