October 15, 1931
INDUSTRIAL AND ENGINEERING CHEMISTRY
of 0.1 N sodium hydroxide. The correction for this error is therefore about 1 drop of 0.5 N alkali. I n another series of experiments, water was added to the phosphoric acid left over from a previous blank determination, and distillation continued repeatedly. The acid in each 800 cc. of distillate was titrated and found to require 0.13 to 0.21 cc. of 0.1 N sodium hydroxide. Apparently a minute amount of phosphoric acid has been mechanically carried over in spite of all the precautions taken. The 1 drop of 0.5 N alkali correction, therefore, covers not only volatile acids, but also phosphoric acid carried over mechanically. A comparison of this method with the benzidine method using thymol blue is given in Table 111. Benzidine Method with Phenolphthalein Indicator
If analysis of lead acetate is made only occasionally, it might not be desirable to prepare a permanent color standard. I n such a case, one would find the benzidine method with phenolphthalein as indicator more suitable. PROCEDURE-Dissolve 0.94 to 0.95 gram of the sample in 20 cc. of water, and add 26 cc. of 0.2 N sulfuric acid and 50 cc. of 95 per cent neutral alcohol. After standing for 30 minutes, filter through a Gooch crucible and wash with 50 cc. of dilute neutral alcohol. Wash the Gooch funnel and the bottom of the crucible with 25 cc. of water. Collect the filtrate and washings in a 300-cc. Erlenmeyer flask of Pyrex glass. Add 5 cc. of benzidine solution and allow to stand for 5 minutes. Filter through a 9-cm. filter paper, and wash with 50 cc. of water. Titrate the filtrate and washings with 0.1 N sodium hydroxide, using 4 drops of phenolphthalein as indicator. ACCURACY OF REsums-The results obtained with phenolphthalein and thymol blue are equally accurate. Table IV gives the results obtained with five samples of lead acetate. DiscussIoN-Twenty-six cubic centimeters of 0.2 N sulfuric acid are sufficient for precipitating the lead in 0.9483 gram of lead acetate [Pb(CzHa02)2.3H20], leaving 1 cc. in excess. Practically all the ordinary lead acetate of commerce is of this composition, though often a minute amount of acetic acid has been lost through absorption of carbon dioxide. Loss of part of the water of crystallization also often occurs during storage. More than 26 cc. of the acid would be required if the sample is a basic salt. The failure of benzidine to produce a precipitate shows that an insufficient amount of sulfuric acid has been added. If the lead content is known, the
381
amount of acid to be added can be found by calculation. A separate rough titration with thymol blue will also furnish the necessary information. The phenolphthalein end point is quite sharp in the presence of alcohol, and no color standard is required. Table IV-Benzidine
SAMPLE
Method w i t h Phenolphthalein Indicator AcOH BY HaPo4 DISTILLATION -BENZIDINE METHOD-METHOD DIFF. Wt. of 0.10706 N sample NaOH AcOH
%
%
%
Gyams
Cc.
Lead acetate, Merck's reagent
0.9463 0.9431 0.9474
46.09 46.00 46.19
31.31 31.35 31.34 Av 31 33
31 38
- 0 05
Lead acetate, com'l crystals, 1
0 9474 0 9464 0 9421
46 30 46 22 45 92
31 31 31 Av 31
41 39 33 38
31 41
- 0 03
Lead acetate. com'l. crystals, 2
(0.10026 N ) 0.9467 49 34 0.9445 4 9 . 2 5 0.9476 49.16 Av.
31.37 31.39 31.23 31.33
31.36
-0.03
Lead acetate, pharmaceutical
0.9457 0.9415 0.9446
49.02 48.97 49.15
31.20 31.31 31.32 Av. 31.28
31.28
-0.00
Leadsubacetate, Merck
0.9272 1.1619 1.1245
38.22 47.58 46.24
24.81 24.65 24.75 Av. 2 4 . 7 4
24.76
-0.02
Acknowledgment
The authors are indebted to G. A. Haley, T. W. Zee, A. T. Bawden, and J. W. Gibb for reading over the manuscript and offering valuable suggestions. Literature Cited (1) Duchemin and Criqueboeuf, Bull. assocn. chim. sucr. disl., 24, 1216 (1907). (2) Fresenius, Z anal. Chem., 5, 315 (1866). (3) Fresenius, Ibid., 18,30 (1874). (4) Fresenius, Ibtd , 14, 172 (1875). (5) Fresenius and Grunhut, Ibid., 47, 597 (1908). (6) Gladding, J. IND. ENG.CHBM., 1, 250 (1909). (7) Gray, Ibid., 1, 802 (1909). (8) Koltho5, Z . anorg. allgem. Chcm., 115, 168 (1921). (9) Popoff, "Quantitative Analysis," 2nd ed., p. 71, Blakiston, 1927. (10) Scott, "Standard Methods of Chemical Analysis," 4th ed., Vol. 11, p. 1547, Van Nostrand, 1927. (11) Stillwell and Gladding, J . Soc. Chcm. I d , 23, 305 (1904).
Stability of Potassium Ferrocyanide Solutions'*z I. M. Kolthoff and E. A. Pearson UNIVERSITY OF MINNESOTA, MINNEAPOLIS, MI".
N CONNECTION with work on the use of potassium ferrocyanide solutions as a reagent in volumetric procedures, considerable difficulty was encountered on account of the instability of the solutions (1/40 molar) with or without some potassium ferricyanide. The decomposition of potassium ferrocyanide solutions when exposed to light or t o air or to both has been the subject of many investigations (I-3,5-7,0-15,17-10, U). However, the total mechanism of the various steps in the decomposition reaction are still not clearly understood. It has been known for some time that a fresh solution of potassium ferrocyanide in water does not redden phenolphthalein; on standing in
I
1 Received May 29, 1931. 1 Part of a thesis submitted by E. A. Pearson to the Graduate School of the University of Minnesota in partial fulfilment of the requirements for the degree of doctor of philosophy.
light it is colored red. If this solution is allowed to stand in the dark, the color again disappears. Therefore, it was thought that the first stage of the decomposition could be represented by the following reversible reaction: dark
2Fe(cN)~---- 4- '/zOa f Ha0
* 2Fe(cN)~--- f 20H-
light
Jimori (8), however, showed that the reaction proceeds in the light without oxygen, and that in the first stage of the decomposition aquopentacyanide is formed which has a yellow color:
or summarized:
ANALYTICAL EDITION
382 T a b l e I-Decomposition
Vol. 3, A-0. 4
of 1/40 M P o t a s s i u m Ferrocyanide a t R o o m T e m p e r a t u r e
TEMP. OF
RECRYS-
COLOR GLASS
TALLIZATION OF
OF
FEREO-
OF
SOLVENT
EXPT. CYANIDE
c.
1 2 3 4 5 6 7 8 9 10 11
100 100 100 80 SO
80
100 80
100 80
12 13 14 15
100 80 100 80 80
16
100
Distilled water p H 5.2 Distilled water: pH 6.2 Conductivity water, p H 6.5 Distilled water, p H 5 2 Conductivity water, p H 6.5 Conductivity water, p H 6.5 Distilled water 0.1 sodium bicarbonate Distilled water 0.1% sodium bicarbonate Distilled water 0.2% sodium carbonate As 8 Distilled water 0 1% sodium hydroxide
+ ++ +
As 11 Conductivity water As 13
+ 0.3% potassiumferricyanide
Distilled water 4- 0.2% 0 2% KsFe(CN)e
As 15
sodium carbonate and
DECREASE IN STRENGTH AFTER
16
37
BOTTLE Days Colorless Brown Brown Brown Colorless Brown Brown Brown Brown Brown Brown Brown Brown Brown
1.7 7 4 0 8 1 1 0.3 0.7 1.3 1 7 1.5 4.8 0.6 0 6 0.4 0.6 0 5 0 9 0 0 0 0 0 0 0 0 0 0 $0 4 0 0 -0 4 1 0 0 9
Brown Brown
0 0 0 0 0 0 +O 3
Fe(CN)s---- -k 2Hz0 [Fe(CN)6HzO]--- -k OH- f HCN Heat and sunlight favor the decomposition; a t low temperatures and in the dark the reverse reaction takes place. The subsequent steps in the decomposition according to Jimori are non-reversible, and in the presence of oxygen lead to the formation of iron oxide and cyanic acid. From the analytical Point of view it is interesting to note that the first stage of the decomposition is reversible, and from the reaction mechanism it may be expected that an of hydroxyl ions will prevent the formation of the aquopentacyanide. Therefore, it may be anticipated that the stability of a ferrocyanide solution will depend largely on the pH of the solution. This was experimentally confirmed. In the beginning of this work 1/40 molar solutions of pure potassium ferrocyanide were prepared in ordinary distilled water having a pH of 5.0 t o 5.4. This water contains more carbon dioxide than is in equilibrium with the air. The strength of the solution decreased 0.4 per cent one day after the preparation, and the decomposition proceeded on longer standing. JT7hen conductivity water with a PH Of 6.0 to 6.7 was used as a solvent, the decomposition was much less, but still amounted to 0.5 per cent after 3 days' standing in diffuse light. In all cases direct sunlight favored the decomposition very much. It was thought necessary to make a more extensive investigation of the stability of ferrocyanide so~utionsin order to get a reagent which does not change on standing. Some of the results are given. (Only part of the work Will be given here. For more details the reader is referred to the thesis of E. A. Pearson.) Potassium Ferrocyanide Used +
The higher the temperature of recrystallization of the ferrocyanide the more aquopentacyanide is formed, according to Briggs (4). Schroder (20) stated that potassium ferrocyanide may be satisfactorily crystallized from a boiling, saturated solution. Some hydrocyanic acid is formed, but the salt when thus recrystallized always had a normal water content, contrary to the statement Of Mu11er and Diefenthaler (16). The results in this laboratory showed only slight differences in the progressive decomposition of salts but more at 8oo and at 'O0" aquopentacyanide is formed at higher temperatures, for the salt recrystallized from boiling solutions had a slightly more yellowish tinge. Also, solutions made from such salts seemed to be initially more yellow. Moreover, such solutions when stabilized with alkali and kept in the dark appeared to gain slightly in strength (0.1 to 0.3 per cent in 3 days) owing to the driving back of the hydrolysis, and coincidentally the solutions became almost colorless. I n the following experiments, shown in Table 1, two samples of potassium ferrocyanide were used. Two samples
REMARZS
Days
dark yellow precipitate of FenOs on bottom bright yellow no turbidity bright yellow: no turb/dity bright yellow, no turbidity dark yellow. brecipitate of FenOa on bottom bright yelloh; no turbidity or deposit bright yellow. no turbidity or deposit Soh. bright yellow' no turbidity or deposit Practically colorles; after 1 day; slightly yellow after 37 days Practically colorless after 1 day: slightly yellow after 37 days Soln. colorless: ferric hydroxide on bottom Soln colorless, ferric hydroxide on bottom
Soln Soln. Soh Soln. Soln. Soh. Soh.
of analytical-grade ferrocyanide recrystallized at 80" to 90" c. were recrystallized, one at 80" C. and the other by saturating at boiling temperature by the method of Schroder. In the latter case the solution was maintained a t boiling for 3 minutes, then filtered, and allowed to crystallize. STABILITY OF FERROCYANIDE SoLunoNs-Portions
of 200 to
250 cc. of the 1/40 molar solutions were placed in well-stoppered flasks and exposed to laboratory light. During the last 21 days
of the experiment (January), the bottles were kept on the window sill facing the sun. The analysis of these solutions was performed by pipetting out 25 cc., adding 10 cc. of 4 N sulfuric acid, one drop of 1 per cent potassium ferricyanide, 3 drops of 1 per cent diphenylamine, and titrating with 0.025 M zinc sulfate until the yellowish green color changed to bluish purple. From these and many other experiments the follo~vining conclusions may be drawn: (1) Ferrocyanide solutions kept in transparent bottles are much less stable than those in brown bottles when exposed to light. (2) The stability of the solution decreases with increasing acidity. In distilled water with an excess of carbon dioxide (pH 5.0 to 5.2), the decomposition is stronger than in water which is i n ~ ~ i 1 $ ~ ~ " , ~ ;&rystallization ~~l$ of the ferrocyanide has no appreciable effect upon the stability of the solution, (4) The presence of small amounts of ferricyanide favors the decomposltion of ferrocyanide solutions. (5) Dilute solutions are relatively less stable than more concentrated solutions. (6) Solutions prepared in 0.2 per cent sodium carbonate in distilled water are stable for long periods of time. Change of the temperature (up to 50' C.) and presence of small amounts of ferricyanide have no or little effect upon the stability in this alkaline medium.
Literature Cited Baudisch~ 62*2706 (lg2'). (2) Baudisch and Bass, Ibid., 64, 413 (1921);56, 2698 (1922). (3) Bauer, Helnr,Chim. Acta, s, 403 (lgo0), (4) Briggs, J . Chem. S O L . , 93, 1571 (1908): 89, 1019 (1911); 117, 1026
(1920). and Hottenhauser! Chem.-Zfg 323 (lggo). (5) (6) Haber, Z . Eleklrochem , 11, 846 (1905). (7) Haber and Foster, Chem.-Ztg., 29, 652 (1905);J . Chem. soc., 89, 912 (1906). (8) Jimori, Z. anow. allgem Chem 167, 145 (1927). (9) Kistiakowsky, 2. physik. Chem., 36, 431 (1900). (10) Kolthoff, Z . anorg. allgem. Chem., 110, 147 (1920);"Volumetric Analysis,'' Vol. I, Wiley, 1928. (11) Kolthoff and Verzyl, Rec. tmu. chzm., 43, 380 (1924). (12) Koninck, de, and ProSt, Z . angem. Chem., 15, 460, 564 (1896). I
i;i; (15)
~
~
~
~
~
~
~
Modenhauer, bid., 15, 223 (1891). (16) Muller and Diefenthaler, z anovg. Chem , 67, 421 (1910). (17) Rossi and Bocchi, Case. chzm. ital., 66, 816 (1925). (18) Schaum and van der Linde, Z. Elekfrochem., 9, 406 (1903). (19) Schbnbein, Fortschrztt Physck, 228 (1846). (20) Schroder, z. anorg allgem. Chem ,72,89(1911). (21) Walker, J . Am Chem. Sac., 23, 468 (1901).
~
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