October 15, 1931
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
379
Benzidine Method for Determination of Acetic Acid in Lead Acetate' J o h n E. S. H a n a n d T. L. C h u Y-1065c NORTH SZECHUEN ROAD, SHANGHAI, CHINA
T h e lead in lead acetate is precipitated by sulfuric 0.1 N ACETICACIDAND 0 2 N HE standard method acid i n t h e presence of t h y m o l blue u n t i l the solution ~ U L F U R I CAcID-These Solufor estimating acetic tions were boiled, cooled under acid in lead and other acquires a distinctlyred-orange color, indicating t h e acid a soda-lime tube, and standacetates involves distillation range. Alcohol is added a n d t h e solution filtered. T h e ardized against t h e 0 1 N with phosphoric acid ( 5 ) . If excess sulfuric acid is precipitated w i t h benzidine a n d sodium hydroxide. BENZIDINESOLUTION-EXagain filtered. T h e acetic acid is directly t i t r a t e d w i t h the d i s t i l l a t i o n is carried s t a n d a r d s o d i u m hydroxide t o a yellowish green color, act1y9.21 grams Of the free base nearly to dryness (a), Some were dissolved in 95 per cent phosphoric acid usually passes indicating t h e alkaline range. This is a quick m e t h o d alcohol and diluted t o 1liter. into the distillate. The passa n d t h e results agree closely w i t h t h e phosphoric acid PBENOLPHTHAI,EIN-~ne distillation method. Equally accurate results can be gram was dissolved in 100 cc. ing Over of phosphoric acid can be reduced to a negligible obtained w i t h phenolphthalein as t h e indicator. of 95 per cent neutral alcohol BLuE-Exactly 0.12 amount by adding water conLead can be conveniently determined by heating t h e gram was mixed with 2.84 cc. tinuously (6, 11) 01 intermitlead sulfate precipitate to c o n s t a n t weight. of 0.1 N sodium hydroxide and diluted with water to 100 cc. tently (5,10) during the procNEUTRAL ALCOHOL-TWO hundred and fifty cubic centimeters ess of distillation. The advantage gained in accuracy is offset of 95 per cent ethyl alcohol were neutralized with 0.1 N sodium by the much longer tirne required for each distillation. steam hydroxide, using about 32 drops of thymol blue or 5 drops of distillation (4) consumes less time, but some of the phosphoric phenolphthalein for indicator. Dilute neutral alcohol was acid is mecilanically carried over ( 5 ) . prepared by diluting 90 cc. of neutral alcohol with 81 cc. of water. Fresenius (3) determined acetic acid in lead acetate by as Indicator Benzidine Method with Thymo1 adding an excess of standard sulfuric acid, filtering, and titratLg an aliquot part of the filtrate. From the total icidity PROCEDURE-Dissolve 0.94 to 0.95 gram of lead acetate found is deducted that due to the excess sulfuric acid in the with 20 cc. of water in a 150-cc. beaker. Add 8 drops of filtrate. This requires a separate determination of sulfuric thymol- blue and sufficient 0.2 N sulfuric acid to cawe the acid. Kolthoff (8) found it possible to obtain an accuracy solution to assume a distinctly red-orange color. (Compare of 0.2 per cent, when acetates of alkalies or alkaline earths with 45 cc. of 0.1 N acetic acid, 1 cc. of 0.2 N sulfuric acid, are titrated with normal hydrochloric acid using reference 8 drops of thymol blue, and some pure barium sulfate in a solution and tropeolin 00 or thymol blue as indicator. With similar beaker. The presence of a white precipitate causes basic lead acetate, Duchemin and Criqueboeuf ( 1 ) recom- the solution to appear more red.) Introduce 50 cc. of 95 per mended a previous neutralization with standard acetic acid cent neutral alcohol and allow to stand for 30 minutes. Filter using phenolphthalein as indicator. through a Gooch crucible with gentle suction into a 300-cc. The authors' method consists of precipitating the lead Erlenmeyer flask of Pyrex glass. Wash the beaker and with sulfuric acid, filtering, and washing through a Gooch crucible with 50 cc. of dilute neutral alcohol. Detach the crucible, precipitating the excess sulfuric acid with an alcoholic crucible from the Gooch funnel and wash the funnel and solution of benzidine, again filtering, washing, and titrating bottom of the Gooch crucible with 25 cc. of water. Add 5 cc. with standard sodium hydroxide. The titration is a direct of benzidine solution, mix, and allow to stand for 5 minutes. measurement of the acetic acid present, and yields equally Filter through a 9-cm. filter paper (S. & S. No. 597), wash accurate results with normal and basic lead acetates. with 50 cc. of water, and collect filtrate and washings in a Lead can be conveniently determined by transferring all 300-cc. Erlenmeyer flask. It is best to use a long-stem the lead sulfate onto the Gooch crucible and heating to Bunsen funnel, and to fill the stem with water before filling constant weight in the usual manner. A correction for the filter with the alcoholic solution. Titrate with 0.1 N insoluble matter should be applied when present. sodium hydroxide until the color matches that of a color standard prepared by dissolving 0.06 gram of chromic chloride Solutions Required crystals and 0.0035 gram of potassium chromate in 270 cc. of CARBONDIOXIDE-FREEWATER-Ordinary distilled water water. was boiled in a flask of Jena glass, fitted with a soda-lime tube, DIscussIoN-If to 45 cc. of 0.1 N acetic acid are added 8 and allowed t o cool. Carbon dioxide-free water was used exclusively for all work, including preparation of solutions. Gooch drops of thymol blue, the solution assumes a yellowish orange crucible sieve plates were used for preventing bumping when color. The solution turns more orange and finally to red water or solutions were boiled. upon additions of 0.2 N sulfuric acid. It is difficult to judge Containers for volumetric solutions were provided with Popoff the color change with certainty until ahout 0.2 cc. of the (9) buret-filling devices and soda-lime' tubes. 0 5 N AND 0.1 N SODIUM HYDROXIDE-The sodium carbonate sulfuric acid has been added. When a white precipitate is present, the color change caused by small quantities of sulin a stock solution of sodium hydroxide was removed with a slight excess of barium nitrate. A portion of the clear super- furic acid can be clearly observed only after settling. A natant solution was siphoned off, diluted to the approximately distinct orange-red color, however, can be easily observed desired normality, and treated with a slight excess of sodium sulfate. The clear solution was siphoned off and standardized when an excess of 0.5 to 1.0 cc. of 0.2 N sulfuric acid has been against potassium acid phthalate. All siphoning was carried introduced. On addition of alcohol, t,he color of the solution out with the exclusion of carbon dioxide. The containers were changes to yellow with a faint orange shade. The alcoholic heavily paraffined. solution changes to a nearly pure yellow color when the excess of sulfuric acid is precipitated by benzidine. 1 Received April 22, 1931. Presented before the annual convention Benzidine sulfate precipitate, when formed in the presence of the Chinese Society of Science and Arts, Nanking, December 5, 1930.
T
380
VOl. 3, KO. 4
of acetic acid, is very fine. On standing,jt gradually coagulates into coarser crystals. To ascertain whether or not this physical difference has any bearing on the purity of the precipitate, various amounts of sulfuric acid were added to 41.60 cc. of 0.10818 N acetic acid, diluted with 75 cc. of 95 per cent alcohol and 95 cc. of water, and treated with benzidine. After filtering and washing with 50 cc. of water, it was titrated with 0.10706 N sodium hydroxide. The results given in Table I definitely show that no acetic acid or benzidine acetate has been carried down by the precipitate. of Benzidine Sulfate i n Presence of Acetic Acid 42.03 cc. 0.10706 N Tu’aOH) (41.60 cc. of 0.10818 N AcOH 0,10706 N NaOH REEXPERIQUIRED FOR TITRATING ERROR MENT 0.2 N HrSO4 0.1 N BENZIDINE FILTRATE^ cc. cc. cc. cc. 1 1 4 42.07 4-0.04 42.03 0.00
Table I-Precipitation
2
2
6
42.04 41.98 42.05
-0.05 f0.02
3
3
8
42.01 42.00
-0.02 -0.03
4
4
10
42.01 42.06
-0.02 4-0.03
5
1
6
42.04 42.02
$0.01 -0.01
1
12
42.06 42.00
$0.03 -0.03
6 a
Corrected for effect of alcohol on indicbtor.
fO.01
If the benzidine sulfate is washed with an additional 50 cc. of water, the washings would require 0.14 to 0.15 cc. of 0.1 N sodium hydroxide for neutralization. Excessive washing should therefore be avoided. With 5 cc. of 0.2 N sulfuric acid, the precipitate of benzidine sulfate formed was heavy and could be filtered only with difficulty. The presence of alcohol decreases the sharpness of the thymol blue end point, making necessary the use of a color standard. The authors took the first change from yellow to yellowish green as the end point, which can be easily recognized by comparing with the permanent color standard. Both the 95 per cent and dilute alcohol should be neutralized shortly before use. When many analyses are to be made, ordinary ethyl alcohol might be used, and the amount of 0.1 N sodium hydroxide required for neutralizing 75 eo. of the 95 per cent alcohol deducted from the buret reading. Usually 0.1 to 0.2 cc. of 0.1 N alkali is required, depending on the purity of the alcohol. Phosphoric Acid Distillation Method The results of the benzidine method were compared with the phosphoric acid distillation method. The distillation method used was a combination of those of Bayer (6) and Scott (10).
PROCEDURE-Etch two lines on a 500-cc. round-bottom flask to indicate volumes of 70 and 170 cc., respectively. Introduce 4.1 to 4.2 grams of lead acetate into the flask and dissolve in water, add 0.3 gram of zinc dust and 20 cc. of phosphoric acid (density, 1.7), and then dilute to 170 cc. with water. Insert a rubber stopper which carries a separatory funnel and a Gray’s distilling bulb (7). The condenser is preferably of the spiral type. Place the round-bottom flask over an asbestos board with a hole in the center. Place 40 cc. of 0.5 N sodium hydroxide in the receiver which is a 1-liter flask of Jena glass. Heat the round-bottom flask with a direct flame. When the 1iquid.k reduced to 70 cc., add hot water from the separatory funnel until it again reaches the 170-cc. line. Repeat the distillation until 800 cc. of distillate have been collected, then change receivers, and boil the 840 cc.
of solution in the first receiver with a Gooch crucible sieve plate under a reflux condenser for 20 minutes. Fit a sodalime tube, cool, and titrate with 0.5 N sodium hydroxide, using about 16 drops of phenolphthalein for indicator. Titrate further distillates until l drop of the alkali produces a permanent pink color. DIscussrox-Under the conditions of the procedure about 50 per cent of the acetic acid is expelled by each distillation. Thus, in one experiment, the nine successive portions of distillates required for neutralization, 21.33, 10.82, 5.41, 2.46, 1.21, 0.55, 0.24, 0.07, and 0.04 cc. of 0.51597 N sodium hydroxide, amounted to 50.63, 25.68, 12.84, 5.84, 2.87, 1.31, 0.57, 0.17, and 0.09 per cent of the total volume (42.13 cc.) of alkali consumed, respectively. During the experiment, each succeeding portion, together with 2 drops of phenolphthalein, was added to the titrated solution and titration continued. The solution was not boiled, but a correction for carbon dioxide as found in ordinary distilled water was applied. Acetic acid is sometimes completely distilled in the first 800 cc., but it also often happened that 1 or 2 drops of 0.5 N sodium hydroxide are required for the ninth hundred cc. Results obtained with this method are given in Table
11. Table 11-Phosphoric SAMPLE Lead acetate, Merck’s reagent
Acid Distillation Method WT. OF SAMPLE 0.51506 N NaOH AcOH Grams cc. % 4.1516 42.09 31 35 4.1168 41 80 31 4 0 4.1127 41.76 31 40 Av. 31 38
Lead acetate, com’l. crystals, 1
4.1178 4.1525 4.1575
41.78 42 22 42.21
Lead acetate, com’l. crystals, 2
4.1178 4.1091 4.1127
41 73 41.67 41.74
Lead acetate, pharmaceutical
4,1794 4.1140 4.1015
42.30 41.62 41.45
Av.
31 38 31.44 31.40 31.41
Av.
31.34 31.36 31.39 31.36
Av.
31.30 31.29 31.25 31.28
Table 111-Comparison of Phosphoric Acid Distillation Method w i t h Benzidine Method using T h y m o l Blue as Indicator AcOH BY HsPO6 DISTILLATION ---BENZIDINE METHODSAMPLE METHOD DIFF. Wt. of 0.10446 N sample NaOHO AcOH Grams cc. % % % 0.9453 47.28 31.37 Lead acetate, com’l. 31.42 0.9468 47.44 crystals, 1 0.9453 47.19 31.31 0.9420 47.11 31.37 31.32 0.9471 47.30 0.9477 4 7 . 4 3 31.39 0.9444 4 7 . 1 5 31.31 Av. 31.36 31.41 -0.05 24.72 24.83 24.69 24.70 24.77 24.67 Av. 2 4 . 7 3 Corrected for effect of alcohol on indicatqr.
Lead subacetate, Merck
a
1.1205 1.1065 1.1449 1.1337 1.1298 1.1346
44.17 43.80 45.07 44.64 44.62 44.63
24.76
-0.03
Precipitate of lead phosphate causes considerable bumping. Zinc replaces lead in the phosphate and causes it to separate in a spongy form, and the spongy lead helps to prevent bumping. I n case bumping should occur, add a little zinc dust through the separatory funnel with the next portion of water. Obviously the tip of the separatory funnel should not be made too small. Phosphoric acid often contains volatile acids. The authors used Merck’s reagent quality and found that 20 cc. yielded, in 800 cc. of distillate, enough acid to neutralize 0.2 to 0.3 cc.
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:
