April, 1922
T H E JOURNAL OF INDCTSTRIAL A N D ENGINEERING CHEAVISTRY
311
The Electrolytic Preparation of Arsenate of Lead' By Herman V. Tartar2 and Gary G. Grant DEPARTMENT O F CHEMISTRY, UNIVERSITY
A mixture of the basic and acid arsenates of lead can be prepared electrolytically from disodium arsenate or arsenic acid. When disodium arsenate is used the product contains a large portion of the basic compound. I f on the other hand, arsenic acid is used the product may be made to contain a large proportion of the acid compound. The object of this investigation was a quantitative study of the process to ascertain what concentration of arsenic acid, current density, distance between electrodes, and kind and concentration of electrolyte served best for the formation of lead hydrogen arsenate; also the eflect of the above factors on current eflciency, electrical energy eficiency, and freedom from accumulations on the anode. Since lead hydrogen arsenate is the chief component of the commercial insecticide, attention was not given to the production of the basic arsenate. The conditions most suitable for the electrolysis involve a concentration of approximately 0.05 per cent arsenic acid, a current density of 1.25 to 1.875 amp. per sq. dm., and a distance between electrodes of 2 . 5 to 5 . 0 cm. The most desirable electrolyte is sodium chlorate at a concentration of 1 to 2 per cent. I f the precipitate is allowed to stand for several days in contact with the 0.05 per cent arsenic acid the proportion of acid arsenate is increased. Current eficiencies are w r y good.
B
ECAUSE of the extensive use of the arsenates of lead for insecticidal purposes, a number of investigations have been made during recent years to ascertain their constitution and properties. The work of Tartar and R ~ b i n s o n ,Smith,4 ~ and McDonnell and Smith: has shown that the commercial material may contain two compounds: uix., lead hydrogen arscnate (PbHAsOJ and a basic arsenate [Pb4(PbOH) (As04)3H20]. At first lead arsenate was prepared for commercial use by the reaction between lead acetate or lead nitrate and disodium arsenate. Several other methods have been proposed during recent years; in fact, some twenty-five patents have been granted covering various processes. So far as the writers are aware the most direct and possibly the most widely used procedure is that proposed by Luther and Volck,G in which lead oxide is suspended in a solution of arsenic acid with a small amount of nitric or acetic acid as a catalyzer; either the baqic or the lead hydrogen arsenate is produced, depending on the relative quantities of lead oxide and arsenic acid used. The literature on the subject affords no reference to an electrolytic method for the preparation of lead a r ~ e n a t e . ~ Nearly two years ago some qualitative tests made in this laboratory showed that a material similar to much that is sold on the market could be prepared electrolytically in a manner somewhat analogous to the Luckow process for white lead. The bath consisted of a solution of an Received September 29, 1921. Associate Professor of Chemistry 8 J . A m Chem Soc., 36 (1914), 1843 4 Ibzd., 38 (1916), 2014. 5 Ibzd., 38 (1916), 2027, 39 (1917), 937. 6 U. S. Patent 892,603 7 The reviewers of this paper have called @e authors' attention to t h e method proposed in U S Patent 870,915 in which the anodic processes are very similar. The method proposed here is, however, more simple and data are given regarding current efficiency, the proper composition of t h e bath, the amount of electrical energy required, and the composition of the lead arcmate obtained 1 2
OF
WASHINGTON, SEATTLE, WASHINGTON
electrolyte t o form a soluble lead salt (sodium chlorate was used) and a second electrolyte to furnish the arsenate ion to precipitate the lead as insoluble lead arsenate. The electrolysis was made with lead electrode.. EXPCRIMEXTAL PART The apparatus consisted of rectangular glass jars 3 x 4 x 6 in. and electrodes of lead 2 5 in. wide which dipped into the electrolyte 2 . 5 in. A copper coulometer was used for measuring the quantity of electricity used and the current was controlled approximately by means of a resistance and a n ammeter. The fall of potential in the bath was ascertained by a voltmeter which could be shunted between the electrodes. A mechanical stirrer was used in some cases to agitate the electrolyte. I n later experiments compressed air was found to be suitable for stirring. After each electrolysis was completed, the precipitate was allowed to settle, filtered on a Buchner funnel, dried a t 115', weighed, reduced to a powder in a mortar, and analyzed. The lead was determined as lead chromate8 and the arsenic by reduction to arsenious acid and titration with standard iodine s ~ l u t i o n . ~From the amount of lead in the precipitate and the quantity of copper deposited in the coulometer the current efficiency was calculated on the basis of Faraday's Law. The arsenic acid used was prepared from arsenic oxide which was found to contain no other impurity than a small amount of arsenious oxide. This was removed by treating the material with a sufficient quantity of concentrated nitric acid and evaporating several times to dryness to remove the excess of the nitric acid. The arsenic acid obtained was made up to volume and standardized. I n this form calculated amounts were added at regular intervals to the electrolysis bath in order to maintain a fairly constant acidity. The sodium chlorate and sodium acetate employed were both guaranteed as C. P. by a reliable manufacturer. EFFECTOF CONCENTRATION OF ARSENICACID Three solutions were made up containing 4 per cent sodium chlorate and three different concentrations of arsenic acid. They were electrolyzed between lead electrodes each having an effective area of 6 25 sq. in. with a current of 0 5 amp. The results are given in Table I. TABLE I-EFFECT Bath number Concentration arsenic acid (as ASSO>),per cent Phenomena a t anode Phenomena a t cathode
OF
CONCENTRATION
OF
ARSEAICACID
1
2
1
2
White pre cipitate Some lead deposited
Little precipitate More lead deposlted
3 5 No precipitate
Much lead deposited
Because of the evident lowering of current efficiency by the deposition of lead on the cathode, these electrolyses were abandoned after a short time. Too high acidity causes the retention of lead in solution and it is deposited on the cathode during electrolysis. While lead is above hydrogen in the electromotive series it is easily deposited under these conditions because of the high overvoltage of hydrogen on lead electrodes. The presence of lead in the solutions was shown by making them alkaline with ammonia, which 8 @
Methods of Analjsis of Official Agric. Chemists, 1920, 57. Ibzd , 58.
T H E JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
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permits the precipitation of basic lead a r ~ e n a t e ;the ~ amounts present corresponded to the relative acidity of the baths. Three more baths were electrolyzed in the manner stated above. Their composition and the results of the electrolyses and analyses are given in Table 11. TABLE11-FURTHER RESULTSON EFFECT OF CONCENTRATION OF ARSENICACID 4
.......
Bath number.. Concentration of AszOs, per cent.. Concentration of "08, per c e n t . . Concentration of NazHAsO4, per cent.. ... Concentration of NaClOa, per cent.. Phenomena a t anode
..........
0.5
... ...
.......... ..... ...
Phenomena a t cathode.
......... .......
.
4.0 Flocculent lead arsenate Some lead deposited 14.9 60.4
5
6
1.0
1.0
1.0
...
...
1.0
4 0
Same ' b u t cr y'stalline iMuch lead deposited Small 59.67
Yield, grams. Per cent lead.. Current efficiency, per 91.0 Small cent Theoretical per cent lead in PbHAsOi.. Theoretical per cent lead in Pbp(Pb0H) (As04)8HzO.
...............
4.0 Slimy lead arsenate No lead deposited 12.5 64.82
..................59.69 ...... .69.6
82.5
Pure crystalline lead hydrogen arsenate can be prepared by using nitric acid, but the current efficiency is very low. The use of disodium hydrogen arsenate to repress the ionization of the arsenic acid\and increase the concentration of arsenate ion results in a lower current efficiency, and the product is more basic than with arsenic acid as the sole source of the arsenate ion. More dilute solutions of arsenic acid were next tried. Iron cathodes were used for these experiments and were found satisfactory; by their use the hydrogen overvoltage is materially lowered. The results are tabulated in Table 111. TABLE 111-FURTHER RESULTS ON EFFECT OF CONCEXTRATION O F
l Bath number.. Concentration of NaClOa, per cent. Concentration of AszOs, per cent, Current density, amp./dm.Z.. Phenomena a t anode.
.................
. . ~......... . .... Phenomena a t cathode Yield, grams. ......... Per cent lead.. .................
Current efficiency, per cent..
A ACID ~ 7 4 0.25 1.24 Finely divided lead arsenate
~
~
~ 8 4 0.05 1.24 Precipitate more finely divided Almost no lead 6.55
... ...
.....
61.66 99.5
The results show that lead arsenate (a mixture of lead hydrogen arsenate and the basic arsenate, the former predominating) is produced ~ i t h o u tappreciable deposition of .lead at the cathode if the concentration of arsenic acid is kept low enough and nearly const,ant during the electrolysis. The product is quite finely divided and on drying gives a very fine powder. EFFECTOF CURRENTDENSITY TNOelectrolyses were made in the same manner as that reported for Bath 8, except that current densities were 0.625 and 1.875 amp. per sq. dm., respectively. The data are recorded in Table IV. TABLE IV-EFFECT
OF
CURRENT DENSITY
......................... ......... ....... ....... ................... .................. .....
Bath number., Concentration of AszOs, per cent. Concentration of NaClOa, per cent. Current density amps./ Voltage with 2.d-cm. di en electrodes.. Voltage with 10-cm. distance between electrodes Yield, grams.. ely), hours.. ................
.............................. ..................................
.................
9 0.05 4.0 0.625 1.0 3.0 9.65 6 62.4 9S.8
10 0.05 4.0 1.87 2.0 6.0 9.5 2 62.05 99.6
The higher current density apparently favors the formation of the acid compound and gives a t the same time a. higher current efficiency. The precipitate also has less tendency to stick to the anode, Considering the voltage drop across the baths, however, the actual energy efficiency is less with the higher current density. This may be offset to some extent by placing the electrodes closer together.
Vol. 14, No. 4
EFFECTOF ELECTRODE DISTANCE AND VOLTAGE These experiments were carried out to ascertain the effect of distance between electrodes on the voltage drop across the bath, the composition of the precipitate, and the deposition of lead a t the cat'hode. The data are given in Table V. TABLEV-EFFECT OF ELECTRODE DISTANCEO N VOLTAGE Bath number.. 11 12 13 Electrode distance, cm.. ............. 10 5 2.5 Concentration of As205 per cent. . . . . . 0,05 0.05 0.05 Concentration of NaClba, per cent 4.0 4.0 4.0 Voltage.. .......................... 4 2 1 Current density, amps.jdm.2.. . . . . . . . . 1,24 1.24 1.24 Yield, grams.. ...................... 9.226 . 9.24 9.48 Per cent lead 65.05 63.7 62.5 Current effici . . . . . . . . . . . 99.5 97.6 98.3 Character of precipitate.. . . . . . . . . . . . . Most Medium Most finely dense dense divided
.....................
The formation of lead hydrogen arsenate is favored, greater energy efficiency is realized, and less lead is deposited a t the cathode with the electrodes close together. The difference in the physical character of the precipit'ates was striking. The one from Bath 13 slid off the electrode quite freely and in a finely divided state suitable for use as insecticide material.
USE
OF
DISODIUM HYDROGEN ARSENATEAND OF SODIUM ACETATEIN ELECTROLYSIS BATH
An attempt was made to use a bath containing disodium hydrogen arsenate and acidified wiOh arsenic acid solution so that the hydrogen-ion concentration was approximately that of 0.05 per cent arsenic acid solution. Under these conditions the acidity could be more easily controlled and kept more nearly constant. The bath was 4 per cent sodium chlorate and 1 per cent disodium hydrogen arsenate, made acid 'to methyl orange, thus giving a solution of approximately ~ ~ the composition NaHzAsOd. The electrolysis gave a dense precipitate which adhered strongly to the electrode; the formation of oxides of lead also occurred a t the anode. The current efficiency was obviously low and the run was stopped after a short time. The writers have no complete explanation to offer for the anodic phenomena observed. A bath containing sodium acetate instead of sodium chlorate was found to give good results in many respects. Much less lead was deposited a t the cathode using even a 1 per cent arsenic acid. It was found, however, that higher concentrations of sodium acetate were necessary than of sodium chlorate. Some of the results are tabulated in Table VI. TABLEVI-RESULTS OBTAINEDWITH SODIUMACETATEIN
ELECTROLYSIS BATH
Bath number. . . . . . . . Concentration of NaC Concentration of Aslos, Current density, amps. Electrode distance, cm
............ ....... .......
Per cent lead ............................. Current efficiency, per c e n t . .
...............
l2 1 1.25 10 4.0 9.25 63.8 98.6
l58
1
1.25 10 3.0 17.3 63.7 98.7
,
-4very uniform product can be obtained by this method. The lead arsenate formed is more basic and less finely divided than that secured from the chlorate bath. EFFECTOF CONCENTRATION OF SODIUM CHLORATE Qualitative experiments indicated that the concentration of this substance might affect the physical properties of the precipitate and its tendency to stick to the electrode. Three baths were tried containing concentrations of sodium chlorate above and below thgge used in previous experiments. These were electrolyzed under similar conditions and the data are reported in Table VII. The more dilute solutions of sodium chlorate favor the formation of the acid arsenate. In these cases. practically
