EQUILIBRIUM I N T H E SYSTEM ARSENIC PENTOXIDE, BARIUM OXIDE, WATER. (ACID SECTION) * BY STERLING B. HENDRICKS
The increasing use of alkaline arsenates as insecticides necessitates a more complete knowledge of the chemical properties of these substances. The following research describes the results obtained from a study of the acid section of the system BaO, A S Z O ~HzO. , Since equilibrium conditions are not rapidly established in basic solutions the basic section of this system will be reported later. Preparation of Malerial~.---Ba(OH)~ . 8 H 2 0 was prepared by recrystallizing “C.P.” reagent from COz-free distilled water. It gave a negative test for Cl-, COS--, and NO;. Asz05. 4Hz0 was prepared by the method previously described by C. M. Smith1. Experimental Procedure.- 100 cc. paraffined cork stoppered oil sample bottles were used €or experimental vessels. Solutions of known concentration, containing some of the solid phase, were placed in these and were then immersed in a thermostat regulated at 30’ =t.IO. Equilibrium conditions were determined by analysis of the solutions at intervals of one week. Analyses were made in the manner given below. Analysis.-The densities of the solution were determined by means of a pycnometer, the temperature being 20’. The barium was determined by the sulfate method, the ignited BaS04 being moistened with conc. HzS04and reignited in order to remove chloride impurities. The arsenic was determined by titrating the 11 liberated from an iodide in conc. HC1 solution, accurately standardized XazSz03solution being used. Since any attempt to dry or wash the solid phase would probably have resulted in hydrolysis or dehydration, i t was necessary to use the method of analysis of wet precipilatesZ in order to determine the compositions of the solid phases. Since there were slight experimental errors involved in the analysis, the exact point of intersection of several lines determining the composition of a solid phase could not be determined by graphical means. The most probable point of intersection of several lines was assumed to be that point a t which the sum of the squares of the perpendicular distances to the lines was a minimum. Since the two points determining a line were very close together in the case of the more acid solutions, it was necessary to identify the solid phases present by microscopic means. The results of the analyses are given on pages 2 5 0 - 2 5 1 . The stable phases and the solutions in equilibrium with two of them are: BaHAs04. HzO and RaH4(As04)z,zHzO, solution, 16.65% BaO and 28.01% As205; BaH4(As04)2.2H20 and 3As206.jHzO, solution 72.04% As205 and *Contributipn from the Kansas State Agricultural College. J. Am. Chem. Soc., 42, 259 (1920). Bancroft: J. Phys. Chem., 6 , 178; Browne: 287 (1902).
ARSENIC PENTOXIDE, BARIUM OXIDE AND WATER
249
a trace of BaO; BaHAsOe. HzO and some more basic arsenate, probably BadAs04)Z. x HzO, solution, .oz& BaO and .037; AszOr,.3Asz05. gH,O was assumed to be the solid phase present in the most acid solution, since the analysis of the wett solid gave proof that As203 4Hz0 was not present,. Menzies and Potter1 reported these oxides, 3 A s z 0 5 .gH20being stable above zgo, thus explaining the conversion in this experiment. The constant solution
.
containing . 0 2 5 % BaO and .03% As205 with BaHAs04. H20 and Ba3(Ae0JZ. x HZO as a solid phase gave no evidence of existing in equilibrium with Ba5Hz(AsO4)4.x HzO, the analogous calcium compound has been recently prepared. These compounds with the exception of the latter have been previously described2. Preparation of Barium Arsenates.-Since most of the methods of preparation of barium arsenates previously described were found to be unsatisfactory, the older methods have been tested and new methods devised. Mono barium arsenate has previously been described by Mitscherlich3 by adding Ba(0H) 2 to conc. H3As04, which is essentially precipitation beyond the invariant point in the previous diagram, Fig. I . The following method gave satisfactory J. Am. Chem. Soc., 34, 1469 (1912). Gmelin Kraut: “Handbuch anorg. Chem.”, 3 11, 584. 3 Gmelin Kraut: loc. cit. 2
STERLING B. HENDRICKS
h 9
ARSENIC PENTOXIDE, BARIUM OXIDE, AND WATER
h
v,
x
v)
N
0
a
*0
h
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H
00
a N
r-
a 00
a a
.3
-42
s
W
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d . y o
kR
I-M
a
0
H
U
H
N
N
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N N
?.?eN
N
100 h W N N N N
01 N
STERLING B. HENDRICKS
252
results; melted Ba(0H)z. 8HzO was poured into an excess of 45% H3As04, the hard, well-crystallized solid, was removed, dired a t 130') and analysed. The analysis gave 36.30% BaO, 55.4570 As206, 8.25% HzO; calculated for BaHd(As04)z 36.58% BaO, 54.83% As20j, 8.57% HzO. The following table summarizes the results obtained in the investigation of BaHAs04. H 2 0 . TABLE I1 Method used
Excess H3As04.Ba(OH)2 added drop by drop. Excess Ba(OH)z. H3As04solution added drop by drop. H3As04.Ba(OH)zsolution added drop by drop until the solution was neutral to phenolphthalein. Refluxed for four hours. Excess BaC12 disodium arsenate solution added to boiling solution. Impure dibarium arsenate dissolved in dilute HC1 and reprecipitated by the slow addition of dilute ammonium hydroxide. Disodium arsenate, dilute solution, saturated with dibarium arsenate in the cold, heated to boiling and filtered. Calc. for BaHAs04. HzO.
%BaO 41.73
% A s z O ~ Ratio BaO/AszOs. 32.89 1.90
51.09
37.56
2.03
51.43
37.71
2.04
51.82
36.68
2.12
51.32
38.20
2.01
51.71
38.68
2.00
51.93
38.93
2.00
The last method was devised on the basis of the decrease in the solubility of BaHAs04. HzO with an increase in temperature. The filtrate from the first solution deposited large crystals, .5 cm. long, having a common index of refraction = 1.635. This table illustrates the necessity of preventing the more basic compounds from precipitating from the solutions. The methods previously described for the preparation of Ba3(As04)2 proved to be unsatisfactory. Alkaline hydrolysis of BaHAs04. HsO, with ",OH, gave BaNH4As04in some cases and but rarely gave Ba3(As04)z1. It was hoped that this system would prove to be similar to a similar system of lead2, but it proved to be quite different. Dibarjum arsenate, prepared by the sixth method given in Table I1 and containing 38.04% Asz03and 51.60% BaO, was subjected to hydrolysis by KHdOH solutions. In order to determine whether ammonia entered the solid phases or not, preliminary hydrolytic experiments were made with solutions varying from I to .OI M. Examination of the solid phases, after careful waehing with acetone, showed the absence of ammonia, the Nessler test was used. In the experiment as summarized in Table I11 one gram samples of the dibarium arsenate were treated with COz-free N H 4 0 H (prepared by distillation in the presence of Ba(OH)2), the arbitrary dilution being .0383 N. 1 2
Salkowski: J. prakt. Chem., 104, I43 (1868); Lefevre: Ann. Chim. Phys., 27,13 (1892). Smith and McDonnell: J. Am. Chem. SOC., 38, 2366 (1916).
ARSENIC PENTOXIDE, BARIUM OXIDE, AND WATER
253
Referring to Table I1one can see that the first additions of ",OH resulted in a constant solution in equilibrium with BaHAsOr . HzO and Ba3(A~04)2. x HzO. The disappearance of the dibarium arsenate is shown by :he breaks in curves; the lower curve is a rectangular hyperbola showing that hydrolysis of B a 3 ( A ~ 0 4 does ) 2 not take place. Analysis of the dried solid phases from several experiments supposed to contain the tribarium arsenate gave the following results: BaO, 66.60%; As?Oj, 3 3 , 1 1 7 ~ ;calculated for B a 3 ( A ~ 0 4 ) 2 ; BaO, 66.67%; Asz05,33.33%.
TABLE 111 Exp. no.
CC.",OH
used.
I
40
2
50
3 4
60
5
90
6
IO0
7 8
I20
80
Grs. AszOa removed. .0425 .os04 .os96 .0830 .088 I
0.9936 I . 0369 0.9788
2.37
I . 0050
2.71
0.9713 0.9722 0.9225 0.8569 0.7612 0.6657 0.6075 0.4360
2.86 2.98 3 .OI 3 .oo 3 .OI 3.04 3 .oo 3.02
I305
0.3350 0.2966
trace
trace
3.03 3 .oo 3.00
. I005
9
130 140
.1165 .1264 ,1291
IO
1.50
.1285
I1
170
. I294
I2
2 00
'
I3 I4
210
1331
I5
400
.I275 . I308 . I340
16
440
*
I7
infinite
300
Grs. As205 removed Mol ratio in solid per liter. phase BaO/AssOj. I .0613 2.25 I . 0084 2.30 2.55
2.59
Control field tests using commercial barium arsenate as a dust are now being carried on by the Southern Field Crops Investigation Laboratory of the Bureau of Entomology, the boll weevil (Anthonomus grandis Boh) being used as the experimental insect. Preliminary results show that its toxicity is as great as that obtained from commercial calcium arsenate, the poison used a t the present time. I wish to thank Dr. H. H. King of the college and Mr. C. M. Smith of the Bureau of Chemistry, Washington, D. C. for assistance given to me during the course of this research. Summary The stable compounds in the acid division of the system BaO, As2Ob, HzO at 30' are BaH(AsO4) . HzO, BaH4(As04)z.2H20, and 3 A s z 0 5 .gHzO. Alkaline hydrolysis of BaHAs04 . HzO gave Ba3(As04)?. x HzO. Methods for the preparation of other arsenates are described. Pasadena, Calijornia.
