A REVISION OF THE ATOMIC WEIGHT O F
ARSEXIC.
297
richs,l Dubreui12 and Leduc.s The electrochemical equivalents of oxygen and hydrogen have been studied by Lehfeldt.' On the silver voltameter and coulometer there are elaborate papers by Duschak and Hulett,' and by Smith, Mather and I,owry.e I n the latter research the electrochemical equivalent of silver is fixed a t I . I 1827 mg. per coulomb. I n the International Table, published in this J O U R N A L for March, there is a serious misprint. Zn should be 65.37 instead of 65.7. WASHINGTON,D. C.
[CONTRIBUTIONS FROM THE CHEMICAL
LABORATORY OF HARVXRD
COLLEGE.]
A REVISION O F THE ATOMIC WEIGHT O F ARSENIC. PRELIMINARY PAPER-THE BY
ANALYSIS O F SILVER ARSENATE.
BAXTER A N D FLETCHER BARKER COFFIN. Received January 16, 1909.
GREGORY PAUL
Below is a summary of the previous work upon the atomic weight cf arsenic,' the results obtained by the several investigators having been recalculated with the use of the following atomic weights:8 0 = 16.003; Ag= 107.880; c1=35.45j; Br=79.916; s=32.07; K=39.096; Na=22.g7j; Cr=52.01; Pb=207.09. 1816 1818 1845 1855
Thornson, Berzelius, Pelouze, Kessler,
Schweigger Jour., 17, 421, Pogg. Ann., 8, I , Compt. rend., 2 0 , 1047, Pogg. Ann., 95, 204,
1859 1859 1861 1896 1902
Dumas, Wallace, Kessler, Hibbs, Ebaugh,
Ann. Chim. Phys. 131, 55, 174, Phil. Mag. (4), 18, 279, Pogg. Ana., 113, 140, Doctoral Thesis, Univ. of Penn., Jour. Amer. Chem. SOC.,24, 489,
AS: A&Ob 2As,O,: 3S0, AsC1,: 3Ag 3A%0,: ZK,Cr@7 3As20,: 2KC10, AsC1,: 3Ag AsBr,: 3Ag 3As20,: zK2Cr20, Ka,As207:4h'aCl Ag,AsO,: 3AgCl 4g,AsO,: 3Ag Pb,(AsO,),: 3PbC1, Pb,(AsO,),: 3PbBr,
76.35 75.03 74.93 74.95 75.23 74 h; 74 2 0 j j .01
74 88 75.oz i4.92 7j.06 74.88
-4 glance a t this rather discordant series of results shows the necessity for a redetermination of the atomic weight of arsenic. Even in the more recent investigations of Hibbs and Ebaugh there exists an extreme variation of nearly two-tenths of a unit in the averages of the five series. Chem. Zent., 1908 (I), 1240;Compt. rend., 146,971;147, 79j, and 147, 1302. Comfit.rend., 147,6 2 7 , 856, and 1300. a Ibid., 147,972. Phzl. Mag. [ 6 ] , 15, 614. 6 Trans. Amer. Electrochem. SOC., 12, 2 5 7 e Phil. Trans., 207, A, 545. Clarke, A Recalculation of the Atomic Weights; Smith, Misc. Coll., Constnnts of Nature, P a r t V, p. 213 (1897). Far a n excellent critical discussion of previous work, by Brauner, see Abegg's Handbuch der anorganzschen Chemie, 3 ( z ) , 491 (1907). 8 Richards, Jour. chim. phys., 6, 130 (1908). a
2 98
GEXER.IL, PHYSICAL AND IXORGIXIC.
In this research silver arsenate was chosen for analysis, first, because the compound is unchanged b y moderate heating, and hence may be dried a t a temperature high enough to expel all but a very small amount of moisture. I n the second place, silver compounds may be analyzed with great ease as well as accuracy by precipitation of the silver as silver halogen compounds. Furthermore, preliminary experiments confirmed the statement by Ebaugh that it is possible completely to convert the arsenate into chloride by heating in a current of hydrochloric acid gas. Such a process has the advantage that no transfer of material is involved. Purification of Materials. Sz'h,er .Irsezate.--All the sampl s of silver arsenate 17ere prepared by adding to a fif teenth-normal solution of silver nitrate a solution of similar concentration of an equivalent amount of an arsenate of sodium or ammonium, the differences between the different saniples consisting chiefly in the nature of the soluble arsenate employed. Precipitation was carried out in a room lighted only with ruby light. After the silver arsenate had been washed by decantation many tinies with pure water, i t was dried in a preliminary way b y centrifugal settiing in platinum crucibles, and then by being heated in m i electric oven a t a temperature of about 1 j o 0 C. The salt was pjwvdered in a n agate mortar hefore tlic final heating in a quarti. tube or platinum l)oot, as esp1,iined later. It was s h o n n Ly tests with diphenylamine that the arsenate cou!d be washed free from niLr:itcs. -41t!iough oiie of the hyclrogenq of arsenic acid resembles the hydrogen of strong acids in its dissociating tendency, the other t w o hydrogens are those of weak acids.: Hence perceptible hydrolysis takes place in solutions of salts of this acid, even when tile base is strong, that of the tertiarq- salts being of course greatest in extent. I t is not an easy matter to predict the effect of this hydrolysis upon the composition of a precipitate of silver arsenate, for while the Phase Rule allows the esistence of only one solid in equilibrium with the arsenate solution except a t certain fixed concentrations, the possibility of the occlusion of either basic or acid arsenates by the silver arsenate still exists. Experiments only are able to throw light on this point. hccordingly arsenate solutions tri different conditions of acidity and alkalinity were used in the precipitations, and the co:iipositions o f the different precipitates were compared. Sample . Per cent.of A x i n .Ag,g.4s0,. , , , . . . . , . , . , , . , , , , , . , . , . , , fq,9601)' ,
SERIES 111 3AgBr Ag?r\sO4 Corrected Corrected Dissolved weight of weieht of Weieht of No. Losson AgBr AgBr Sample AgyAs04 Agkr in Weight of Of in vacuum. vacuum. asbestos from filtrate. fusion. in vacuum. anal- of Gram, Grams. Gram Gram Grams. ys1s. Ag8ASO+ Grams.
o 00008
o 00004
0 0002.+
0 0000;
Ratio gAgBr: AgsASO4.
n.ooo40 10.66jj3 I. 2 17s; 8,24545 I . 2 1 7 0 6
0.0O0Ij
o.oonr; o oooo9 0.0000j 6.32j90 1 . 2 1 ; 8 ; o.0000g o.oooo6 o.oooo8 6.50258 I . 2 I j ? ) I 0.000;3 o .om14 0.00012 10.035jz I , 2 1 7 5 2 0 00021
0.0O0Oj
O . W ? h
0.oOOOj
0.00013 9.2314; I. 2 17'): 0.00003 ;.3;106 1.21789
Average . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.21789 I'er cent.of .4g in Ag,AsO,. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69.962~~ r\verage per cent,of Ag in Ag,XsO,. . . , . . . . . . . . . . . . . . . . . 69.9616 1
As: AgCI
=
0.7 52632: x.oOono0.
Richards and Wells, Tiixs
0,574453: I . r m o 0 .
Raxter, Ibid., 28,
JOI,RNAI,,
(1905).
* Ag: Agnr
=
1322
(r9oh)
27, 450
A REVISIOX OF THE ATOMIC WEIGHT OF ARSENIC.
307
SERIES IV. 3AgCl: AgSAsO,.
No.
of Sample analof ysis. Ag,AsO,. 1.5
F
16
F
17
18
G G
19
(3
20
G 6
21
Corrected weight of AgaAsO4 iu vacuum. Grams.
Weight of AgCl Residual in vacuum. AgC1. Grams. Gram.
4.67268 7.7~883 5.28049 4.25346 3.47340 5.17269 4.10766
4,34393 ;.1j602
4.90908 3.95422 3.22892 4,80877 3.81856
Corrected weight of AgCl Volatilized AgCl. in vacuum. Gram. Grams.
0.00006 o.ooo07
o.ooo02
o.oo001
O.OMX)I
o.ooo02
o.omoo
o.oo001
o.ooooo
o.ooooz o.oooo2
0.00000
4.34389 7.17597 4.90908 3.95424 3.22893 4.80879 3.81858
o.ooo02
o.oocoo
Ratio 3AgCI: AgsAsO,.
Average ..................................
0.929636 0.92gF,72 0.929664 0.929652 0.929616 0.929650 0.929624 0.929645
SERIES V. 3AgCI: k A s 0 , . Corrected weight of Weight of of Sample AgaAsO, AgCl i n analof in vacuum. vacuum, ysis. Ag,AsO+. Grams. Grams.
No.
22
G
5.47133
5.08686
Dissolved Weight of ApCl from asbestos. Eltrate. Gram. Gram.
Corrected weight of AgCl in
t o s s on
fusion. Gram.
vacuum. Grams.
o.oooog o.ooo14 0.00066
Ratio SAgCl: AgSAsO+
5.08643 0.929652
............
Average of all analyses in Series V and VI. Per cent. of Ag in AgJsO,. ..........................
0.929646 69.9681
SERIES VI. 3AgBr: Ag,AsO,. Corrected weight of Weight of Dissolved of Sam le Ag8As04 AgRr. Weightof AgBr Losson analo? in vacuum. in vacuum. ashestos. from tiltrate. fusion. Grams. Gram. Gram. Gram. ysis. Ag,AsO+. Grams. NO.
23
24 25 26
G G G G
Corrected -eight of AgBr
Ratio
Grams.
AgaAsO4.
in vacuum. 3AgBr:
O.QOOIO 0 . 0 0 0 1 2 6.04440 4.96261 6.04438 0.5.31743 6.47645 o.ooo15 0 . m 9 O.MX)II 6.47658 4.46882 5.44273 0 . ~ 2 6 0 . ~ 0 0 1 1 0 . ~ 3 0 1 0 5.44300 4.16702 5.07j3.3 0.00010 0.ooo04 0.ooo08 5.07539
1.217988 1.21799t 1.217995 1.217990
Average .......................................... 1.21799~ Per cent. of Ag in Ag,AsO,. ......................... 69 * 9678 Average per cent. of Ag in Ag,AsO,. ................. 69,9680
Discussion of Results. The first point to be noted in the foregoing tables is that the results can be divided into two distinct groups according to the samples of arsenate employed, Series, I, 11, and 111, with Samples A to E, giving values for the per cent. of silver in the arsenate lower than Series IV, V, and VI, with Samples F and G. I n the second place, both methods for determining the ratio of the arsenate to the chloride give essentially identical results. This is shown by the agreement of Series I and 11, and that of Series IV and V.
308
GEKERAL, PHYSICAL AKD INORGANIC.
Finally, the per cent. of silver in silver arsenate found in Series I and I1 agrees within less than two thousandths of a per cent. with that found in Series 111. This agreement. together with that of the individual analyses of each series, indicates both uniformity in the material employed and purity of the hydrochloric and hydrobromic acids, as well as accuracy in the analytical work. The agreement of Series 11- and 1with Series VI is closer still. I n the following table are given the sources of the various samples of silver arsenate : Sample A Na,KH,AsO, Sample E Ka,NH,SsO, Sample 15 Ka,H-lsO, Sample I: Xa,AsO, Sample C Na,HAsO, Sample G Xa,dsO, Sample D (SH,),SsO, I t is to be expected that the basicity due to hydrolysis would be most marked with Samples F and G, less in the case of Samples -4 and E, still less with Sample D, and least in the case of Samples B and C. I n the case of Samples B and C, acid accumulates in the solution during the precipitation of the silver arsenate. I n comparing the results from the different samples of silver arsenate it must be noted that occluded basic salt would increase the apparent percentage of silver in the arsenate. In the case of Samples F and G the conditions are most favorable for the occlusion of basic salts, and these two samkles actually yield a higher p e l centage 0 ) silver than the other samples. On the other hand, accumulation of acid in the solution in which the precipitation of the silver arsenate is taking place is found to have no tendency to promote occlusion of acid salts, since Samples B and C give results agreeing closely with those of Samples A, D, and E. These two facts lead to the conclusion that Samples A to E represent normal trisilver arsenate, and that Samples I; and G contain basic impurities, I n order to calculate the atomic weight of arsenic from the per cent. of silver in silver arsenate a knowledge of the ratio of the atomic weights of silver and oxygen is necessary. Some uncertainty exists as to this ratio, hence calculations have been made upon the basis of several possible values for silver, oxygen being assumed to have the value 16.000. This has been done only with the results of Series I, 11, and 111, since, as has been pointed out, they are probably nearer the truth than those of Series I T v , IT, and VI. The difference between the two sets of results amounts only to six one-hundredths of a per cent. in the atomic m i g h t of arsenic. Series I a n d 11.
If Ag If Ag If iig
930, XS 580, As
=
7 j ,026 74.961
= 107.& j O , i i S =
74.923
= IO;. = IO,.
=
Series 111.
-, 3-. 0 1 :
74.953 74.914
CALCULATION OF THE CRITICAL TEMPERATURE.
309
When the results of Series I and I1 are averaged with those of Series 111, i t is found t h a t AS = 75.02 I If Ag = 107.930 If Ag = 107.8t’o As = 74.957 As = 74 918 If Ag = 10j.8jo The atomic weight of arsenic will be further investigated in this laboratory. The most important results of this research may be briefly summed up as follows: I . Methods for the preparation of normal trisilver arsenate were devised. 2. It is shown that trisilver arsenate precipitated by means of trisodium arsenate probably contains occluded basic impurity. 3. It is shown that silver arsenate cannot be completely dried witho u t fusion. 4. The specific gravity of unfused trisilver arsenate is found to be 6.66 at 25Oj4O. 5. The per cent. of silver in silver arsenate is found to be 69.9616 by three closely agreeing methods. 6. With several assumed values for the atomic weight of silver referred t o oxygen 16.000,the atomic weight of arsenic is found to have the following values: If Ag = 107.93 As = 75.02 If Ag = 1 ~ 7 . 8 8 AS = 74.96 If Ag = 107.85 A S = 74.92 A grant from the Carnegie Institution of Washington has been of great assistance in the pursuit of this investigation. We are also indebted to the Cyrus M. Warren Fund for Research in Harvard University for much indispensable platinum apparatus, CAMBRIDGE, MASS., November
4 4
198.
[CONTRIBUTIONS FROM THE HAVEMEYER
LABORATORSES OF COLUMBIA
UNIVERSITY,
NO. 160.1
THE CALCULATION OF THE CRITICAL TEMPERATURE OF AN ASSOCIATED LIQUID FROM SURFACE TENSION RESULTS. BY J. LIVINGSTON R. MORGAN. Received December 16, 1908.
Theore tical. Accordiag to Ramsay and Shields1 liquids may be divided into two great classes. One of these, the so-called non-associatad liquids, following the law 2.Fhysik. Cleem., 12, 433-75 (1893).
