T H E MOLAR WEIGHT O F SELENIUM DIOXIDE I N ETHYL ALCOHOL BY EDMUND BRYDGES RUDHALL PRIDEAUX AND GERALD GREEN
The Molar Weights of Oxides in Solution There are few oxides of the non-metals, solid or liquid a t ordinary temperatures, which dissolve as such in solvents other than water without chemical action. The following may be quoted.’ Oxide
Formula Weight M
92 I08 220
22 0
3 94 3 96 182.9 I27
Solvent
acetic acid phosphoryl chloride naphthalene benzene sulphuric acid benzene phosphoryl chloride phosphoryl chloride
Molar Weight observed
M’ 90.3- 94.6 108.7-108.2 218 227
288-455
associated
-
167 I93 135- 1 3 6 ~
From the foregoing figures, obtained by the cryoscopic method, it appears that association, which is usual in the vapour state in the case of acid anhydrides, exists also when the same compounds are in solution. Selenium Dioxide in Alcohol The case of selenium dioxide is particularly favourable, as being the oxide of a strongly electronegative element, which is readily soluble in alcohols (unlike tellurium dioxide) and does not, according to our experience, dehydrate them appreciably (unlike sulphur trioxide). v. infra. Preparation of the Selenium Dioxide A sample of the element in the form of powder, supplied by the courtesy of the Baltimore Copper Smelting Company, as arranged by Professor Victor Lenher of the University of Wisconsin, was oxidised by the different methods which are described in the literature. (I) The method of Divers and Hagas. The selenium is oxidised by concentrated nitric acid, evaporated several times with more nitric acid and, after heating nearly to its subliming point, is resublimed in a Current of dry oxygen. The product consists for the most part of very small, pinkish crystals. 1
a
See Turner’s “Molecular Association.” Worsley and Baker: J. Chem. SOC.123, 2872 (1923). J. Chem. SOC. 75, 538 (1899).
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EDMUND BRYDGES RUDHALL PRIDEAUX AND GERALD GREEN
( 2 ) By direct combination with oxygen. This gives products which vary according to the temperature of sublimation and other conditions from pink to red, and also greyish needles of a gritty nature. (3) By oxidation in presence of oxides of nitrogen according to the procedure of J. Meyerl. A current of oxygen was passed through a wash-bottle containing nitrite and acid. The selenium burnt a t first to give a chocolate or dirty grey product. ThiF gradually became lighter and finally gave a mass of colourless crystals like snow. When this material was heated again in pure oxygen it lost its crystalline form and brownish fumes were observed. On sublimation it gave a greenish vapour which condensed in slender needles. A part sublimed as the pink compound, but sufficient of the colourless was also obtained for use in the experiments. An analysis gave the following results. An analysis gave the following results: Se 0 2 0.4046 0.4561 0.2362 Se 0.2886 0.3246 0.1693 % Se 71.33 71.17 71.25 Theory requires 7 I .22.
The Preparation of Dry Alcohol Thorough dehydration of the alcohol was necessary, in order to ensure that no appreciable amount of selenious acid should be present, in the more dilute solutions. I n the more concentrated the conversion of a small proportion into selenious acid would have little effect. The alcohol used in most of the experiments had been refluxed over lime for about 8 hours and allowed to stand in contact with aluminum amalgam according to the procedure of Wislicenus and Kauffman which, contrary to the opinion of Konowalow2, we found t o be effective. The alcohol boiled a t 78.35' under normal pressure. After treatment with calcium carbide, refluxing and redistillation, it boiled When dry selenium dioxide is djssolved in alcohol which has been a t 78.38'. purified thus, it reddens litmus paper. We attribute this to a trace of moisture, which forms selenious acid. We have found that the most effective drying agent is selenious oxide itself. The dry oxide is dissolved in the sample which is then distilled through freshly ignited lime. When the oxide is dissolved in this distillate it does not redden litmus. This alcohol, free from SeOz, was used in some of the experiments. Determination of the Molar Weight. Experimental The type of Beckmann apparatus employed consisted of a double walled glass solvent vapour jacket and an inner boiling tube, into the bottom of which was sqaled a stout platinum wire. Into the slanting side tube there passed a water cooled condenser. The selenium dioxide was introduced through a short vertical tube passing through the cork with the thermometer. The dry alcohol, prepared as already described, boiled a t 78.39' under normal 1
Ber. 55, 2052 (1922). Pharm. 2. Russ. 35, 328.
SELENIUM DIOXIDE IN ETHYL ALCOHOL
I275
pressure. About 2 5 C.C. was weighed into the corked tube and boiled for about a,n hour until the temperature remained sufficiently constant. The selenium dioxide, weighed in a glass tube was sent straight into the liquid by tapping. The side condenser was fitted with a drying tube.
First series. The weight of the alcohol was 21.338grams and from this grams has been deducted in making the calculations. The molecular weights have been calculated from a constant of 11.9 and the individual variations do not show any definite trend, except in the last three results, which refer to very concentrated solutions. The arithmetical mean of these molar weights is I2 2 . Second series. The weight of alcohol was 23.508-0.2 grams, the barometric pressure was normal and pra.ctically constant. The molar weights again show no trend. The most concentrated solution is that a t which the molar weight begins to decrease in the first series. The arithmetical mean of these molar weight,s is I2 I. 0.20
First Series At
Grams 8e02 in 100 alcohol
Second Series Grams SeOz in IOO alcohol
At
0.43
0.043
1.31
0.I 2
1.77 2.925
0.162 0.29 0.36 0.46 0.54 0.612 Q . 877 I.048 1.395 1.77 I .88
19.7
0.735 0 985 1 * I95 1.385 1.63 1.915
22.0
2.105
2.23
24.48
2.335
27-27
2.601
32.67 37.9 44.25 48.6 54.4
3.04 3.475 3.995 4.515 5.c97
3.71
4.85
5.77 6.80 9.30 11.1
13.7 17.85 19.05 23.I 27.3 31.0 35.0 38.0 41.6 45.8 49.5 54.1 56.6 64.9 69.5
2.64 2 * 995 3.363 3 e675 3.929 4.249 4.731 5 -312 5.875
6.444 7.054
I .62 2.85 4.39 6.02 7.59 9.60 12.I5
14.53 17.1
0.165 0.295 0.425 0.585 9
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EDMUND BRYDGES RUDHALL PRIDEAUX AND GERALD GREEN
Results of Molar Weight Determinations If the values of ‘w’ or IOO/W= grams of SeOz to IOO grams of alcohol are plotted against At=elevation of the boiling point the best line for each series is obtained graphically, the following values of W / A t are deduced. First Series I O .46
Second Series IO.
7
Dilute Solutions to w=25 10.28
The third value reproduces most accurately all the observations and we have taken w / A t as 10.3. The constant molar elevation in alcohol (100grams) is given in the most recent edition of Landolt and Bornstein’s Tables as 12.0 (from substances of known molar weight) and 11.9 (from thermal data). Since the former value represents an average, and may include an excess of positive or negative deviations, we have chosen the latter. Hence our observed molar weight is 122.6, as compared with a formula weight of I I I - 2 .
Possible Formation of Ethyl Selenite We have observed few signs of chemical action. There is a slight reduction after prolonged boiling, shown by the formation of a small reddish deposit. There is no formation of bubbles of ethylene, nor is the solution acid when both materials are perfectly dry. The question whether there is any formation of mono-ethyl selenite by the reaction : Se02+Cz H, 0 H = Se 0 ( OH) 0 C2 HEis somewhat, more difficult to decide. This substance however would probably react acid to litmus. Hinsberg’ claims to have prepared a solid of this composition by evaporation of an alcoholic solution in presence of calcium chloride. But the alcohol is very loosely combined, it is lost when the substance is kept in a vacuous desiccator and is difficult to distinguish from alcohol of crystallisation. The molar weight in the more dilute solutions would not be affected by the formation of this compound, and if it were progressively formed in the more concentrated solutions, there would be a very decided fall; the molar weight corrected for solvent thus abstracted would be over 160. Although perhaps the lowness of the values in the case of the three most concentrated solutions may perhaps be attributed to a slight degree of combination with the solvent, yet the const,ancy of the remaining results seems to render it unlikely that this effect is important. If this solvation is ruled out, the results show a constant degree of polymerisation of the selenium dioxide over a wide range of concentration, quite similar t o that observed in the case of boric acid2 dissolved in water. The extrapolation of these results to high dilution led .4nn. 259, 40 (1890).
* Beckmann: Z. physik. Chem. 6 , 460 (1890).
-
SELENIUM DIOXIDE IN ETHYL ALCOHOL
I277
to molar weight of 67.2 instead of 6 2 for H3B03. It was considered that they showed a normal molar weight, and the apparent association was attributed to the volatility of the solute. No correction however was introduced. I n the present case we have obtained some additional data with a view to supplying this correction. It is derived from a consideration of the vapour pressures of the pure solvent, the solution, and the partial pressures of solvent and solute. Using the symbols and equations as given in 'Problems in Physical Chemistry':
At (observed) =
RT2 (rL- r,). n
h!
This may be transformed intoIOOW
m =At, /At X -X (1-rV/rL); in which capital letters refer to W the solvent, and rv are the weight ratios of solut,e to solvent in the vapour, which must be found by analysis.' The first series of distillations was carried out in the Walker-Landsberger boiling point apparatus. After some practice it wa.s found possible to keep the volume of the liquid in the graduated tube constant by regulating the heating of the flask which rupplied the alcohol vapour, and of the water bath which surrounded the outer jacket. A series in which the tube leading away the vapours projected a short distance outside the heated area before sloping downwards, showed very distinctly the effect of refluxing, the values of r,/r,, being only 1/5 to 1 / 1 0 of those in the other experiments. In the second series the whole still-head was enclosed in a steam jacket, the solution being heated by the vapours of boiling alcohol as before. The solutions a t the beginning and the end of the distillation, and the distillates were analysed, and their densities were determined. The total weight of SeOzfound in residue and distillate, 16.10 grams agreed well with the amount taken, 16.13 grams, and the agreement was also good in the other experiment,s. Composition of solution a t beginning is LI " " " end LI " " mean
go. 5 SeOz in 82
'(
5
86. 5
I'
"
IOO "
alcohol "
'I
The other experiments were checked in a similar manner. An upper limit to the values of r, may be obtained from the vapour pressures2 of pure SeOz. In the most concentrated solutions w=87, p = 15 m.m, P-p=735 r,=0.0204, rL= '0.359, I -rv/rL=o.g43 and m = 115.9. But at lower concentrations the partial pressure of selenium dioxide in the vapour In the determination of ry/rL the point, a t issue is assumed ie. that the solut,e has a normal molar weight in the dissolved state. I n cases of only slight association, this gives a first approximation, which is well within the limits of experimental error in the present case. Jannek and Meyer: Z. anorg. Chem. 83,5r (1913).
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EDMUND BRYDGES RUDHALL PRIDEAUX AND GERALD GREEN
is far lower than that of the vapour in equilibrium with the solid; for at w = 1 2 , m"79.5, follow from the above calculation.
w = 3 1 , m=104 & a t Solution SeOz in IOO
Distillate SrOz in IOO
rv/rL
-rY/rL
m(corrected)
0.9525 0.943 0.9574
115.9
0.9665 0.945 0.925
118.8 115.8 113.6
0.987 0.975 0,971
I21
I
First Series 61.9 31.05 11.9
2.93 1.77 0.509
0.0475 0.057 0.0426
116.7 117.2
Second Series 86.5 45.7 15.35
2.90 2.72 1.15
0.0335 0.0595 0.075
Third Series '17.1 29.0 8.91
1.02
0.73 0.255
0.01325 0.0251 0.0286
119.5 119.2
These results on the whole show a diminution in molar weight with diminution in concentration. If equal weights are assigned to each, the mean of the expressions I -rv/rL is 0.958, giving a corrected molar weight of 117.5. This corresponds to an association of IO% (double molecules). It will only be possible however t o determine by this method the precise amount of the correction by the use of a more perfect distillation a.pparatus and larger quantities of solution than were available. Summary and Conclusions The molar weight of pure selenium dioxide dissolved in dry alcohol has been determined by the elevation of the boiling point. The means of two series agree well with one another, and the most probable molar weight from all results is 122.6, when the constant is taken a t 11.9. There is no trend in the molar weights up to a concentration of 55 grams in 100. The relations between the amounts of the dioxide in solution and vapour has been determined; the mean correction based on these reduces the molar weight to 117.5. University College, Nottingham May, 1924
