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INDUSTRIAL AND ENGINEERING CHEMISTRY
saving once standards have been set up. It is also not necessary to make the usual tedious determination of the alkalies, which must be made if the quartz is to be determined by calculation from the mineral composition of the spar.
Literature Cited (1) Bausch & Lomb Optical Co., Rochester, N. Y., Cutelog 1859-C, 1930.
VOL. 12, NO. 6
( 2 ) Booze and Klein, J. Am. &ram. Sot., 6, 698 (1923). (3) Chamot and Mason, “Handbook of Chemical Microscopy”, Vol. 1, p. 441, Kern York, John Wiley & Sons, 1931. (9), 651 (1927). (4) rnsley, H., J , Am. Ceram. sot., (6) McCaughey, W. J., Trans. Am. Ceram. SOC.,15, 381 (1913). (6) Parmelee, C. W., and McVay, T. N., Univ. Illinois Eng. Expt. Sta., Bull. 233, 1931. (7) Thompson, C . L., J . Am. Cemm. SOC.9 17 (8)~ 257 (1934). (8) Watkins, Enamelist, 3, No. 2, 11 (November, 1926).
Volumetric Determination of Selenate GLENN C. SOTH
AND
JOHN E. RICCI, Xew York University, New Yorlr, N. Y.
IK
C O S N E C T I O N with certain phase rule studies requiring numerous and readily duplicated accurate determinations of dissolved selenate, some simplifications in the volumetric method first suggested by Pettersson (9) were effected, which i t seems worth while to describe. This direct method, studied in detail by Gooch and coworkers (3, 4,6 ) , seemed, especially when compared with the gravimetric methods available (determination as lead selenate or as elementary selenium), the best suited for rapid and accurate analysis. The reactions involved are: reduction of the selenate to selenite by means of some halogen acid, distillation of the liberated halogen into potassium iodide solution and titration of the equivalent iodine with standard thiosulfate solution. As described by Gooch and co-workers, however, and also as studied by Moser and Prinz (S), the analysis requires a cumbersome apparatus and has several sources of error, so that i t seemed desirable to modify and simplify the procedure further. The chief difficulties are the necessity of passing a current of carbon dioxide through the apparatus during the distillation of the bromine to ensure its complete removal, the need of two or more traps of potassium iodide solution to absorb the bromine completely, and the uncertainty of controlling the treatment with hydrochloric or hydrobromic acid without causing either further reduction of the selenite to elementary selenium or the formation of volatile selenium bromide. I n the procedure described by Gooch the distillation is continued until the volume in the distilling flask is reduced by one third or until color begins to reappear in the residual liquid. Gooch and Pierce (5) in fact recommended indirect titration, not of the liberated bromine but of the selenite formed. The necessary operations for the direct determination, using hydrobromic acid for the reduction, can be carried out accurately and rapidly by means of the all-glass apparatus shown in Figure 1, consisting of a 200-ml. round-bottomed flask with a sealed-in dropping funnel, a Kjeldahl trap, a condenser, and a receiving flask. In use, a small amount of stopcock grease is applied to the ground-glass joints to prevent sticking. The sample is laced in the distilling flask and diluted to about 125 ml., and a pew glass beads are added to prevent bumping. The solution is heated t o boiling and the air driven from the apparatus. When no more bubbles pass through the potassium iodide solution in the receiving flask, the water in the condenser is turned off for a few seconds. After a few more bubbles escape, the water is turned on again, drawing the solution up into the condenser to a height of 5 or 7.5 cm. (2 or 3 inches). Twenty milliliters of concentrated hydrobromic acid are added slowly to the boiling liquid through the dropping funnel, and from time to time thereafter water is added t o maintain the volume in the distilling flask. The distillation is continued until the contents of the distilling flask are colorless, after which the condenser is washed into the receiving flask and the solution is titrated immediately. I n place of hydrobromic acid (which must be free of bromine), sulfuric acid may be mixed with the sample in the dis-
tilling flask a t the start, and potassium bromide solution added through the dropping funnel. Discussion The distillation as described requires about 20 minutes and the entire analysis about 30 minutes. Although the disappear-
W
FIGURE 1. APPARATUSFOR
VOLUMETRIC DETERMISATIOX OF
SELEKATE
JUNE 15, 1940
ANALYTICAL EDITION
TABLE I. DETERMINATION OF SELENATE hl e t h od
Sample Grama Analysis of MgSeOd. 6H20 0.2504 Mg-8-hydroxyquinolate 0.2737 0.2662 0,2020 Lead selenate 0.2011 0.1996 Distillation method 0.8161 Using H B r 0.8254 0,8098 Using HISO, and KBr 0.6675 0. a
----hIgSeO4Found
m
0.8175
Theoretical
70
70
60.65 60.78 60.77 60.70 60.78 60. a6
60.75
60.74 60.74 60.79 60.68 60.71 60.81
... ... ...
... ... ... .. .. .. ...
... ...
Analysis of Solution of hIgSeO4.6HzO Distillation method, using HBr 2.8192 35,73 35. i5S 1,7397 35.56 ... a Known from determination of Mg by quinolate method, 36.74 and 35.76 %.
ance of the bromine color in the residual liquid is the best indicator for the completion of the reaction and of the distillation of the bromine, the distillation can also be followed by observing the color of the vapor in the condenser, so that the analysis may be used for colored solutions. The current of carbon dioxide employed by Gooch has been found unnecessary, inasmuch as the steam from the distilling flask is sufficient to carry over all the liberated bromine. I n addition to permitting a simpler setup, this Kjeldahl type of procedure eliminates several possible sources of error. The absorption of bromine by potassium iodide solution is not instantaneous, so that even with the two traps employed by Gooch the current of carbon dioxide passing through the potassium iodide solution may carry with i t some bromine. The present method of operation forms a liquid seal to the apparatus, so that no vapor can escape and all the bromine must dissolve in the receiving solution. With only one trap, furthermore, both time and possible loss in transfer are saved, because the contents of several traps need not be combined for the final titration. When excess of hydrobromic acid is mixed directly with the sample in the distilling flask before boiling, as in the original procedure of Gooch, there is an initial rapid evolution of bromine which is difficult to control. By adding the hydrobromic acid through the dropping funnel the evolution of the bromine is regulated so that none escapes before i t can react with the potassium iodide solution. Finally, since the volume of residual liquid is not used as an indication of completion of the reaction, the initial volume being maintained throughout, and since the addition of bromide can be so regulated as to avoid great excess of halogen acid in the boiling solution, there is no danger either of the formation of elementary selenium as stated by Gooch or of the distillation of selenium tetrabromide from concentrated solution. The procedure described should be useful for the detection and determination of selenate in the presence of selenite and for the determination of purity of salts. Sulfate ion, which must be absent in the lead selenate gravimetric method, does not interfere. The chief limitation, however, is the interference by oxidizing agents other than selenate itself; no other oxidizing agent (including nitrate ion) capable of liberating bromine from hydrogen bromide in boiling solution may be present.
Test of Procedure MATERIALS. Magnesium selenate hexahydrate, MgSeO,. 6H10, prepared by recrystallization from magnesium oxide and selenic acid, was used for all analyses. The selenic acid was pre-
329
pared from elementary selenium according to the method of Gilbertson and King (2). Two analyses are included on solutions of pure magnesium selenate whose composition was known from determination of magnesium by the method described below. Hydrobromic acid was a Merck 40 t o 42 per cent solution, free of bromine. The potassium iodide solution in the receiving flask was made up just before use and contained 8 grams of c. P. salt and 30 ml. of 4 N sulfuric acid in 150 ml. of solution. The liberated iodine was titrated in the usual manner, using starch indicator and 0.15 N thiosulfate previously standardized against potassium dichromate. The purity of the magnesium selenate was checked by precipitation of lead selenate according to the method of Ripan-Tilici (IO, 11) and by precipitation of magnesium as the o-hydroxyquinolate and subsequent titration with standard potassium bromate (Berg’s method, I ) , using the procedure of Kolthoff and Furman ( 7 ) , with indigo as indicator.
Literature Cited (1) Berg, 2.anal. C h e m , 71,23 (1927). (2) Gilbertson and King, J. Am. Chem. SOC.,58, 180 (1936). (3) Gooch, “Quantitative Analysis”, p. 197, New York, John Wiley & Sons, 1916. (4) Gooch and Evans, 2.anorg. Chem., 10, 253 (1895). (5) Gooch and Pierce, Ibid., 11, 249 (1896). (6) Gooch and Scoville, Ibid., 10, 256 (1895). (7) Kolthoff and Furman, “Volumetric Analysis”, Vol. 11, p. 483, New York, John Wiley & Sons, 1929. (8) Moser and Prina, 2.a n d . Chem., 57, 277 (1918). (9) Pettersson, Ibid., 12, 287 (1873). (10) Ripan-Tilici, Ibid., 102,343 (1935). (11) Spacu, Bull. SOC. chim., (5) 3, 159 (1936).
Jacketed Receiver for Vacuum Distillation J. B. CLOKE, Rensselaer Polytechnic Institute, Troy, N. Y.
T
HE accompanying figure represents a well-known re-
ceiver for distillation under diminished pressure, which has been provided with a jacket for the circulation of ice water or other cooling medium. This all-glass receiver (made by the Ace Glass Co.) is especially useful for handling low-boiling compounds which must be distilled under diminished pressure. The joints are f 24/40.
