INDUSTRIAL AND ENGINEERING CHEMISTRY
32
ses by other laboratories have shown excellent agreement, which is gratifying in view of the fact that they had no previous experience with the method. In Table IV are shown typical results obtained on analysis of commercial crude and dynamite grades of glycerol in comparison with similar analyses by the modified Rojahn method.
TABLE IV. DETERMINATION OF WATERIN COMMERCIAL GLYCEROL (Comparison of the new method with the Rojahn method) r Per Cent of Water Found Deaiocation over Po06 for: Vacuum distillation 24 48 72 Sample 1 2 Av. hours hours hours Dynamite No. 1 0.44 0.45 0.45 0.75 0.81 0.90 0.74 0.74 0.82 0.93 1.04 Dynamite No. 2 0.73 Crude No. 1 5.77 5.78 5.57 5.92 5.92 5.79 4.43 4.42 3.89 Crude No. 2 4.52 4.77 4.41
This is further evidence that the desiccation over Ps05 is unreliable. One of the dynamite glycerols showed fairly good agreement with the new method in 24 hours, while the other gave a result almost 70 per cent too high. In the crudes both were somewhat low a t 24 hours and gave more nearly oorrect values at 48 hours. The longer time required appears to be caused by the formation of a layer of phosphoric acid over the surface of the phosphoric anhydride, thus reducing its efficiency as a desiccant. Over activated alumina, a desiccant which is granular and porous but admittedly of lower efficiency than PZOS, a value of 4.50 per cent of water was obtained on crude No. 2 in 24 hours.
VOL. 8,NO. 1
the water vapor. The water vapor is absorbed in a desiccant and weighed. The vacuum distillation method is shown to give the true water content of glycerol with a relative error of not over 3 per cent on samples containing 0.5 to 5 per cent of water, Results are readily duplicated and are available in a maximum time of 2 hours. The attention of the chemist is required during only a fraction of this time. The method is applicable to crude and refined glycerol.
Literature Cited (1) Berth, T., Chem.-Ztg., 51,975-6 (1927). (2) Bosart, L. W., and Snoddy, A. O., IND. ENG.CHEM.,19, 5 0 6 ~ (1927). (3) Griin, A., and Wirth, T., 2.angew. Chem., 32,59-62 (1919). (4) Hoyt, L. F., IND.ENG.C H ~ M26, . , 329 (1934). (5) Hoyt, L. F., and Clark, P. C., Oil & FutInd., 8, 59-61 (1931). (6) Kameyama, N., and Semba, T., J. SOC.Chem. Ind. Japan, 30, 10 (1927). (7) Langmuir, A. C., et al., J. IND.ENG.CHBM.,3, 679-86 (1911). (8) Lawrie, J. W., "Glycerol and the Glycols," p. 298, New York, Chemical Catalog Co., 1928. (9) Rojahn, C. A., 2.anal. Chem., 58,433-42 (1919). (10) Schmidt, M. R., and Jones, H. C., Am. Chem. J., 42,37 (1909). RECI~IWD September 14, 1935. Contribution No. 18 from the Eastern Laboratory of E. I. du Pont de Nemours &Company.
Calculating the Blank
Precaution
BARTHOLOW PARK
Anhydrous magnesium perchlorate should be classed as a potentially dangerous chemical, just as are sulfuric acid, metallic sodium, etc. Accidents have resulted from the accidental admixture of sulfuric acid with this compound to form perchloric acid, which, in turn, exploded in contact with organic matter. In view of this, the following precautions must be followed:
Michigan College of Mining and Technology, Houghton, Mich.
1. Never use sulfuric acid to replace potassium hydroxide in drying tower H. 2. The chemical should be keDt free from organic material as far as possible. Never use a cotion plug to rephce glass wool in the U-tube. 3. The chemical should be handled and the spent material disposed of only by the chemist, and never should be left t o a janitor or clean-up boy. 4. The spent chemical should not be thrown into waste jars or sinks. It should be dissolved in water and disposed of in a drain, ditch, or other safe place outside the laboratory.
Where these precautions are observed, there should be no trouble resulting from the use of anhydrous magnesium perchlorate as a desiccant.
Summary
A critical study has been made of the modified Rojahn method, showing that it only approximates the true water content. It gives varying results due to inadequate control of the following factors: (1) the temperature of the room as it affects the volatility of glycerol a t the reduced pressure; (2) the presence of varying amounts of volatile impurities other than water; and (3) the loss of efficiency of the desiccant when comparatively large quantities of water are present, as in crude glycerol. A new method, designated the vacuum distillation method, is described. It is based on the removal of water from glycerol a t 100" C. and 2- to 3-mm. pressure. The volatility of the glycerol and low-boiling impurities in glycerol is controlled by a reflux condenser, which separates these ingredients from
M
ANY volumetric determinations require an excess of the reacting solution to produce a visible end point.
For accurate work this excess must be subtracted from the total amount added in order to obtain the correct titer. The magnitude of this excess may be determined directly by titrating a blank, on which the end point is apt to be uncertain, or it may be calculated in the following manner. If two or more samples of different weights are run under exactly the same conditions, the amount of excess reagent should be the same in each case and simple equations may be used to calculate it.
Example
A zinc solution was titrated with ferrocyanide, using uranium nitrate as outside indicator and a volume of 2001 cc. a t the end point with the following results: 10 cc. 20 cc. 30 cc. 40 cc.
From which
28.50 28.50 19.13 37.87 37.87 37.87
of of of of
-x -x -x -x -x -x
zinc = 9.75 cc. of zinc = 19.13 cc. of zinc = 28.50 cc. of zinc = 37.87 cc. of
ferrocyanide ferrocyanide ferrocyanide ferrocyanide
= 3(9.75 - x) and x = 0.375 = 3/2(19.13 - x) and x = 0.40 x) and x = 0.37 = 2 9.75 = 419.75 - 2) and x = 0.38
-
=
=
4/3(28.50 - x) and x = 0.39 2(19.13 - x) and x = 0.39
Average, 0.38 cc. The ratio of the zinc solution to the ferrocyanide solution, after subtracting the blank of 0.38 cc. figures out as 1 to 0.937. RECEIVED July 15, 1935.
