ANALYTICAL CHEMISTRY
416 which makes it possible to observe the precise end point of one acid fraction and the beginning of the next.
Table 1. Recovery Tests
PROCEDURE
The material to be analyzed is ground to pass a 100-mesh screen and thoroughly mixed. A sample containing 0.2 t o 0.4 me. of total organic acids is taken for analysis. Usually 0.1 gram of cured tobacco is sufficient. I n this case 0.4 ml. of 4 N sulfuric acid is added and thoroughly mixed with the sample in a small beaker. T o this mixture 0.3 gram of silica gel,.prepared according t o Isherwood, is added and stirred until the whole appears to-be a dry powder. Three grams of silica gel are mixed with 3 ml of 0 5 S sulfuric acid and sumended in 25 to 30 ml. of the 15% solvent. The suspension is poured into the chromatograph tube and the excess solvent forced out with gentle air pressure until the solvent level just disappears below the top of the silica gel. The prepared sample in the beaker is suspended in about 5 ml. of the 15% solvent and transferred t o the top of the silica gel column. The beaker ip then rinsed two or three times with small amounts of solvent and finally niped clean with a small plug of cotton, which is pushed down into the chromatograph tube until it rests on the sample. The column is filled with the 15% solvent and connected with the solvent reservoirs through the two-way pressure stopcock. External indicator, O.O20j, solution of thymol blue prepared according to Isherwood, is permitted t o flow a t the rate of about 1 drop prr 10 drops of solvent emerging from the column. The 15% solvent is forced through the column under about 5 pounds' air pressure until the oxalic acid fraction is completely washed through. Two very small acid fractions precede the oxalic acid fraction in most cured tobacco samples. The stopcock is then adjusted to force the 30% solvent through the column until all of the malic acid and citric acid fractions are collected. If the sample is unusually high in citric acid, better separation of malic and citric acids is obtained if the 1.5% solvent is forced through until all the malic has been collected. The acid fractions are titrated with 0.01 iV harium hydroxide. During the titration the solvent and water mixture is shaken vigorously, and as the end point is approached the two layers are allowed to separate for observation. Barium hydrovide has less tendency to form stable emulsions under these conditions than either sodium or potassium hydroxide An excess of 0.01 N barium hydroxide is added and the mixture is back-titrated with 0.01 N hvdrochloric acid until tlie chloroform-butanol layer is definitely pink.
O.P. Acids Alone, hle.
Added 0.112 0,075 0.078 0.265
Oxalic Malic Citric Total
Found 0.116 0,072 0.076 0,264
0.05 G r a m of Cured Tobacco, Me. Plus C.P. Acids % Alone Added Found Recovery 0 080 101.3 0.023 0.079 0.059 96.7 0.061 0.024 98.2 0.112 0.110 0.073 0.249 98.8 0.120 0.262
Table 11. Reproducibility Tests Acids Found in 0.1 Gram Samples of Cured Tobacco, Unknown Unknown 1 2 Oxalic Malic Citric 0.008 0.005 0.046 0.054 0.130 0,008 0,004 0,044 0,056 0.131 0.004 ... 0.047 0.031 0,050 0.004 0.049 0.034 0.050 0:004 0.073 0.041 0.100 0.011 0.004 0.073 0.044 0.098 0.010 0.008 ... 0,066 0.020 0.058 0.008 0 064 0.020 0.058 0,007 0:003 0.059 0.029 0.086 0.006 0.004 0.054 0,027 0.089
Lab. NO. 2
73 3 67
75
Me. Total 0.293 0,293 0.132 0.137 0.229 0.229 0.152 0.152 0.180 0.184
An examination of Tables I and I1 indicates the accuracy and precision to be expected with this technique. Fair precision is obtainable when as little as 0.004 me. of an acid is present. The limiting factor in the upper range is the ability of the column to separate large acid fractions, but generally the best separations on the size of column described here are obtained when the quantity of the total acids does not exceed 0.4 me. LITER4TURE CITED
(1) Isherwood, F. A., Biocheni. J., 40, 688 (1946). ( 2 ) Pucher, G. W., Vickery, H. B., and Wakeman, A. J., IND. EX. CHEM.,ASAL. ED.,6, 140 (1934).
R E C ~ I V EAugust D 27, 1951. The investigation reported in this paper is in oonnection with a project of the Kentucky Agricultural Experiment Station and is published b y permission of t h e director.
Technique for Noting Combustion in Oxygen Bombs CHARLES HARRISON HALE, Continental Oil Co., Ponca City, Okla. ERTAIN widely used oxygen bomb methods for sulfur and chlorine ( 1 ) specify the ignition of the sample charge in a platinum cup by means of nylon thread or absorbent cotton supported on a platinum firing wire. Passage of current through the firing wire is regulated so as t o ignite the nylon or cotton and thus the oil sample without melting the wire Ignition in platinum and eliminating the use of iron fuse wire avoid contamination by iron oxide, but ignition by this procedure sometimes fails because the magnitude of the current may be too low t o ignite the nylon or cotton. I n such a case it is necessary t o try the ignition again, still keeping the current low enough that the platinum wire is not melted. Determination of firing or of failure t o fire has been made by placing the hand on the outside of the bomb and noticing the rise or lack of rise in temperature. This is not considered a safe practice because the operator is exposed t o the chance of a ruptured bomb at the time of its peak pressure development. A yery simple method of quickly checking the firing consists of attaching a small thermocouple t o the side of the bomb and noting the deflection of a suitable potentiometer or millivoltmeter as the surface temperature increases. Even when the bomb is immersed in ire water a definite deflection is obtained. The procedure used in these laboratories for the past several months is as follom: The bomb (Parr Instrument Co. double valve, Model 1101) is
charged with up to 0.8 gram of sample and oxygen to a pressure of 35 atmospheres. A heavy rubber band is tightly stretched around the bomb cylinder and the bead of an iron-constantan thermocouple, B. & S. gage 22, is t,hrust under the band to effect firm contact with the bomb cylinder. Leads from a variable transformer, set to a predetermined vokage, are attached and the bomb is placed in a small pail of ice water. This in turn is placed inside a steel shield consisting of a 13-inch length of 12-inch steel pipe Tvhich stands vertically on a circular steel plate to which it has been welded. The e.m.f. developed by the thermocouple is balanced by a portable L. & N. potentiometer without regard to its actual value. The preselected current is passed through the bomb for an appropriate interval-e.g., 4 seconds-and if firing occurs this is shown by a definite deflection. Typically, the deflection may be 5 scale divisions, corresponding to approximately 0.5 to 1.0 millivolt, roughly equivalent to 10 O to 20' C. Should the lack of voltage change indicate a failure t o ignite the sample, a further effort can be made promptly by adjusting time or magnitude of current without removing the bomb from its water bath and protecting cylinder. The convenience with which repeated combustion effort;. can be made encourages the use of moderate firing currents and has decreased the number of melted firing T-iires. LITERATURE CITED
Soc. Testing Materials, Philadelphia, "A.S.T.AZ. S t a n d n u i ~ on Petroleum Products," D 129--50, D SO8-SOT.
( 1 ) ilm.
RECEIVED July 23, 1951.
,
