ANALYTICAL EDITION
IND U STRIAL andENGINEERING CHEMISTRY Harrison E. Howe, Editor
Determination of Glycol or Glycerol J
J
In Dilute Solutions Containing Oxidizable Impurities WILLIAM E. SHAEFER Hercules Powder Company, Experiment Station, Wilmington, Del.
G
LYCEROL and glycol are so similar in their chemical
2 hours. The latter value, although surprising because it was lower than the former, was not confirmed because the author was interested primarily in the determination of glycol. The glycol used in these and subsequent experiments had a specific gravity of 1.1172 a t 15.6"/15.6" C., which corresponds to a purity of 99.5 per cent. The results of acetylation experiments on weighed samples of this glycol are shown in Table I. On the basis of these results it was concluded that in the acetylation of glycol for 15 minutes under these conditions the reaction p'roceeds only to the extent of about 97.9 per cent of the stoichiometric value. Pyridine hydrate, CsH5N.3Hz0, has a constant boiling point of 94.4" C. I n a number of experiments a 50-cc. sample of dilute glycol solution was distilled slowly through a 3bulb Snyder column until its volume was reduced to about
behavior that, generally, a chemical method suitable for the determination of one is also applicable to the determination of the other. Regardless of the type of substance being analyzed for glycerol, the two commonly used methods for its quantitative determination are the dichromate oxidation method and the acetin method, wherein the acetate is formed and saponified. Before the dichromate method can be applied to a sample, all oxidizable impurities and all chlorides must be removed. This is more difficult and t h e consuming than is desirable. The acetin method, according to some writers (3,6),cannot be applied to a glycerol solution less concentrated than 40 or 50 per cent. The problem a t hand was to analyze dilute (1to 5 per cent) aqueous glycol solutions that also contained oxidizable impurities. Since the direct and simple pyridine acetic anhydride method (1,b, 4 6 ) possesses manifest advantages, the writer wished to employ it if possible in solving this problem. Evidently such a method cannot be applied directly to the analysis of a 5 per cent glycol solution because the water in a 2-cc. sample, containing 100 mg. of glycol, would require 10.8 grams of acetic anhydride. There seemed to be no hope of removing a sufficiently large proportion of the water by distillation of an aqueous glycol solution through a column, without a serious loss of glycol, so that the pyridine-acetic anhydride method could be applied to the residue. However, it proved possible to remove the water completely by distillation when a suitable liquid was added to form an azeotropic mixture.
TABLEI. EXTENTOF ACETYLATION OF GLYCOL BY ACETICANRPDRIDBREAGENT Refluxing
7
1 hour
15 min.
10 min.
2 min.
~RIDINE-
Standing at Room Tern ersture for I &ur
%
%
%
%
%
97.0 97.7
97.2 97.7 98.2 97.1 98 7 98.7 97.9
95.9 94.9 96.4
93.0 83.9
70.9
.. .. .. ..
Av.
.. ....
..
.. .. .. .. I
.. ....
.
TABLE11. PRELIMINARY ANALYSESOF SYNTHETIC GLYCOL
Experimental Work
SOLUTIONS
Experiment No.
Several preliminary experiments were made in an effort to remove practically all the water from a dilute glycol solution by distillation after the addition of xylene to form a constant-boiling mixture with water. I n further experiments, pyridine was employed to form a one-phase solution for the acetylation. Since results were encouraging, the work was continued and the procedure described below was developed. Before working out the details, two determinations of glycerol and a number of determinations of glycol were made by the direct pyridine-acetic anhydride acetylation method. The specific gravity of the glycerol used in this work indicated In two that it was 98'6 per cent ments, acetylation was found to proceed to the extent of 97.8 per cent rn 30 minutes and to the extent of 96.6 per cent in
1 2 3 4 5 6 7 8 9
10 11 12 13 14 15
Glycol Present' Qrame 2.40 2.46 2.46 2.46 0.49 0.49 0.49 0.49 0.49 2.46 2.46 0.49 0.49
0.10 0.10
Glycol Found Urams 2 42 2 45 2:44: 2:43 2,45,2.47 2.44,2.42 0.50 0.49 0.47 0.48 0.46 2 42 2 43 2:45: 2:45 0.49 0.49
0.10 0.09
a Aliquot portiona of a solution containing 6.92 grams of glycol per 100 cc. were measured with pipets.
449
INDUSTRIAL AND ENGINEERING CHEMISTRY
450
10 cc. Then it was treated with 50 cc. of pyridine and distillation was continued until the temperature reached 99” C. It was assumed that a negligible amount of water was left in the sample after this distillation. If the residue contained more than 0.5 gram of glycol, it was diluted to 100 cc. with pyridine and an aliquot was analyzed. Some results for glycol obtained by this procedure, calculated on the assumption that the acetylation values were only 97.9 per cent of the theoretical ones, are shown in Table 11. In an effort to obtain results of higher accuracy, the distillation of the pyridine-water solution was continued in some cases until the distillation temperature reached 110’, Better results were obtained, which indicated that the more complete removal of the water was beneficial and did not result in an increased loss of glycol. Results obtained by several different analysts following the improved procedure are given in Table 111. The analyses reported in Table I V were made on portions of an aqueous glycerol solution which contained 4.88 grams Qf glycerol per 100 cc. The average accuracy of the results obtained by this method was found to be about 1 per cent when applied to samples containing 0.5 gram or more of glycol. The average precision of the results shown in Table 111, where added impurities had to be removed, is about 1.5 per cent; that shown in Table IV is about 1.0 per cent.
VOL. 9, NO. 10
chosen temperature. This reagent is prepared by treating 154
cc. of acetic anhydride with 1 liter of dry pyridine. Acetylate the glycol by boiling the reaction mixture gently for 15 minutes. Heating for 1hour doe3 no harm. Treat the hot reaction mixture with 20 t o 30 cc. water to convert the excess acetic anhydride to acetic acid and t o rinse down the condenser, cool the flask somewhat t o avoid loss of acetic acid, remove it from the con-
denser, and further cool it with tap water. Titrate with approximately N sodium hydroxide solution, using phenolphthalein as the indicator. Certain precautions should be taken during this titration t o avoid the danger of saponification of the glycol acetate or glycerol acetate. The kolution should be shaken continuously while being titrated to prevent the accumulation of a local excess of alkali and care should be taken t o avoid overtitrating it. Treat 25-cc. portions of acetic anhydride-pyridine reagent by the procedure just described to obtain blank values. This method is believed to be applicable to the analysis of dilute solutions of glycol or glycerol in water or any solvent boiling below 110’ C., unless the solution contains a higher boiling or nonvolatile impurity that can be acetylated.
Calculation
- B)
X normality of alkali X 0.03103 X 100 - % glycol cc. of sample X density x 0.979 where A = cc. of sodium hydroxide required for blank B = cc. of sodium hydroxide required for sample 0.03103 = gram of glycol equivalent t o 1 cc. of N alkali 0.979 = factor t o represent the extent t o which stoichio-
(A
metric results are approached in the reaction
Procedure
If the glycol sample contains hydrochloric acid which is Measure out a sample of 50 cc. or less which is thought to conneutralized before the distillation is started, and if an aliquot tain about 2.5 grams or less of glycol and remove any salt which of the residue is analyzed, the volume of the sodium chloride can form a hydrate or an addition compound with glycol. Neuformed on neutralization must be calculated and the result tralize the treated sample with dilute acid or alkali, using pkenolused in calculating the volume of pyridine solution in the phthalein as the indicator. Transfer the sample t o a pearshaped acetylation flask and distill slowly through a 3-bulb volumetric flask. The volume so calculated, divided by the Snyder column until tbe volume of liquid in the flask is reduced to size of ths aliquot acetylated, is then applied as a factbr to about 10 cc. Remove the thermometer and stopper at the top of the column . “ the expression used in calculating Per cent of glycol. If 8 dilute glycerol solution is being analyzed, the factor for the and add 50 cc. of pyridine, dried over sodium hydroxide. Continue the distillation slowly until the temperature rises to 110” C. glycerol equivalent to 1 cc. of N alkali-viz., 0.03069-shouId Rinse the Snyder column with approximately 10 cc. of pyridine. be used in place of 0.03103. To the acetylation flask containing the residue, or to another acetylation flask containing an aliquot of the residue after diluSummary tion t o 100 cc. with pyridine, add from a Lowy automatic pipet, which need not be calibrated, 25 cc. of approximately 2.6 N A method for the determination of glycol or glyceroI in pyridine-acetic anhydride reagent, always measured a t the same dilute solutions that may contain oxidizable impurities con’ , sists essentially in the removal of the water or other lowGLYCOL SOLUTIONS boiling solvent after the addition of pyridine, and subsequent TABLE111. ANALYSESOF SYNTHETIC acetylation of the residue. The method cannot be applied Experiment Glyool Analyst YO. Present Found in the presence of high-boiling impurities that can be acetyG./100cc. G./100 cc. lated. Since the acetylation reaction proceeds only to about 1 98 per cent of the stoichiometric value, a constant correction 2 3 must be applied in calculating the results. When applied to 4 synthetic mixtures, the average accuracy and precision of the 6 6 method are about 1 per cent. 7 8
9
Acknowledgment
10
Solution also contained 4% of HC1. This was neutralized before distillation waa begun. b Solution also contained HCl, ethylene dichloride, and chlorohydrin, The HC1 was neutralized before distillatlon w a ~begun, 6 Solution also contained 4% of CaClt and chlorohydfin. A calculated amount of sodium sulfate solution waa added and precipitated calcium sulfate was removed by filtration before distillation was begun. a
OF SYNTHETIC GLYCEROL SOLUTION TABLEIV. ANALYSES
Experiment No.
Glycerol ‘Present Grams
Glycerol Found Crams
The writer wishes to thank two of his colleagues, V. A. Aluise and W. R. Tomlinson, Jr., for their assistance in ,some of the analytical work.
Literature Cited (1) Freed, M., and Wynne, A. M., IND.ENG.CHEY.,Anal. Ed., 8, 278 (1936). (2) Marks, S., and Morrell, R. S., Analyst, 56, 428 (1931). (3) Meyer, Hans, “Nachweis und Bestimmung organischer Verbindungen,” p. 44, Berlin, Julius Springer, 1933. (4) Peterson, V. L., and West, E. S., J.Bioi. Chem., 74, 379 (1927). (5) Sherman, “Organic Analysis,” 2nd ed., p. 281, New York, Macmillan Go., 1917. (6) Verley, A., and Bolsing, Fr., Be?., 34, 3354 (1901). RECEIVED August 2, 1937.
