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Some comments on the mercurimetric determination of iodine
MICROCHEMICAL
JOURNAL
32, 123-124 (1985)
LETTERS
TO THE EDITOR
Some Comments on the Mercurimetric Determination of Iodine Sir: The determination of halogen in organic compounds by oxygen-flask combustion and subsequent mercurimetric titration is widely practiced. In its original form (I, 2), reduction of the halides by hydrogen peroxide, it was restricted to chlorine and bromine since the reduction of iodine by hydrogen peroxide is incomplete in the time allowed. Later it was found that hydrazine reduces all three halogens completely and on that basis a unified method was proposed (3). Unfortunately, this method requires boiling with H,O, and subsequent cooling, which greatly diminishes its simplicity and elegance. In our laboratory, this step was once forgotten; the result nevertheless was good. A brief investigation revealed that, without the boiling step, results were good only when relatively large amounts of iodine were present. For small amounts and blanks the endpoint was uncertain and late. Because the purpose of the boiling step is the destruction of excess hydrazine, we concluded that hydrazine could not interfere with the titration if its concentration was low enough, and that it might be possible to remove most of it by reaction with carbonyl. Acetone had no effect but the addition of formaldehyde resulted in sharp endpoints and the usual blanks (0.15 ml). EXPERIMENTAL
Reagents: Saturated solution of hydrazine sulfate in water; formaldehyde, 37% solution in water, stabilized with methanol; 0.05% bromphenol blue in ethanol; 1.5% sym-diphenylcarbazone in ethanol; 0.01 N mercuric nitrate: 1.7 g Hg(NO,), * H,O in 500 ml water containing 2 ml coned HNO,. Bring to 1 liter, standardize against NaCl. Charge the flask with 2 ml 0.5 N KOH, 10 ml water, and 4 drops (approx 0.2 ml) hydrazine sulfate solution. Flush with oxygen for 1 min. After combustion shake briefly and let sit for 20 min. Rinse stopper and flask with 5 to 10 ml water and add 2 ml of the formaldehyde solution. Follow with 100 ml 95% ethanol and 2 drops of bromphenol blue. Titrate with 0.1 N nitric acid until yellow and add 0.5 ml excess. Add 8 drops diphenylcarbazone and titrate with mercuric nitrate to pale violet. RESULTS
AND DISCUSSION
o-Iodobenzyl alcohol (Aldrich, 54.22% I) was first analyzed by the method given in Ref. (3). Ten samples of 3 to 12 mg gave a mean of 54.12% and a standard deviation of 0.53%. Then 22 samples of 4 to 14 mg were analyzed with addition 123 0026-265X/85 $1.50 Copyright All rights
0 1985 by Academic Press, Inc. of reproduction in any form reserved.
124
LETTERSTOTHEEDITOR
of formaldehyde instead of the boiling step. Here the mean was 54.19% and the standard deviation 0.34%. The difference in standard deviations is statistically significant (F test, 95%). We used a lo-ml buret with relatively large drop size and graduation only to 0.05 ml. We are certain that the use of a proper microburet with submersed tip and graduation to 0.01 ml would improve both standard deviations quite considerably. In order to check the application of the new method to other halides, 10 samples (3.9 to 11.6 mg) of p-chloroacetanilide (20.90% Cl) were analyzed with addition of formaldehyde instead of boiling with H,O,. Here the mean was 20.96% and the standard deviation was 0.16%. We also did some experiments on interrupting the procedure. This can be done up to and including the addition of formaldehyde. Long standing after the addition of alcohol brings back large blanks and poor endpoints. REFERENCES 1. Cheng, E W., A rapid microdetermination of halogen in organic compounds. Microchem. J. 3, 537-542 (1959). 2. Cheng, E W., Simplification of the mercurimetric procedure for organo-halogen determination. Microchem. J. 21, 125-128 (1976). 3. Lalancette, R. A., Lukaszewski, D. M., and Steyermark, A., Determination of iodine in organic compounds employing oxygen flask combustion and mercurimetric titration. Microchem. J. 17, 665-669 (1972).
E KASLER
R. JALKIAN Departmenr of Chemisrty University of Maryland College Park, Maryland 20742 Received February 1, 1985; accepted March 27, 1985
JOURNAL
32, 123-124 (1985)
LETTERS
TO THE EDITOR
Some Comments on the Mercurimetric Determination of Iodine Sir: The determination of halogen in organic compounds by oxygen-flask combustion and subsequent mercurimetric titration is widely practiced. In its original form (I, 2), reduction of the halides by hydrogen peroxide, it was restricted to chlorine and bromine since the reduction of iodine by hydrogen peroxide is incomplete in the time allowed. Later it was found that hydrazine reduces all three halogens completely and on that basis a unified method was proposed (3). Unfortunately, this method requires boiling with H,O, and subsequent cooling, which greatly diminishes its simplicity and elegance. In our laboratory, this step was once forgotten; the result nevertheless was good. A brief investigation revealed that, without the boiling step, results were good only when relatively large amounts of iodine were present. For small amounts and blanks the endpoint was uncertain and late. Because the purpose of the boiling step is the destruction of excess hydrazine, we concluded that hydrazine could not interfere with the titration if its concentration was low enough, and that it might be possible to remove most of it by reaction with carbonyl. Acetone had no effect but the addition of formaldehyde resulted in sharp endpoints and the usual blanks (0.15 ml). EXPERIMENTAL
Reagents: Saturated solution of hydrazine sulfate in water; formaldehyde, 37% solution in water, stabilized with methanol; 0.05% bromphenol blue in ethanol; 1.5% sym-diphenylcarbazone in ethanol; 0.01 N mercuric nitrate: 1.7 g Hg(NO,), * H,O in 500 ml water containing 2 ml coned HNO,. Bring to 1 liter, standardize against NaCl. Charge the flask with 2 ml 0.5 N KOH, 10 ml water, and 4 drops (approx 0.2 ml) hydrazine sulfate solution. Flush with oxygen for 1 min. After combustion shake briefly and let sit for 20 min. Rinse stopper and flask with 5 to 10 ml water and add 2 ml of the formaldehyde solution. Follow with 100 ml 95% ethanol and 2 drops of bromphenol blue. Titrate with 0.1 N nitric acid until yellow and add 0.5 ml excess. Add 8 drops diphenylcarbazone and titrate with mercuric nitrate to pale violet. RESULTS
AND DISCUSSION
o-Iodobenzyl alcohol (Aldrich, 54.22% I) was first analyzed by the method given in Ref. (3). Ten samples of 3 to 12 mg gave a mean of 54.12% and a standard deviation of 0.53%. Then 22 samples of 4 to 14 mg were analyzed with addition 123 0026-265X/85 $1.50 Copyright All rights
0 1985 by Academic Press, Inc. of reproduction in any form reserved.
124
LETTERSTOTHEEDITOR
of formaldehyde instead of the boiling step. Here the mean was 54.19% and the standard deviation 0.34%. The difference in standard deviations is statistically significant (F test, 95%). We used a lo-ml buret with relatively large drop size and graduation only to 0.05 ml. We are certain that the use of a proper microburet with submersed tip and graduation to 0.01 ml would improve both standard deviations quite considerably. In order to check the application of the new method to other halides, 10 samples (3.9 to 11.6 mg) of p-chloroacetanilide (20.90% Cl) were analyzed with addition of formaldehyde instead of boiling with H,O,. Here the mean was 20.96% and the standard deviation was 0.16%. We also did some experiments on interrupting the procedure. This can be done up to and including the addition of formaldehyde. Long standing after the addition of alcohol brings back large blanks and poor endpoints. REFERENCES 1. Cheng, E W., A rapid microdetermination of halogen in organic compounds. Microchem. J. 3, 537-542 (1959). 2. Cheng, E W., Simplification of the mercurimetric procedure for organo-halogen determination. Microchem. J. 21, 125-128 (1976). 3. Lalancette, R. A., Lukaszewski, D. M., and Steyermark, A., Determination of iodine in organic compounds employing oxygen flask combustion and mercurimetric titration. Microchem. J. 17, 665-669 (1972).
E KASLER
R. JALKIAN Departmenr of Chemisrty University of Maryland College Park, Maryland 20742 Received February 1, 1985; accepted March 27, 1985