NOTE ON ANALYTICAL PROCEDURES Ground Starch as an Indicator in Iodometry H.A . CONNER AND R. W. BOVIK, Johnson Suture Corporation,
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H E useful life of the solution of soluble starch used as a conventional indicator in iodometry may be prolonged with any one of a number of disinfectants. Holyever, hydrolysis of the starch etentually renders the reagent unsuitable for use, and therefore it has been found advantageous in many cases to replace the solution with a dry preparation of ground starch, The starch is prepared by a procedure similar to that recommended by illsberg, Griffing, and Field ( I ) and by Schoch (3). -1 convenient quantity is suspended in about twice its weight of ethyl alcohol and ground in an efficient ball mill for at least 80 hours, at which time a microscopic examination will reveal few, if any, granules that have escaped disintegration. The ground starch is filtered off, dried, and then reground. The resulting preparation, which is almost completely soluble in cold water, is ready for use. An inexpensive salt or pepper shaker has been found to be a convenient container and dispenser for the starch. If desired, a solution of starch may be prepared without the use of heat by simply adding the ground starch to cold water. Presumably any available starch may be used, but soluble starch is particularly satisfactory. The time required for grind-
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Ill.
ing will depend on various factors, including the particular mill used, the charge, the kind of starch, and the starch-alcohol ratio. The preparation may be kept indefinitely without charge. Iodometric titrations are carried out in the usual manner, a small amount of the ground starch being added near the end point. Titers obtained with this indicator were checked against those obtained with a solution of soluble starch prepared according to the directions of Lange ( 2 ) . Aliquots of a potassium dichromate solution were titrated in the conventional manner with 0.01 N sodium thiosulfate.. An average of five titers using a soluble starch solution gave a value of 32.40 ml.; with ground starch the same average of 32.40 ml. was obtained. LITERATURE CITED
(1) Alsberg, C. L., Griffing, E. P., and Field, J., J . A m . Chem. Soe., 48, 1299-300 (1926). ( 2 ) Lange, N. A., "Handbook of Chemistry", 5th ed., p. 1171, Sandusky, Ohio, Handbook Publishers, 1944. (3) Schoch, T. J., Cereal Chem., 18, 121-8 (1941).
CORRESPONDENCE o-Cresol in Phenol SIR: We have subjected the "cloud-point" method of Seaman, ENG.CHEM.,ANAL.ED.,15, 159 (1943)l Korton, and Foley [IND. to brief examination and, while we found it an eminently practicable and useful procedure within the limits laid down, we also found that exact confirmation of results was not obtained. The root cause of the discrepancy was discovered in the figure observed for the "cloud-point temperature" of pure phenol and water when mixed in the prescribed proportion; this is quoted as 66.40" in the article, whereas we obtained a figure of 66.10' C. The phenol used in our short series of experiments had a 10111 point (Bell and Herty, Standardization of Tar Products :'Pat: Committee procedure) of 41.0" C. and the o-cresol had a cloud point of 31.0" C. The results of our tests are given here.
SIR: Regarding discrepancy between the cloud point of 66.40" C. for pure phenol and water which we reported [IND. ENG.CHEM.,ANAL.ED., 15, 159 (1943)] and the figure which Kay and Haywood have obtained, 66.10" C.: We have repeated some of the experiments and think that we have found the explanation for the discrepancy. Instead of one abrupt change in appearance, there are really two. We took the first one (at 66.40' C.) and evidently they must have taken the second one (at 66.10' C.). I t is impossible to describe these points in words, but after one has observed both, one can always detect either point with good precision. Three of us have checked each other quite well at both points. Furthermore, there is a similar difference when determinations are made with phenol containing added o-cresol, although we have not done sufficient Observed o-Cresol Calculated Cloud-Point o-Cresol in Mixture by Equations Given work to know whether the difference will remain constant or not Temperature with Phenol Below over a range of concentrations. I t may be that a variation i n e. % 7c .. the magnitude of the differences with a variation in o-cresol con66.10 0.00 0.00 07.96 1.45 1.45 centration may explain the fact that the slopes of the lines cal70.40 3.39 3.3Y 73.00 5.52 5.52 culated from the four determinations which were sent us differ from those which we reported (1.273 and 1.222 instead of 1.326 I t was found necessary, in order to bring observed and caland 1.167, respectively). culated o-cresol contents into line, to modify the equations given I t would seem likely that either point can be used, provided in the article as follows: that the analyst constructs his curve on the basis of that point; For cloud-point temperatures up to 70.25' C.: but since we have been able consistently to get reproducible cloud point (" C.) - 66.10 o-cresol = values with the point which we had chosen, it might be desirable (1) 1.273 t o have the analyst look for that point, in which case he could For cloud-point temperatures 70.25' to 73.5' C. : use our equations, rather than to use the other point and have to repeat the large number of determinations which would be Too-cresol = cloud point (' C.) - 66.25 (2) 1.222 necessary to establish accurate equations. J. KAYAND P. J. C. HAYROOD Wu. SEAMAN t
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ALMORA 24 BEECELEV ROAD WREXHAM DENBIQHUHIRE GREATBRITAIN
CALCO CHEMICAL DIVIEION -4MERICAN CYANAMID C O M P A N Y BOUNDBROOK, N. J .
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