INDUSTRIAL ASD ENGINEERISG CHELIISTRY
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reagent, and taking 0.4 ml. of the supernatant liquid for transmission analysis. The resulting transmission percentage can be compared with the standard curve, and the result multiplied by the number of milliliters of reagent used.
standard curve which may be used for the determination of a wider range of concentrations. The concentration of sodium should be relatively constant, but variations of * 10 per cent or so in the concentration will have little effect on the accuracy of the determination. An example of the use of such a curve is in the determination of potassium in blood plasma, in which the sodium concentration seldom varies more than 10 per cent from the normal values.
Aclinowledgment
Application to Larger Quantities
The author would like to thank Maurice B. Tisscher, head of the Department of Physiology, for his constructive suggestions and aid in preparing this paper. This work was made possible by a grant from the Rockefeller Foundation.
The method outlined above can be used for the determination of larger quantities Of potassium. Quantities as large as 80 mg. may be determined by using larger quantities of the
(1) Kolthoff, I. h l , and Bendix, G . H., IND. ENG.C H E v . , A i n a l .E d , 11, 94 (1939).
Literature Cited
Iodine in Thvroid J
Elimination of Uncertain End Point and Blank in the U. S. P. XI Assay R . S. BURSETT AND R. F. WARKOW, Laboratories of the Cudahy Packing Company, Omaha, Sebr.
I
S COhlNOT with other laboratories the authors had
difficulty several years ago obtaining consistent results in determining iodine in thyroid by the old U. S. P. X method. At that time the senior author found that simple pH adjustment of the solution to be titrated eliminated the indefinite end point and no blank was obtained on the reagents. Mter the U. S. P. XI method was available, this refinement greatly improved its accuracy and precision also. The work of Beal and Szalkowski ( I ) , upon which the present U. s. P. XI method is based, improved upoii the older method considerably. They devoted most of their efforts t o keeping the chlorate content of the solution low. This was clone by preparing hypochlorite from bleaching powder and sodium carbonate and by controlling the acidity with the use of definite amounts of hypochlorite and acid. I n so doing they obtained a solution with a higher p H than \\-as ordinarily obtained by the U.S. P. X method. This higher pH and cooling the solution to a definite temperature before titrating were also important factors in their improved procedure. However, by the present U. S.P. XI method, as Beal and Szalkowski (1) point out, “a blank of 0.4 to 0.6 cc. of thiosulfate will be regularly obtained by the experienced operator. . . . ITe believe that if the solution does not s h o ~a return of the blue color until after a t least 30 seconds the true end point has been reached.” Hilty and Wilson (3) recently proposed a new procedure based on cerate oxidimetry and expressed the opinion that in-
consistent results obtained by the present U. S. P. XI method are due to technical difficulties in eliminating excess chlorine. Recent’ difficulties in checking with other laboratories and their eventual ability to check the authors’ results when the suggested modification was used lead the authors t o believe that a note should be published regarding this refinement. If the uncertain end point and blank can be eliminated, there is an advantage in retaining the present method because of the recognized sharpness of the &arch-iodine end point. This modified U.S. P. XI thyroid method has recently been found more accurate and less time-consuming than existing procedures for determining iodine in mineral feeds for livestock. It is, however, necessary to use a 5.0-gram sample and to remove the insoluble material present by filtration after the soluble material has been dissolved from the fused mineral feed sample.
Modified Method REAGESTS.These reagents are required in addition to those given in the U. S. P. XI assay for iodine in thyroid. Thymol blue, 0.4 per cent. One gram of thymol blue powder is ground in an agate mortar with 21.5 cc. of 0.1 AT sodium hydroxide plus enough Tvater t o dissolve the dye and is finally diluted to 250 cc. This is ten times the concentration of the ordinary Clark and Lubs indicator solutions. Sodium hydroxide, approrimately 50 per cent solution. PROCEDURE. The procedure is the same as the present U. S. P. XI assay for iodine in thyroid up to the addition of the potassium iodide. Before it is added, the pH of the solution is
PH T ~ B LI.E EFFECTOF T.ARYING (Comparison of results obtained on 1.0 gram of powdered thyroid a t different hydrogen-ion concentrations of the solution titrated. used except for refinements mentioned.) Color of Solution Glass Elec- jY/200 X 1.046 (Thymol B l u e Used trod? p H Sodium for Indicator) Reading Thiosulfate B!ank Iodiue Procedure CC 24.6 25.1
cc . 0. 2
U. J. P. XI method
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