ANALYTICAL CHEMISTRY
838 11.38%), after 1.5-hour digestions, in 39 trials yielded an average of 10.70y0 of nitrogen (94%), with maximum of 11.39% and minimum of 9.27%. For 6-methoxysalicylaldazine ( N = 8.98%) five trials, involving 2.5-hour digestions, gave an average of 9.03%, with maximum 9.15% and minimum 8.9070. Some of the low results may be attributed to loss caused by irregular but sometimes violent bumping during the reductions. Sodium and Methanol. Nine analyses of nicotinic acid, R-ith 30-minute digestions, yielded results all of wh,ich are acceptable, averaging 11.247, (maximum, 11.487,; minimum, 11,0470); adenine treated in the same way liberated ammonia during reduction. For general use a trap to retain volatilized ammonia would be needed, but a selective determination of nitrogen so liberated and total nitrogen might be possible. Electrolytic Reduction of nicotinic acid, in 80% sulfuric acid, using platinum electrodes and a current of 0.7 ampere a t 4 volts, followed by digestions for 30 to 90 minutes, yielded in three trials one good result (11.41%) and tn-o Ion. results (10.82,10.52).
case of basic stannic sulfate, it is further indicated that the extent of coprecipitation is negligible, once supersaturation is relieved, until one of the ions constituting the carrier has been quantitatively removed from the liquid phase. The complete results of these investigations will be reported to ANALYTICAL CHEMISTRY a t a later date. ACKNOWLEDGMENT
This investigation is supported in part by grants from the Atomic Energy Commission and the Research Corp. LITERATURE CITED
(1) Elving, P. J., and Van Atta, R. E., AXIL. CHEM.,22, 1375
(1950). (2) Gordon, L., Ibid.,24, 459 (1952). (3) Wagner, W.F., and Wuellner, J. A , Ibid.,24, 1031 (1952). (4) Wahl, A. C., and Bonner, N. A . , "Radioactivity Applied to Chemistry," New York, John Wiley 8r Sons, 1951. (5) Willard, H. H., AXAL.CHEX.,22, 1372 (1950). (6) Willard, H. H., and Gordon, L., Ibid.,25, l i 0 (1953).
This exploratory study is reported thus incomplete because none of the authors will be able to continue it, and in the hope that someone else may be inclined to do so. Grateful acknowledgment is made to the Committee for the Advancement of Research of the University of Pennsylvania for a grant to support the study, made during the summer of 1951.
LOGISGORDOK DEPARTMEXT OF CHEMISTRY CARL.C REIVER SYRACUSE UXIVERSITY HARRY TEICHER SYRACUSE 10, S . Y. RECEIVED for review March 9, 1953. Accepted April 1, 1953.
*
LITERATURE CITED
(1) Grunbaum, Schaffer, and Kirk, ANAL.CHEX,24,1487 (1952). (2) Ogg and Willets, J . Assoc. Ofic.Agr. Chemists, 24, 641 (1941). (3) Wagner, IXD. ENG.CHEM.,S l v . 4 ~ ED., . 12, 771 (1940). (4) White and Long, ASAL. CHEM.,23, 363 (1951).
SARAH AI. ROODS DEPARTMEXT CHEMISTRY DAVIDSCHEIRER PEXSSYL~ANIA E. C. WAGNER PHILADELPHIA, PA. RECEIVED December 18, 1952. Accepted March 30, 1953. OF ~ N I V E R B I T YOF
Coprecipitation from Homogeneous Solution laboratory is utilizing precipitation from homogeneous Tsolution . ( 2 , 6) to study the distribution of foreign ions beHIS
tween the precipitate and liquid phases. Coprecipitation is being studied as a function of the fraction of carrier precipitated; this is similar to the fractional crystallization techniques utilized in studies of the homogeneous and heterogeneous distribution laws as described by Wahl and Bonner (4). This communication describes the preliminary results obtained in the coprecipitation of manganese(I1) by basic stannic sulfate (6) and of strontium by barium sulfate (1, 3). I t has been found, as would be normally expected, that the extent of coprecipitation of manganese( 11) on basic stannic sulfate precipitated by the urea method is increased with increasing rate of precipitation. However, a t a given rate, except in the case of a very slow precipitation rate, much of the foreign ion is coprecipitated during the separation of approximately 25y0 of the total carrier present. Only a negligible amount of the foreign ion is then further coprecipitated as the remainder of the carrier is precipitated up to .the point where approximately 99.9% of the total initially present has been separated. This latter observation is in accord with the Paneth-Fajans-Hahn rule. After complete precipitation has occurred, the adsorption of foreign ions by the carrier becomes appreciable and causes a sharp rise in the total amount of coprecipitation. I n the case of coprecipitation on barium sulfate precipitated a t a slow rate with dimethyl sulfate ( I ) , the quantity of strontium found with the initial fraction of precipitated carrier was sharply reduced Lvhen vigorous stirring was employed. This is not the case with the sulfamic acid procedure (3). These experiments indicate, except possibly in the case of (S), the presence of a supersaturated condition during the early stages of a slow precipitation process, as is provided by the technique of precipitation from homogeneous solution. I n the
Precautions in the Determination of Lead in Biological Material by Diphenylcarbazide SIR: Experience during the past decade Rith the method for determination of lead in body fluids described by Letonoff and Reinhold (1, 2 ) has indicated that occasional analyses were unsatisfactory. K i t h few exceptions, one of three causes has been responsible. Losses of precipitate may result from the use of centrifuge tubes that are asymmetrical or unsuitable for other reasons. However, proper tubes have been made available recently especially for this and similar methods by the Corning Glass Forks. A second cause of low results has been traced to the use of animonium acetate wash solution of p H below 7.0. Elevated content of carbon dioxide in distilled water, carbon dioxide uptake by the wash solution on standing, or excessively acid preparations of ammonium acetate have been responsible. A modified viash solution to replace the 0.470 ammonium acetate solution of the original method is prepared by dissolving sufficient ammonium acetate (of low lead content) to make a solution approximately 1% in water containing 1.5 ml. of 0.1 S ammonium hydroxide per 100 ml. This solution is filtered through Whatman S o . 42 paper, stored in a refrigerator, and used at a temperature of about 10" C. Each tube is nearly filled by using 15 ml. of wash solution. If a refrigerated centrifuge is available, a significant gain in precision is obtained by centrifugation at 5" C. Addition of l5Y0 acetic acid for pH adjustment prior to precipitation seldom is required with chemicals no\T available. The third and most important source of error has been contamination, not so much xith lead from extraneous sources as with chromate residues. Special care should be taken to remove these thoroughly from centrifuge tubes, stirring rods, stoppers, etc., before proceeding 1% ith color development. After the mother liquor from the precipitate has been decanted and dis-
