In Figure 6 the transmittance of bromothymol blue a t 6150 A. in pH 8.6 veronal buffer is plotted as a function of concentration.
fabrication of parts of the apparatus and electronic circuits. LITERATURE CITED
(1) Craig, R., Bartel, A,, Kirk, P. L., ACKNOWLEDGMENT
The authors are most grateful t o J. L. Oncley for his advice and encouragement, and to Frederick Gilchrist and Ross Schubarth for assistance in
Rev. Sci. Instr. 24, 49-52 (1953). (2) Holter, H., L@vtrup,S., Compt. rend, trav. lab. Carlsberg, SBr. chim. 27,
27 (1949). (3) Krueglis, E. J., Ibid., 27, 203 (1950). (4) Oldenberg, O., Broida, H. P., J. Opt. SOC.Am. 40, 381 (1950).
(5) SR-ift, H., Rasch, E., in “Physical Techniques in Biological Research,” Oster, G., Pollister, A. W., eds., Vol. 111, pp. 353-400, Academic Press, New York, 1956. RECEIVED for review August 12, 1957. Accepted October 11, 1957. WORKsupported by grants from the Milton Fund of Harvard University and the U. S. Public Health Service (H-2440); and b a fellowship of the American Cancer Sbciety to D. F. H. W.
Determination of Trace Amounts of Total Nitrogen in Petroleu m Distillates by Adsorption Improved Modification SIR: Since publication of the original paper on “Determination of Trace Amounts of Total Kitrogen in Petroleum Distillates” [ANAL. CHEM. 29, 177 (1957)l by Bond and Harriz, several analysts have reported difficulty in controlling violent bumping during the Kjeldahl digestion, with occasional breakage of the flask. This difficulty has been investigated further and traced largely to the fact that some operators fail to cut the silica gel column into lengths of less than 1 inch (preferably 0.75 inch) as specified in the method. Longer sections tend to bridge across
the bottom curvature of the flask and bumping then occurs, whereas with the shorter lengths digestion proceeds smoothly. These observations have also led to a simplification of the procedure for blank determinations. Early attempts to perform a blank determination on the loose gel were unsuccessful because of bumping, necessitating the use of a magnetic stirrer. The gel for the blank is now placed in a regular adsorption column and then wetted with about 5 ml. of nitrogen-free iso-octane or other suitable hydrocarbon previously percolated
through silica gel. This technique completely eliminates the need for magnetic stirring during digestion. The usual blanks of 0.3 to 0.6 ml. of 0.01N acid are equivalent t o 0.1 to 0.3 p.p.m. of nitrogen for 500 ml. of the typical naphthas tested. The authors wish to thank John C. Tomlinson, of these laboratories, for his suggestions for improvements to the method. GEORGE R. BOND,JR. CLIFFORD G. HARRIZ Houdry Process Corp. Marcus Hook, Pa.
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Volumetric Determination of Potassium in t-ertilizers Modifications and Comments SIR: Schall (2) recently described a volumetric method for determining potassium in fertilizers. The method consists of precipitation of potassium with sodium tetraphenylborate, removal of the insoluble salt, and titration of excess tetraphenylborate with standard quaternary ammonium salt solution, using bromophenol blue as indicator. The method, because of its speed and relative ease as compared with other potassium methods ( I ) , has stimulated considerable interest among fertilizer chemists; however, as with any new method, minor modifications such as indicator and strength of solutions warrant investigation. In our laboratory, 1882
ANALYTICAL CHEMISTRY
the following refinements and modifications have been made: Dilute tetraphenylborate solution to one half recommended strength. No dilution of quaternary ammonium salt is needed. Add 7-ml. excess of tetraphenylborate solution instead of 2 ml. Use a 5-ml. microburet instead of the recommended 10-ml. semimicroburet. Carry out precipitation in 100-ml. rather than 50-ml. volumetric flasks, with no increase in complexing reagents. Take a 50-ml. aliquot portion for titration. Where sample contains more than 50% KzO, use one-half sample weight (1.25 grams). This is in lieu of doubling
the flask size, which would be from 100 to 200 ml. for us, or from 50 to 100 ml. in the original procedure. Bromophenol blue, although an acceptable indicator, does not give an easily detectable end point. Fertilizer mixtures containing large amounts of natural organic materials and so-called minor or secondary elements often give dark-colored solutions in which the end point is partially obscured. An investigation of indicators has shown that a 0.04’% solution of Clayton Yellow (C.I. 813), also called Titan or Mimosa yellow, in distilled water gives a sharp color change from pale yellow to intense pink. In the presence of
