Anhydrous Copper Sulfate in the Kjeldahl Nitrogen Determination CROOM BEATTY 111 Oberlin College, Oberlin, Ohio
T
HE Association of Official Agricultural Chemists (1) recommends the use of crystallized copper sulfate (pentahydrate) in the official Kjeldahl-Gunning-Arnold method for the determination of organic nitrogen. Bradstreet (2) in a survey of the Kjeldahl method of analysis mentions the use of copper sulfate as a catalyst for the Kjeldahl digestion, but in every case this is the copper sulfate pentahydrate. Niederl and Niederl(3) recommend, for the micro-Kjeldahl technique, the use of 1 part of potassium sulfate, 3 parts of copper sulfate pentahydrate, and a small amount of selenium. This latter procedure, somewhat modified for semimicroquantities, was used for routine determinations of nitrogen in organic compounds. It was found, however, that if the copper sulfate pentahydrate were replaced by anhydrous copper sulfate several advantages were apparent. The anhydrous copper sulfate did not dissolve completely in the sulfuric acid (unlike the pentahydrate) and the fine crystals served as “boiling stones”, allowing a much more rapid and smooth digestion, free from all bumping. The size of the
flame used was of less importance, since there was no tendency to bump. Copper sulfate as a catalyst is likewise useful in the distillation of the ammonia as an internal indicator, since the blue color disappears when the solution is made basic, while black cupric oxide is formed. The semimicromethod of analysis used calls for a sample size of 20 to 35 mg., to which are added 8 ml. of concentrated sulfuric acid, 2 grams of copper sulfate-potassium sulfate mixture and about 20 mg. of powdered selenium. The mixture is digested for 15 to 25 minutes. d finely ground mixture of 1 part of anhydrous copper sulfate and 2 parts of potassium sulfate, with a small amount of selenium is used as catalyst in the semimicro-Kjeldahl analyses run in this laboratory.
Literature Cited (1) Assoc. Official Agr. Chem., Official and Tentative Methods of
Analysis, 4th ed., 11, p. 25, 1935. (2) Bradstreet, R. B., Chem. Rev., 27, 331 (1940). (3) Niederl and Niederl, “Organic Quantitative Microanalysis”, New York, John Wiley & Sons co., 1938.
Recommended Specifications for Microchemical Apparatus Correction in the Design of the Dumas Nitrogen Stopcock
cT-v
C
R
I/
FIGURE 1.
FIGURE 2.
INCOHRECT
I
T HAS been called t o our attention that the design of the stopcock published in our previous report [IND. ENG.CHEM., ANAL. ED., 13, 580 (1941)l is incorrect, in that it reduces the safe zone of possible leakage from one fourth of the circumference t o one eighth or less. To overcome this, we suggest that the stopcock be designed as shown in Figure 2 instead of the previous incorrect procedure shown in Figure 1. I n the incorrect view, the groove on the lower end opening of the straight bore is directed toward the opening of the bent out476
/I(1,
CORRECT
let bore. In the correct form, Figure 2, the direction of the groove on the lower end of the straight bore must be away from the opening of the bent bore, so that a safe zone of at least one fourth of the circumference of the stopcock plug remains. The groove of the bent bore may remain as specified. G. L. ROYER,Chairman H. K. ALBER L. T. HALLETT J. A. KCCK,Secretary
