NOVEMBER 15. 1940
657
ANALYTICAL EDITION
before or after the actual analysis, since temperature and capillary constants have a marked effect on the diffusion current. I n this %ray careful thermostating and differences due to different capillaries can be eliminated. A comparison between chemical and polarographic analyses of lead, zinc, and cobalt is given in Table I.
Acknowledgment The authors wish to thank JJ-. c. pierceof tile University Chicago for his assistance.
of
Literature Cited (1) Kolthoff, I.
M.,and
Lingane, J. J . , ('hem. Rei.. 24, 1 (1939).
Effect of Selenium on the ICjeldahl Digestion R. B. BRADSTREET 17 Hamilton Ave., Cranford, N. J.
T
HE use of selenium as a catalyst in the Kjeldahl digestion \vas first proposed by Lauro ( 4 ) in 1031. Since then, many investigators have reported on its use, alone or in combination with other catalysts. Poe and Salder ( 6 ) , comparing the digestion times of various catalysts and combinations of catalysts, found that a mixture of selenium, copper sulfate, and mercuric oxide was a rapid and efficient catalyst, and further that there was no loss of nitrogen. On the other hand, Davis and Kise ( 2 ) state that the use of selenium is not to be recommended. Other workers (5, 7 , 8) have also noted a loss of nitrogen. The writer feels that this is unfortunate, since selenium is a n efficient catalyst, and possesses the advantage of not forming a complex with ammonia, as does mercury.
TABLE I. EFFECTO F Selenium used, grams
SELESIUM O S S I T R O G E X D E T E R X I S A T I O K
0.10
R Catalyst Se Se Se Se
Se Se
0.25 c;
0.50
0 75
1.00
1.25
5
s
53
x
++ FeSOd.7Hz0 CuSO4.5HzO
10 00 9.92 10.11
9.78 9.07 9.87
++ CuSOa.5HzO FrSOr.7H20
-Anthranilic Acid (10.21% 5 2 ) 10.25 10.25 9.99 9.97 9.84 10.25a 10.25 9 . 9 9 R ;i9 9.23 10.24b 10.26 10.05 10.04 9.18
9.91 9.95 9.82
( 2 0 . 3 0 5 Xd-
,--p-Xitroaniline
Se Se Se Se
Se
Se 0
b
Nd-
-Acetanilide (10.36% 1 0 . 4 2 10.34 10.00 1 0 . 0 7 10.37Q 10 31 10.25 10.10 10.36b 10.26 10.07 10.10
10.10
19.98
19.89 1 9 . 9 s 19.25 20.02 20.26 2 0 . 0 4 20.30 20.13 20.64 19.87 20.07 20.27
++ FeS01.7H20 CuS04.5H10
20.285 20.26 20.27b 2 0 . 2 7
FeS01.7Hz0 ++ CuS04.5Hz0
-m-Dinitrobenzene 1 6 . 6 4 16.66 16.57 16.675 16.64 16.36 i 6 . 6 5 b 16.64 16.16
Catalyst, 0.1 gram Se Catalyst, 0 . 1 gram Se
(16.687, n'd16 37 16.65 16.12 16.37 16.22 1 6 . 3 4 14.62 16.19 18.60
++ 0.1 gram FeS01.7H20 0 . 1 gram CuSOr.5H20
The customary amount of selenium used in digestions varies between 0.1 and 0.25 gram. It is proposed in this paper to ascertain the limiting amounts of selenium alone and in combination, beyond which a loss of nitrogen will occur. Conditions, as far as possible, were standardized to minimize all errors. Gas burners were used, and the flames carefully regulated. Very little difference in clearing time was noted, and all samples were given 1 hour afterboil. Four organic compounds, representing four different combinations of nitrogen were used : acetanilide, anthranilic acid, p-nitroaniline, and m-dinitrobenzene.
Procedure Appropriate samples were n-eighed into Kjeldahl flasks, and 35 ml. of concentrated sulfuric acid containing 35 grams of salicylic acid per liter were added and allowed to stand in the cold for 10 minutes, after which 6 grams of anhydrous sodium thiosulfate were added to each flask. After the reaction had subsided, gentle heat was applied until the mixture blackened. The
flasks were cooled, 10 grams of potassium sulfate containing varying amounts of catalyst were added, and vigorous heat was applied. After the digestion mixture had cleared, the heat was reduced so that the contents of the flasks boiled gently, and only a small amount of sulfuric acid vapor escaped. The flasks were then cooled and 126 ml. of distilled water added. To the diluted and cooled digest, 165 ml. of 35 per cent sodium hydroxide were carefully added, so that two distinct layers were formed. A small piece of lox-melting paraffin and several pieces of mossy zinc were added and the flasks were connected to the distillation rack. Davisson (3) distilling bulbs were used. The flasks were sxirled gently to mix the two layers and heat was applied. Distillation was continued for 1 hour after boiling started, and the distillate was collected in 500-ml. Erlenmeyer flasks containin 50 ml. of distilled water, 25 ml. of 0.1 N hydrochloric acid, a n t 4 drops of a 0.1 per cent solution of methyl red. At the end of the distillation, the flasks were disconnected and the condensers and delivery tubes carefully washed out with distilled n-ater. Titration of the distillate was made with carbonate-free 0.1 N sodium hydroxide. Blank determinations were run and suitable corrections applied. Three different catalysts were used : selenium alone, selenium and ferrous sulfate ( I ) , and selenium and copper sulfate. I n the second and third cases the amounts of ferrous sulfate and copper sulfate ranged from 0.10 to 0.25 gram, whereas the selenium was varied from 0.25 to 1.25 grams. Selenium alone ranged from 0.1 to 1.25 grams. I n the det'ermination of nitro compounds by the Kjeldahl method, reducing agents are necessary to transform nitro nitrogen to amino nitrogen which, in turn, is easily converted to ammonium sulfate. Two of the compounds used in this investigation contain nitro nitrogen. and for this reason salicylic acid and sodium thiosulfate were used in the digestion mixture. Table I shows the results of numerous digestions. A survey of these results shows that with an increase of selenium there appears to be a loss of nitrogen. T h a t this follows in no direct ratio to the amount of selenium used is also evident. The limiting amount of selenium is 0.25 gram, whether alone or in combination with ferrous sulfate or copper sulfate. Even with 0.25 gram of p-nitroaniline, however, selenium alone gave lo^ results. It may be concluded from these results that t'he quantity of selenium used as a catalyst should not exceed 0.25 gram. This limit also applies to combinat'ions with other catalysts, such as ferrous sulfat'e or copper sulfate.
Literature Cited (1) (2) (3) (4)
(5) (6)
(7) (8)
Bradstreet, R. B . , ISD. ENG.Cmxr., Anal. E d . , 10, 696 (1938). Davis, C . F., and Wise, hl., Cereal Chem., 10, 488-93 (1933). Davisson. B. S., J. ISD.ESG. CHEM.,11, 465 (1919). Lauro, M. F., I b i d . , Anal. E d . , 3, 401-2 (1931). Osborn, R. A , a n d Krasnite, -1.. J . Assoc. Oficial Agr. Chem., 17, 3 3 9 4 2 (1934). Poe, C . F., and Nalder, hl. E., IND.ENG.CHEM.,Anal. E d . , 7, 189 (1935). Sandstedt, R. M.. Cereal Cium., 9, 15&7 (1932). Snider, S. R., a n d Coleman, D. 4.,Ibid., 11, 414-30 (1934).
