Dee,, 1920
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY
TABLEI-RESULTS O F ADDING EQUAL VOLUMES FURIC
ACID TO
OF CONCENTRATED SULSOLUTIONS O F NAPHTHALENE-1,6- AND 2,7-DISULFONIC
Acrns .-.
Concentration of Salt Solution Used G. per 100 cc.
2,7-Acid No precipitate Crystals No precipitate Crystals No precipitate 12.88 Crystals Crystals’ 1 The mixture showed an immediate slight turbidity. On standing for 40 hrs. there was formed a heavy, silky, crystalline product. 1.63
1,B-Acid Turbidity
3.22 6.44
SUMMARY
An alternative method of detecting naphthaleneand 1,6-disulfonic acids is given, which does not depend upon t h e use of t h e microscope. 2,7-
THE INFLUENCE OF POTASSIUM PERMANGANATE ON KJELDAHL NITROGEN DETERMINATIONS By Donald C. Cochrane INSTITUTE O F
ANIMALN U T R I T I O N ,
PENNSYLVANIA STATE COLLEGE, STATE COLLEGE, PA
Received August 17, 1920
I n a paper read before t h e Association of Official Agricultural Chemists a t t h e meeting of November 1915, Frear questioned t h e accuracy of t h e Kjeldahl method for nitrogen as commonly used in fertilizer analysis. The d a t a which Frear presented showed t h a t a large loss of nitrogen occurred in determinations on fertilizers when potassium permanganate was added; t h a t t h e time elapsing between cessation of boiling and t h e additipn of t h e permanganate influenred t h e results; and t h a t t h e presence or absence of nitrates made no appreciable difference in t h e loss occurring on t h e addition of t h e permanganate. It was therefore deemed advisable t o check t h e method upon substances commonly met with in nutrition work, paying particular attention t o t h e influence on t h e results of t h e addition or non-addition of potassium permanganate. TECHNIQUE
PERMANGANATE-weigh b y difference from a glass-stoppered weighing bottle directly into a 300-cc. Kjeldahl digestion flask about I g. of substance, and add 0.3 g. of metallic mercury and 3 0 cc. of concentrated sulfuric acid, rotating t h e flask during t h e addition of t h e acid in order t o wash down a n y particles of t h e sample adhering t o t h e neck of t h e flask. Heat cautiously over t h e naked flame of a Bunsen burner until frothing has ceased, then increase t h e temperature until vigorous boiling commences, and maintain a t this temperature until t h e solution becomes colorless, and for one hour thereafter. T h e flask should be shaken occasionally during digestion, which will require a total of about 2 . 5 hrs. Next, remove t h e flask from t h e flame and add in small portions sufficient potassium permanganate t o produce a constant pink or green color. If t h e permanganate is too finely powdered flashing will occur with a resultant loss of nitrogen. About 50 mesh permanganate gives t h e best results. It is important t h a t t h e permanganate be added immediately after removing t h e flask from t h e flame, not more t h a n I O sec. elapsing between t h e turning out of t h e flame and t h e completion of t h e addition of t h e permanganate. Allow t o cool, dilute with zoo cc. of water, let stand 15 METHOD I.
WITH P O T A S S I U M
min., and transfer from t h e Kjeldahl flask t o an Armsby copper pot containing 2 5 cc. of potassium sulfide solution (40 g. per liter), washing the flask with 2 jo cc. of water in three portions. Make strongly alkaline with saturated sodium hydroxide solution, inclining t h e pot so t h a t t h e hydroxide will flow t o t h e bottom without mixing, and connect a t once with t h e distillation apparatus. Shake vigorously t o insure thorough mixing. Collect t h e ammonia in standard sulfuric acid and complete t h e titration i n t h e usual manner. It is of t h e utmost importance t h a t t h e delivery tube dip into t h e standard acid until t h e first rush of ammonia is over; also t h a t t h e standard acid be stirred frequently in order t o prevent t h e lighter ammoniacarrying distillate from collecting on t h e top, thus causing loss of ammonia. M E TH 0D
2.
W I T H 0U T
P 0TA S S I U M
,
P E R M A N G A hTAT E
-The manipulation in this method is identical with Method I , except t h a t t h e permanganate is omitted. DISCUSSION
The results of nitrogen determinations on twentyfour samples are shown in Table I. The time of digestion in both methods was t h e same for each sample. Blanks were run on each lot of reagents and t h e nitrogen results were corrected for t h e blanks. I n every case except one (Oat Straw A) t h e average percentage of nitrogen is lower where permanganate was not added. The allowable difference between extremes on triplicate nitrogen determinations on feeds and feces in these laboratories is 0 . o j per cent. I n this series, however, all determinations are taken into consideration. Attention is directed t o the wide variations between extremes where t h e permanganate was omitted. I t is a well-recognized fact t h a t a loss of nitrogen occurs where flashing results from t h e addition of permanganate. If a loss of nitrogen occurs, as is stated by numerous authorities,l on addition of t h e permanganate even where flashing does not ensue, it is possible t h a t t h e result obtained is in reality a balance figure representing compensation between a loss due t o t h e permanganate and a gain through a more effective oxidation of t h e organic matter through t h e addition. Assuming such t o be t h e case, t h e discrepancy between t h e conclusions drawn from determinations on this class of substances and t h e work reported b y Frear on fertilizers may be explained. The possible presence of cyclic compounds containing nitrogen in t h e ring should not be overlooked.2 There is a wide difference of opinion amoqg analysts as t o t h e proper interpretation of “immediately’> as used by Kjeldahl3 in his original description of t h e method. It was found t o be impossible with t h e equipment at hand t o determine t h e rate of cooling of t h e sulfuric acid solution after removal of t h e flame. However, a series of determinations was made i n which the, variant was t h e time elapsing between cessation of boiling a n d t h e addition of permanganate.
e
1
Z. a i d . Chem., 22, 375; 67,
2
J . Biol. Chem., 17, 275. Z . 9h3’siol. Chem., 6 7 , 5 1 6 ; 69, 439.
524; 69, 439.
