T H E JOUR.VAL O F I N D C S T R I A L A S D E N G I N E E R I N G C H E M I S T R Y
914
COMPARISON OF THE KJELDAHL-GUNNING-ARNOLD METHOD WITH THE OFFICIAL KJELDAHL AND OFFICIAL GUNNING METHODS OF DETERMINING NITROGEN
plete oxidation. T h e reagents have been checked b y blanks b y all methods. - C o l u m n 2 gives t h e n u m ber of determinations which were m a d e on a typical sample of each of t h e substances indicated i n column I .
By T. C. T R E S C O T ~ Received September 3, 1913
It will be noted t h a t t h e results b y t h e KjeldahlGunning-Arnold method carried o u t with a one a n d one-half-hour period of digestion are higher t h a n those b y t h e Gunning method for three or four hours, excepting in t h e cases of cheese a n d flour, in which t h e y are practically t h e same. Compared with t h e results of t h e Kjeldahl method with three or four hours digestion t h e y are practically t h e same, excepting i n t h e cases of leather, leather waste, linseed meal, a n d
It has been brought t o t h e attention of t h e Nitrogen Laboratory of t h e Bureau of Chemistry t h a t t h e modified method for determining nitrogen, known as t h e Kjeldahl-Gunning-Arnold method,2 can be carried out i n a shorter t i m e t h a n t h e Gunning method or t h e Kjeldahl method a n d with as accurate results as either of t h e m . This method is identical with t h e official Kjeldahl method3 except t h a t I O grams of
DETERMINATION O F NITROGEN BY GUNNING,KJELDAHL, Gunning method
y01. j, NO. I I
AND
KJBLDAHL-GUNNING-ARNOLD METHODS
Kjeldahl method
Kjeldahl-Gunning-Arnold method
-L___
No. of analySUBSTANCE ses Max.
Min.
Av.
so.of analyses Max.
4 hours. Hair waste. . . . . . . . . . . . . Dried blood.. . . . . . . . . . . Bone meal.. . . . . . . . . . . . Leather. . . . . . . . . . . . . . . . Leather waste. . . . . . . . . . Cottonseed meal.. . . . . . . Linseed meal.. . . . . . . . . . Gelatin,. . . . . . . . . . . . . . . Cyanamide.
10 10 9 9 10 9 10
............
8 9
Beef extract(a). . . . . . . . . Desiccated meat(b). . . . .
3 3
Flour. . . . . . . . . . . . . . . . . . Bread . . . . . . . . . . . . . . . . . Gluten b r e a d . . . . . . . . . . . Macaroni, . . . . . . . . . . . . . Ground rye h a y . . . . . . . . Shorts. . . . . . . . . . . . . . . . . Bran. . . . . . . . . . . . . . . . . .
8 2 2 2 2 2 3
.........
2 3
Molasses feed.
Milk . . . . . . . . . . . . . . . . . Condensed milk. , . . , , .
.
Evaporated milk..
Cheese.
......
...............
2
2
2 3 3 3 3 3 3 3 3 3
9.09 8.93 3.26 6.18 7.69 6,i9 4.97 15.90 13.64 9.14 12.74
8.93 8.70 3.14 6.01 7.64 6.68
4.86 15.78 13.47 9.10 12.74
hlin.
Av.
No. of analyses Max.
4 hours. 9.01
8.80 3.17 6.07 7.67 6,74 4.94 15.87 13.56
6 10 10 10 8 9 8 8 7
...
3
12.74
Z
3 hours. 1.80 1.74 1.76 1.98 1.99 1.96 10.44 10.36 10.40 2.25 2.25 2.25 2.47 2.47 2.47 2.49 2.50 2.47 2.78 2.75 2.77 2.84 2.84 2.84 1.88 1.85 1.87 1.71 1.60 1.66 0.578 0,528 0.528 1.30 1.30 1.30 1.19 1.17 1.18 1.19 1.17 1.18 1.09 1.10 1.08 1.18 1.17 1.18 1.06 1.05 1.06 1.07 1.07 1.07 4 hours. 3.87 3.89 3.90 4.54 4.57 4.52 3.50 3.47 3.49
9.09 8.93 3.26 6.23 7.80 6.96 5.08 16.11 13.59 9.22 12.86
9
Min.
h'o. of analyAv. ses
1112 hours.
8.98 8.81 3.14 6.06 7.69 6.79 5.00 15.72 13.53
9.04 8.88 3.20 6.12 7.74
6.88 5.03 15.98 13.57
...
9.11 12.86
12.86
3 hours. 1.80 1.74
1.76
9 10 9 9 9 9 10
8
5.01 16.06
9.03 8.90 3.19 6.18 7.79 6.88 5.08 16.08
3
21,/2 hours. 13.59 13.59
13.59
3 3
2 hours. 9.27 9.18 12.91 12.86
12.89
11 ' 2 hours. 1.77 1.74 2.08 2.05 10.58 10.58 2.27 2.25 2.58 2.53 2.53 2.50 2.81 2.75 2.92 2.89 1.94 1.94 1.77 1.74 0.550 0.547 1.33 1.29 1.25 1.22 1.24 1.19 1.10 1.09 1.17 1.14 1.08 1.07 1.08 1.08
l.i6 2.07 10.58 2.26 2.55 2.52 2.79 2.91 1.94 1.75 0.549 1.31 1 23 1.22 1.10 1.15 1.08 1.08
2 2 2 3 2
2 3 2 3 3 3 3 3 2 3
6.85
Min.
.4v.
4 hours.
9.09 8.98 3.26 6.26 7.80 6.96 5.14 16.17
9 2 2
8.98 8.81 3.14 6.12 7.75
Max.
...
IO 10 8 9 9 IO 10
8
9.09 8.93 3.26 6.23 7.80 6.96 5.14 16.17
8.87 8.81 3.14 6.06 7.75 6.85 5.02 16.06
8.98 8.89 3.21 6.16 7.77 6.91 5.07 116.10
7
13.64
13.59
13.61
3 3
3 hours. 9.27 9.18 12.91 12.86
12.89
9
1.80
1.74
... 1.77
3.87 3 3.93 3.83 4.58 3 4.60 4.56 3 3.49 3.47 3.48 ( a ) The maximum and minimum of 3 analyses for 4112 hours by the Kjeldahl-Gunning-Arnold method were 9.27 and 9.18, respectively. ( b ) Two analyses for 41/2 hours b y the Kjeldahl-Gunning-Arnold method gave 12.91 a n d 12.86, average 12.89.
crystallized potassium sulfate are added a s i n t h e Gunning method, a n d potassium permanganate is omitted. Investigations were carried o u t on a variety of substances i n order t o compare more fully this modification with t h e s t a n d a r d Gunning a n d Kjeldahl methods. T h e following table gives t h e maximum, minimum, a n d average percentages of nitrogen found i n various substances b y means of t h e different methods, employing t h e periods of digestion which t h e experience of this laboratory indicates are necessary t o secure com1 2
3
Chief, Nitrogen Laboratory, Bureau of Chemistry. of Chemistry, Czrc. 108, p. 15 (1912). G S. Dept. Agr., Bureau of Chemistry, Bd1. 107, Rev., p. 5.
U.S. Dept. Agr., Bureau
gelatin, i n which t h e y are slightly higher. I n t h e c a s e of cyanamide t h e one a n d one-half-hour digestion b y t h e Kjeldahl-Gunning-Arnold method gave results (not given i n t h e table) which indicated incomplete oxidation a n d i t was found necessary t o heat t h i s substance for t w o a n d one-half hours. With this longer period of oxidation t h e results of t h e modified method were t h e same a s those of t h e Kjeldahl a n d Gunning methods. T h e general conclusion from these results is t h a t t h e Kjeldahl-Gunning-Arnold method with one a n d one-half hours' oxidation, except i n t h e case of cyanamide, which requires t w o a n d one-half hours, gives more concordant a n d reliable estimations of nitrogen
XOT-..
THE JO
I913
R-V-4L 0 F I LVD 17sT RI d L il S D E S G I LVE E RI S G C H E M I S T R k'
t h a n do t h e official Gunning or official Kjeldahl methods, both of which require from three t o four hours for osidation, depending upon t h e material. BmEAu
L-.
OF
CHEMISTRY
s. DEP.AnT.\lErT O F .kGRICLT,TURE
,
R'ASHIXCTOS
ORTHO-TOLIDINE AS A REAGENT FOR THE COLORIMETRIC ESTIMATION OF SMALL QUANTITIES OF FREE CHLORINE By J. XV. ELLMSA X D S. J. HAUSER Received August 25, 1913
K i t h t h e increasing use of very small quantities of chlorine, either free or in t h e form of hypochlorites for t h e disinfection of m-ater for drinking purposes, a colorimetric method for t h e detection a n d estimation of small a m o u n t s of free chlorine is desirable. S T .IR CH- I O D I D E 31 E T H 0 D
T h e volumetric method in which iodine is liberated f r o m potassium iodide b y t h e chlorine, a n d t h e iodine titrated with sodium thiosulfate. using starch a s a n indicator, offers some difficulties when very small quantities of chlorine are t o be determined. Both t h e sodium thiosulfate a n d t h e potassium iodide solutions deteriorate on standing. Frequent standardizat i o n of t h e sodium thiosulfate is always necessary if accurate estimations are t o be made. Free iodine is liberated from solutions of potassium iodide on standi n g , a n d errors from this source must be constantly guarded against in estimating small quantities of free chlorine. Starch as a n indicator in this method is open t o a n objection on t h e score of not being sensit i v e . unless freshly prepared solutions are used. T h e indefiniteness of t h e color end point, d u e t o t h e liberation of free iodine from t h e potassium iodide, a n d t h e consequent deepening of t h e color are defects which make t h e detection of small a m o u n t s of free chlorine uncertain. Under t h e most favorable conditions t h e .starch-iodide method is capable of detecting quantities of free chlorine n o t lower t h a n 0.03 p a r t per million. This method, therefore, for t h e foregoing reasons lacks reliability for use in determining t h e residual free chlorine which m a y be left in t h e treated water. A C E T I C ACID S O L U T I O N O F O R T H O - T O L I D I S E
T h e ortho-tolidine t e s t for free chlorine, a s suggested b y Earl B. Phelps, in which a n acetic acid solution of this reagent is used. furnishes a delicate qualit a t i v e test. b u t is open t o certain objections for quantitative purposes on account of variations in color produced b y waters of different composition.' According t o this method a one-tenth per cent solution of otolidine in I O per cent acetic acid is employed. With two drops of this reagent in jo cc. of t h e water containing about 0 . 0 ; p a r t per million of free chlorine a yellow color is supposed t o be developed. Dittoe a n d Van Buskirk found t h a t instead of a yello\\- color developing with very small quantities of chlorine, a green color was produced which changed from green t o yellow a n d finally t o deep red as t h e concentration of t h e free chlorine was increased. I n one instance "Report on t h e Public Water Supply of Cleveland with reference to the Treatment with Calcium Hypochlorite," by W. H . Dittoe and L. H. I'an Buskirk, Ohio State Board of Health, Bull. 3, iYo. 1 (January, 1913).
915
when testing for chlorine in a sample of t h e water supplied t o t h e city of Columbus, Ohio, a light blue color developed on adding o-tolidine. S o explanation for these color changes is a t t e m p t e d b y t h e above writers. It is a p p a r e n t , however, t h a t o-tolidine could not be very well employed in a colorimetric estimation, unless these color changes were e h i i n a t e d . X limited a m o u n t of s t u d y of t h e various factors affecting these color changes has been undertaken b y t h e authors, a n d a reliable mode of procedure for a colorimetric determination worked out. -4 possible esplanation of t h e reactions inl-olJ-ed is offered, which seems t o account for some of the color changes. although t h e authors h a r e not had t h e opportunity of making a sufficiently thorough investigation t o enable t h e m t o esplain completely all of these complex reactions. In using t h e acetic acid solution of o-tolidine with small a m o u n t s of chlorine. it was found t h a t different shades of color were produced. ranging from a yellowish green t o a blue. -Although in this test t h e dyes are produced in a n acid solution. nevertheless t h e variations in t h e colors formed appeared t o be intimately associated with t h e original degree of alkalinity of t h e water. Waters t h a t are naturally alkaline from t h e carbonates of calcium and magnesium, which t h e y contain, or those rendered artificially so b y a n y of the fixed alkalies, act similarly. T h e higher t h e original alkalinity of t h e water containing free chlorine, t h e bluer is t h e shade of color produced. T h e more nearly neutral is the water being examined, t h e yellower is t h e t i n t . In natural waters of moderate hardness, which have not been materially modified b y some method of purification, t h e usual t i n t obtained is a yellowish green. B y increasing t h e concentration of t h e acetic acid a more yellowish green color is produced; b u t only b y adding very large quantities of acid is a yellow color formed. On t h e other h a n d t h e addition of a very small quantity of a highly dissociated acid, such as hydrochloric or sulfuric, produces a deep yellow color with small amounts of free chlorine. This yellow color is not affected b y t h e original degree of alkalinity of t h e water being tested, nor is t h e t i n t modified b y a n y change in t h e concentration of the acid. By increasing t h e a m o u n t of free chlorine, t h e acetic acid solution of o-tolidine produces first a solution with a yellowish green color, changing t o an orange a n d t h e n t o a deep red. Still larger quantities of chlorine produce a d a r k red precipitate. Small quantities of bromine a n d iodine produce, with an acetic acid solution of D - t oli dine, green- c ol or e d solutions. K i t h larger a m o u n t s bromine acts as does chlorine, but iodine tends t o f o r m a precipitate more readily. a n d i t is of a bluish color. K i t h increasing concentrations of solutions of potassium bichromate or potassium permanganate. b u t with no halogens present t h e acetic acid solution of o-tolidine produces first green-colored solutions, then yellow solutions and finally deposits brown-colored precipitates. These precipitates are soluble in hydrochloric acid a n d give yellow- colored solutions. On t h e addition of nitric acid t o a n acetic acid solution
