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 C H E M I S T R Y
Jan., 1915
t h a t the Kjeldahl-Gunning-Arnold method reduces the time of digestion over the original Kjeldahl or Gunning methods from four hours t o one and one-half hours. This is a very important item in laboratories where a large number of nitrogen determinations are being made. The Gunning-Copper method has been in use in this laboratory for the past nine years, and t h e general practice has been t o digest from two t o three hours after the solution becomes clear. Since this method possesses several advantages in manipulation over the Kjeldahl-Gunning-Arnold method, a comparison of the two methods with different periods of digestion was undertaken, using bone meal, dried blood, cyanamide, and linseed meal as representative substances. I n Table I , the period of
39
this point, determinations were made on a number of substances such as are encountered in a fertilizer or food laboratory. Three determinations were made on each substance by each method. The data are given in Table 11. The results obtained for these substances by the two methods show a very close agreement, and in point of, accuracy there is no choice between them. The Gunning-Copper method possesses several distinct advantages over the other which may be enumerated as follows: I-The use of potassium sulfide is unnecessary, and thus one detail of manipulation (addition of K2S) is eliminated, slightly shortening the time required for the determination. Also the presence of hydro-
TABLE I-EFFECT OF DIFFERENTPERIODS OF DIGESTION Period of digestion after solutions became clear
ON PERCENTAGES OF NITROGENFOUND 11/2 hours 2 hours '
1 hour No. of analyses Max. Min.
METHOD BONE M E A L : Gunning-Copper ............................ 6 Kieldahl-Gunninn-Arnold.. . . . . . . . . . . . . . . . . . . 6
.......... ,. 6 . . . . . . . . .... . . 6 . . . . . . . . . .. . . 6 ... 6 LINSEED MEAL: Gunning-Copper ............................ 6 Kjeldahl-Gunning-Arnold., . . . . . . . . . . . . . . . . . . 6 ,
2.18 2.25
2.16 2.19
Av. 2.16 2.20
No. of analyses Max. Min. 4 4
2.20 2.20
2.18 2.18
Av. 2.19 2.19
No. of analyses Max. Min. 6
6
2.20 2.23
2.18 2.19
Av. 2.19 2.20
3 hours
No. of analyses Max. Min.
6 6
2.23 2.22
2.20 2.20
Av. 2.20 2.20
14.11 14.03 14.07 14.32 14.11 14.24
6 14.24 14.07 14.13 6 14.32 14.19 14.29
6 14.32 14.15 14.22 6 1 4 . 3 0 14.19 14.25
6 14.40 14.23 14.29 6 14.34 14.15 14.26
15.75 15.37 15.52 15.62 15.46 15.55
6 15.62 15.50 15.53 6 15.54 15.41 15.49
6 15.67 15.46 15.55 5 15.62 15.50 15.55
6 15.58 15.50 15.53 6 15.62 15.50 15.55
5 4
6
~
5.53 5.62
5.45 5.53
5.49 5.56
digestion in all cases refers t o the length of time after the solutions became clear. I n the length of time required t o become clear, no difference was noted in t h e two methods. Using a moderate flame, the time required for the sulfuric acid solution t o become clear was as follows: Cyanamide.. . . . . . . . . . 20 min. Bone meal.. . . . . . . . . . . 25 min. Dried blood.. . . . . . . . . . 25 min. Linseed m e a l . . . . . . . . . 35 min. The results seem t o indicate t h a t a digestion of one and one-half hours is sufficient t o produce a quantitative yield of ammonia in either method, except in the dried blood, where from two t o three hours are necessary in the Gunning-Copper method. Hibbardl found three hours t o be necessary, using this method. On the whole, the results are slightly in favor of t h e Kjeldahl-Gunning-Arnold method, as a quantitative yield of ammonia is produced after a digestion of from one t o one and one-half hours. However, it is doubtful if there are many substances for which the Gunning-
5.62 5.62
5.50 5.59
5.57 5.61
6
5.62 5.59
5.50 5.53
5.55 5.58
6 6
5.64 5.62
5.56 5.56
5.62 5.58
gen sulfide in the laboratory from this source is avoided. 11-In adding the sodium hydroxide before distilling, the copper sulfate acts as a n indicator, so t h a t a large excess of alkali is easily avoided, and the bumping during distillation is much lessened. 111-The difference between the price of copper sulfate and metallic mercury, and the elimination of potassium sulfide, makes the Gunning-copper method somewhat cheaper. SUMMARY
A quantitative yield of ammonia in dried blood is produced sooner in the Kjeldahl-Gunning-Arnold method t h a n in the Gunning-Copper method. I n the case of all the other substances studied, a digestion of one and one-half hours proved equally efficacious for either method. The Gunning-Copper method possesses advantages in manipulation which make it preferable t o the Kjeldahl-Gunning-Arnold method, especially where a TABLE11-DETERMINATIONS OF NITROGEN ON VARIOUS SUBSTANCES large number of determinations are t o be made. METHOD Gunning-Copper Kjeldahl-Gunning-Arnold Period of digestion
SUBSTANCE
Max. Gelatin. . . . . . . . . . . . . . . 15.14 E g g albumin (dried). . . . 12.86 Peptone. . . . . . . . . . . . . . . 14.49 Casein. . . . . . . . . . . . . . . . 13.87 Feathers. . . . . . . . . . . . . . 14.49 Leather. . . . . . . . . . . . . . . 4 . 8 3 Fish s c r a p . . . . . . . . . . . . . 6 . 4 8 Animal t a n k a g e . , . . . . . . 6.32 Garbage tankage.. . . . . . 2.99 Beef scraps.. . . . . . . . . . . 9 . 1 4 Castor bean pomace. ... 4.66 Cocoa. . . . . . . . . . . . . . . . . 4.01 Cottonseed m e a l . . . . . . . 7.33 Cottonseed meal.. . . . . . 6 . 4 3 Silage (dried). . . . . . . . . . 1 . 3 0 Flour. . . . . . . . . . . . . . . . . 2 . 4 3 Bran, . . . . . . . . . . . . . . . . . 2.60 Bone meal.. ........... 3 . 0 6 Peat. . . . . . . . . . . . . . . . . . 2.41
11/2 hours Min. Av. 15.12 15.13 12.75 12.80 14.38 14.45 13.81 1 3 . 8 3 14.43 14.47 4.72 4.76 6.35 6.43 6.31 6.26 2.96 2.98 8.97 9.04 4.55 4.59 3.94 3.97 7.30 7.31 6.40 6.42 1.26 1.28 2.41 2.42 2.55 2.58 3.02 3.04 2.37 2.38
Max.
11/z hours Min. Av.
Copper method will not give a n equally high yield with the same length of digestion. I n order t o test 1
THISJOURNAL,a , 464.
CHEMICALLABORATORY STATION MICHIGANEXPERIMENT EASTLANSING
COMPARISON OF A FEW METHODS FOR TOTAL PHOSPHORIC ACID IN SUPERPHOSPHATE' B y C. A. PETERS Received October 5, 1914
A 'comparison of several methods for the determination of total phosphoric acid in superphosphate has been made, emphasizing the interference of silica in the gravimetric method, and showing t h a t shorter methods give as accurate results as the customary double precipitation as phosphomolybdate and ammonium 1 T a k e n from a thesis b y Arthur G. Weigel, optionally presented for t h e degree of B.S. at the Massachusetts Agricultural College a t Amherst.
T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
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magnesium phosphate. Nothing new is claimed for t h e work. T h e superphosphate used was home-made from ground rock. All b u t a b o u t three-quarters of one per cent of t h e phosphoric acid was water-soluble. Aqua regia was used t o dissolve the phosphoric acid. For the determination of t h e phosphoric acid various procedures were carried out as follows: A-The Official Method.’ B-The Official Method modified b y evaporating t h e liquid on a water b a t h , taking u p with dilute nitric acid and filtering through paper t o remove silica before precipitation. C-Direct weighing of t h e yellow precipitate as outlined b y Baxter.2 D-The volumetric process of Pemberton as described by wile^.^ E-The direct precipitation of ammonium magnesium phosphate in t h e presence of ammonium citrate. F-Same as E, substituting citric acid for ammonium it rate.^ None of these methods require description. However, t h e apparatus used for heating the yellow precipitate t o 2g0°-3000, as outlined b y Baxter, will be described. This consisted of two 6411. iron dishes, deep form, such as are used for sand baths. T h e edges of one dish were cut back a/4 in. every half inch a n d bent in so as t o make a cover for t h e uncut dish. A hole in t h e t o p of t h e cover held a cork a n d thermometer. Triangles inside were arranged t o hold t w o or three crucibles, each covered with a small watch glass. An asbestos jacket consisting of a side a n d t o p prevented t h e radiation of heat. The whole was heated with a Bunsen burner, t h e flame being protected from draft by a n iron conical shield 8 in. high. The results obtained by t h e different methods are given in Table I, each determination made being listed. TABLE I-TOTAL PHOSPHORIC ACID IN SUPERPHOSPHATE Average DUPLICATES Per METHOD Per cent cent A-Official Method.. . 15.30 15.25 15.28 B-Official Method (modified) ......... 15.07 14.97 15.02 C-Direct weight of 14.81 14.81 yellow pp t 14.82 14.85 1 4 . 8 2 C1-Same as C but overheated. 14.53 1 4 . 5 4 1 4 . 5 4 D-V o 1 u m e t r i c method. .......... 14.80 14.86 14.83 E-Pptd. as NH4Mg14.85 1 4 . 9 4 Pod.. 14.85 14.87 1 4 . 8 8 F-Pptd. as NHaMgPod.. 16.50 16.29 16.39
.........
.......
.............
.............
Variation from B Per cent REMARKS f 0 . 2 6 No silica removed. 0.00
Silica removed
-0.20
Temp. 29O0-3OO0
-0.48
Temp. 30Oo-33O0
-0.19
KOH : HNOi
-0.14
In am. citrate
f1.37
I n citric acid
From t h e comparison of t h e results given under A a n d B in Table I i t would be inferred t h a t there was 0.26 per cent of silica in the pyrophosphate when the solution of total phosphoric acid before precipitation with molybdate was not evaporated t o dryness. T h e results indicated under C, D a n d E, which were Bull. 107, p 2. A m . Chem. J . , 39 (1902), 298. 3 Wiley. “Principles and Practice of Agricultural Chemistry,” I1 (1908), p. 160. 6 Wiley. I b i d . . I1 (1908). p. 88. b Wiley, I b i d . . I1 (1908). p. 98. 1 1
Vol. 7, No.
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obtained b y direct weighing of t h e yellow precipitate by t h e volumetric method a n d b y precipitation in ammonium citrate, are, on t h e average, 0.17 per cent less t h a n those listed under B. If t h e results obtained under B-silica-free-are t a k e n as standard, then i t is apparent t h a t either of three more rapid methods (C, D or E) give as accurate results as t h e Official Gravimetric Method. No a t t e m p t was made t o find silica in a n y of t h e precipitates a s its presence is generally acknow1edged.l T h e results given under CI show t h e decomposition of ammonium molybdate by temperature over 300 O a n d the resulting negative error. c 0 NC L U S I 0N The above work emphasizes what is already known: I-That the Official Gravimetric Method of determining total phosphoric acid gives high results when phosphoric acid is determined in superphosphate witho u t evaporation of the solution t o dryness on a steam bath t o remove silica. 11-That several methods other t h a n the Official Gravimetric give equally good results in half t h e time. DEPARTMENT OF GENERAL AND AGRICULTURAL CHEMISTRY MASSACHUSETTS AGRICULTURAL COLLEGE, AMHERST
CLEANING SOILS FOR MlCROSCOPIC EXAMINATION By W. H. FRY AND JOHN A. CULLEN Received Oct. 22, 1914
I n m’icroscopic mineralogical work on soils i t is necessary t h a t t h e grains be transparent or a t least strongly translucent a n d t h a t the grain-surf aces be free from all coatings sand adhering material. Where light is not transmitted, t h e optical properties of t h e minerals cannot be determined in soil preparations; a n d where t h e material carries an appreciable coating of some foreign substance the refractive index determined will be t h a t of the coating instead of t h e mineral, even though t h e determination of other optical properties of t h e grain under examination may remain unaffected. Many soils have their constituent grains more or less stained and coated b y various substances. Often simple washing a n d drying serve t o render them adaptable t o microscopic examination. B u t in a great number of cases t h e grains are so coated with iron oxides, hydroxides, or both, t h a t other means of cleansing have t o be resorted to. T h a t cleansing agent would be most efficient which, while entirely removing the iron a n d other stains, a t t h e same time had t h e least decomposing action on the minerals themselves. Hydrochloric a n d nitric acids act very energetically upon apatite, t h e determination of t h e presence of which is sometimes t h e primary object of t h e examination. Sulfuric acid cannot be used for t h e same reason, a n d in addition has t h e disadvantage of decomposing biotite, a common soil constituent. Several readily available organic acids were tested on various soils, different concentrations of the various acids being used in order t o ascertain t h e most efficient 1
Wiley, loc. c i f . , p. 78.
