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Comparative results by the U. S. D. A. and liberation methods on four samples are given in Table 11. TABLE 11. DETERMINATION OF TARACIDS Sample
U.S. D. A. Method %
Liberation Method
21 3 4
1 0 10 . 20 100.4 99.6
97 8.5 3,97 8.4 96.8,96.8 97.7,97.5
%
CHEMISTRY
VOL. 11, NO. 8
checked using U. S. P. cresol containing 0.2 per cent of water and no neutral oil, the tar acids obtained being 99.8 and 99.9 per cent. It is firmly believed that the adoption of these improved methods of testing will result in a greater degree of accuracy, and more closely define quality in terms of specification requirements.
Literature Cited An accuracy of 0.2 per cent is obtainable by the liberation method, as is demonstrated by the foregoing results. As each sample contains both water and neutral oil, the u. s. D. A. values are obviously high, since tar acids alone show 100.0 per cent or more, When per cent of water and neutral oil is added to the liberation method values, the totals are well within 99.5 and 100.0 per cent. The liberation method was
commercial Analysis,,, 5th Val, 111, p, 296, Philadelphia, P. Blakiston’s Son & Co., 1933. (2) Assoc. Official Agr. Chem., “Official and Tentative Methods of Analysis,” 4th ed., p. 420, 1935. (3) Chapin, R *M., U.s. Dept. Am., Bull. 1308 (1924). (4) Standardization of Tar Products Test Committee, “Standard Methods for Testing Tar and Its Products,” 2nd ed., p. 212, London, 1938.
Determining Organic Matter in Paddy Soils Reliability of Rapid Titration Methods C. K. LEE National Geological Survey of China, Chungking, China
T
HE volumetric determination of the soil organic matter by measuring the reducing power of the carbonaceous substance with a certain oxidizing solution is generally recognized as being much less laborious than gravimetric methods. In the former case, soils subjected to analysis are digested in a strongly acid solution with a standard solution of either potassium permanganate (2) or chromic acid (4),and the excess of oxidizing agent is then titrated with a reducing solution, frequently oxalic acid or ammonium ferrous sulfate. Previous investigation (1) has shown that results obtained by such methods agree fairly well with gravimetric ones. Owing to their simple and rapid operation, these methods are often employed when a large number of samples is a t hand and high accuracy is not desired. Rapid titration methods, such as those mentioned above, are not a t all adaptable for water-logged soils, where high reduction potential usually reduces a part of the oxidizing solution. The presence in these soils of ferrous iron, which is frequently liberated in the boiling solution of strong sulfuric acid, also introduces a serious error into the resulting data.
As the total content of ferrous iron is still not measurable quantitatively, it is rather difficult to distinguish among the portions of the total oxidizing solution consumed by organic matter, ferrous salts, or other reducing substances. A number of paddy soils have been used as testing samples, and results are given in Table I. The approximate content of ferrous salts in the soils is estimated according to Morgan’s method (3). From the data, it is obvious that erratic results have been yielded most frequently in paddy soils with gleied subhorizons, which are usually characterized by low organic matter content and relatively high ferrous salt content. These methods are limited in value for such soils.
Literature Cited Soil Sci., 1, 114-27 (1935). (2) Istscherikov, V., and Tuirina, E., Pedology (U.S . S . R.),22, 61-80 (1927). (3) Morgan, M. F., Conn. -4gr. Expt. Sta., Bull. 372 (1935). (4) Tiurin, I. V., Pedologv (U. S.S . R.), 26, 36-47 (1931). (1) Crowther, E. M., Trans. 3rd Intern. Congr.
TABLE I. ORGANICMATTERCOSTENT OF PADDY SOILS (Determined by dry combustion and rapid titration methods) -KKMnOa Carbon by Dry Combustion Method
Sample used Gram
5-8-87 0 t o 18 om.
% 2.60
18 to 50 om.
0.55
Samples
90 t o 110 om. 5-8-80 0 t o 20 om. 45 t o 60 om. 7-7-6 0 t o 20 cm. 20 to 45 om.
0.06
0.75 0.11 0.65 0.09
Method-Carbon oalcd. from result of dry combustion method
,--Chromic
Acid MethodCarbon calcd. from Carbon result of dry obtained combustion by CrOs method method ‘Mg. Mo.
MO.
Carbon obtained by KMnO, method M8.
Sample used Gram
0.1546 0,1501 0.2508 0.2611 0.5043 0.5082
4.02 3.91 1.38 1.43 0.30 0.31
4.20 4.25 1.44 1.60 1.01 0.84
0.1490 0.1487 0.2513 0.2544 0.4991 0.5024
3.88 3.88 1.38 1.40 0.30 0.30
3.9s 3.94 1.58 1.50 1.20 1.01
0 1508 0.1611 0.4981 0,5084
1 13 1.21 0.56 0 56
1.25 1.39 1.58 1.81
0.1613 0.1837 0.6108 0.4931
1 13 1.15 0.56 0.54
1.12 1.28 1.72 1.63
High Very high
0.2004 0.2087 0.5044 0.5078
1.30 1.35 0.45 0.46
1.61 1.52 1.31 1.18
0.1948 0.1976 0.5132 0.4958
1.29
1.44
Very high
01 .. 43 60 0.45
1 . 46 0 1.31
Very high
Ferrous Salts High Very high Very high
