THE ESTIMATION OF CARBON IN SOILS. 595

LXIV.--The Estimation o f Carbon in Soils and Kindyed Substances.

By ALFRED DANIEL HALL, NORMAN HARRY JOHN MILLER, and NUMA MARMU.

THE method of estimating the organic carbon present in soil by oxidising to carbon dioxide with a mixture of chromic and sulphuric acids was originally suggested by Wolff, and was subjected to a critical examination by Warington and Peake (Trans., 1880, 37, 6 17), who

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596 HALL, MILLER, AND MARMU: THE ESTIMATION OE'

found that it invariably gave results 10 to 20 per cent. lower than were obtained by direct combustion in oxygen. Cameron and Breazeale (J. Arner. Chern. SOC., 1903, 26, as), who recently examined the process, obtained similar results. The deficiency appears to be due, not to any failure of the chromic acid to attack the organic matter in the soil, but to the incompleteness of' the resulting oxidation, a certain amount of aldehyde, acetic acid, and other substances less oxidised than carbon dioxide being produced. The authors have re-examined the method and find that by the addition of a short tube containing red-hot copper oxide to complete the combustion the whole of the carbon in the soil can be obtained as carbon dioxide.

The authors also prefer to absorb the evolved carbon dioxide by means of dilute caustic alkali in a Reiset tower, and determine its amount by the method of double titration with phenolphthalein and methyl-orange which was suggested by Hart and worked out more

recently by Brown and Escombe (Phil. D a m . , 1900, 193, series B, 289). This is by far the most sensitive method for estimating small quantities of carbon dioxide, and is rapid and easy of manipulation when the necessary solutions have been prepared and standardised. A further advantage in the suggested method is that an estimation can first be made of the carbon dioxide combined as carbonate in the soil (see Amos, Jour. A g ~ i c . Sci., 1905, 1, 322), and then, on the same sample and in the same apparatus, after the addition of the copper oxide tube and fresh alkali in the Reiset absorbing tower, the organic carbon can be estimated. The apparatus employed is shown in the figure ; A is a Reiset tower containing alkali to remove all traces of carbon dioxide from the incoming gas, .B is a flask containing the soil, fitted with a side delivery tube and a stoppered funnel, the tap of which makes connection with the Reiset tube, A, or with the liquid in the funnel. A small condenser, C, is inserted to avoid risks of cracking the combustion tube with condensed water, B is a combustion tube

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CARBON IN SOILS AND KINDRED SUBSTANCES. 597

containing about 6-8” of granular copper oxide and heated by the large Bunsen burner E ; P is a U-tube containing a little silver sulphate to arrest any chlorine or volatile chlorides, after which the gas is passed through the Reiset tower, G, to the water-pump. The procedure is as follows :

Five to twenty grams of finely-powdered soil are placed in B and 100 C.C. of 4 per cent. sodium hydroxide solution in G ; after allowing the pump to operate for a little time, G is introduced and sufficient sulphuric acid diluted with an equal volume of water is poured on to the soil. As the air current carries the evolved carbon dioxide forward, the acid is boiled gently to complete the decomposition of the carbonates and the expulsion of the dissolved carbon dioxide, until in twenty to thirty minutes the Reiset tower can be detached and the alkali titrated (see Brown and Escombe, Zoc. cit .) . Another 100 C.C. of alkali are then run into the tower, the copper oxide tube is introduced into the circuit and heated to redness, and 20 to 30 C.C. of strong sulphuric acid are allowed to flow on to the soil. A drop or two of water are then passed in to clean the tap of the funnel, followed by 10 C.C. of saturated chromic acid solution. Oxidation now begins and the process is completed as before. Carbon compounds in the soil appear to oxidise completely in a short time, as do the carbohydrates and other pure carbon compounds examined. A few substances, however-ring compounds containing nitrogen-are very resistant, betaine, for example, being only oxidised to the extent of 86 per cent. after one hour’s heating. The substitution of phosphoric for sulphuric acid is of no particular advantage, neither hastening the reaction nor rendering i t more complete in the few cases where the substance resists attack. The following table shows a comparison between the results obtained by this modification of the chromic acid wet combustion and the ordinary process of combustion in a stream of oxygen.

Carbon, per cent,

Substance. Found. Calculated. Cellulose ...... 44 ‘35 44‘44 Mannitol.. ....... 39 ‘46 39-56 Benzoic acid ... 69’03 68-85 Quinol ......... 65.87 65.45 Diphenylamine. 85.75 85’21

Chromic Combus- acid tionin

Soils. method. oxygen. Broadbalk waste 0-9” 1 *67 1.57

2 , ,, 10-18” 0’71 0.67 > > ,, 19-27” 0.50 0.51

Chromic Combus- acid tionin

Soils. method. oxygen. 1 Geescroft waste A. 0-9” 1.21 1-13 ,, B. 0-9” 1’58 1’51 ,, A. 10-18” 0‘60 0.64

1 > > ,, B. 10-18” 0.46 0.45

I Pasture ......... 0-9” 3-84 3’60 ,, ......... 0-9’’ 3.75 3.50 ,, ......... 10-18“.07 2-09 i

THE ROTHAMSTED EXPERIMENTAL STATION, LAWES AGRICULTURAL TRUEJT.

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