the determination of organic carbon in soils

3
I 640 J. H. PETTIT ASD. I. 0. SCHAUB. tion on a single evaporation, but for technical purposes it is a negligible proportion of that contained in . ; grams. The filtrate from the silica is made up to a liter and tivo por- tions, of IOO cc. each, are measured into two Becher glasses. 'I he average technical analyst has no time to redistil ammonium hydroxide. It is much more economical of time to assume a contamination \\it11 ammonium carbonate and reprecipitate. This is not a long operation, especiall!, with a filter pump, and is al~zwgs safe. For technical purposes the lime and magnesia are easily determined with one precipitation each, although each may be contaminated with a trace of the other. It is rare that for technical purposes a determination of carbon dioxide or alk-alies is necessary. \I'hile thanking Dr. Hillebrand and Mr. Blount for all the good things they have said and done, I am confident that the scheme above sot forth will be found to furnish all the information neces- sary for the solution of a great number of technical problems. for which the elaborate niceties of Dr. Hillebrand's scheme were never intended. and ior which they should never be recommende!. I,Al<(lKAT<>KS 01" TtIC cOM>IISSIONEK5 01' .\<LOI.ST? SEU TOKK Septeniher jo iooi [CONTRIBVTION FRO31 THE VNlVERSITY OF ~I,t,IiYOIS .%GRICULTCR.\L EX- PERIMENT STATIOS NO. 12.1 THE DETERnINATION OF ORGANIC CARBON IN SOILS. BY J. H. PETTIT ASD I. 0. SCIIACH. Received 5ei'tcrn:ier 16. 190~. IN COSSECTIOS ivith the chemical side of the soil investigations now being carried on at this Station, it was desirable to know the organic carbon content of a large number of soils. The copper oxide combustion method was too long and tedious, and the wet combustion method, using potassium dichromate and concentrated sulphuric acid, did not give complete combustion on chemically pure organic componnds like sugar. Accordingly, Parr's' method for carbon in coal was tried for soils. -4fter some modifications this method gave results which compare very favorably with the copper oxide combustion method, and it has the ad.r-antape of being much inore rapid that; other methods used. 1 This Jour:ia:, 26, ?ai

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Page 1: THE DETERMINATION OF ORGANIC CARBON IN SOILS

I 640 J. H. PETTIT A S D . I. 0. SCHAUB.

tion on a single evaporation, but for technical purposes it is a negligible proportion of that contained in .; grams.

The filtrate from the silica is made up to a liter and tivo por- tions, of IOO cc. each, are measured into two Becher glasses. 'I he average technical analyst has no time to redistil ammonium hydroxide. I t is much more economical of time to assume a contamination \\it11 ammonium carbonate and reprecipitate. This is not a long operation, especiall!, with a filter pump, and is a l ~ z w g s safe. For technical purposes the lime and magnesia are easily determined with one precipitation each, although each may be contaminated with a trace of the other.

I t is rare that for technical purposes a determination of carbon dioxide or alk-alies is necessary.

\I'hile thanking Dr. Hillebrand and Mr. Blount for all the good things they have said and done, I am confident that the scheme above sot forth will be found to furnish all the information neces- sary for the solution of a great number of technical problems. for which the elaborate niceties of Dr. Hillebrand's scheme were never intended. and ior which they should never be recommende!.

I , A l < ( l K A T < > K S 01" TtIC c O M > I I S S I O N E K 5 0 1 ' . \<LOI.ST?

SEU T O K K Septeniher j o i o o i

[CONTRIBVTION FRO31 THE VNlVERSITY O F ~I,t ,I iYOIS .%GRICULTCR.\L EX- PERIMENT S T A T I O S NO. 12.1

THE DETERnINATION OF ORGANIC CARBON IN SOILS. BY J. H. PETTIT A S D I. 0. S C I I A C H .

Received 5ei'tcrn:ier 16. 1 9 0 ~ .

IN COSSECTIOS ivith the chemical side of the soil investigations now being carried on at this Station, it was desirable to know the organic carbon content of a large number of soils. The copper oxide combustion method was too long and tedious, and the wet combustion method, using potassium dichromate and concentrated sulphuric acid, did not give complete combustion on chemically pure organic componnds like sugar. Accordingly, Parr's' method for carbon in coal was tried for soils. -4fter some modifications this method gave results which compare very favorably with the copper oxide combustion method, and it has the ad.r-antape of being much inore rapid that; other methods used.

1 This Jour:ia:, 26, ? a i

Page 2: THE DETERMINATION OF ORGANIC CARBON IN SOILS

ORGANIC CARBON IN SOILS. 1641

Poor determinations on chemically pure s u p - gave the follow- ing results bv this method:

Weight carbon -__. c_-- Carbon Weight taken. Calculated Found 1 recovered

mg. mg mg Per cent 150 63.15 62.81 99.46 1.50 63.15 62.85 99 5 2 ' 5 0 63.15 63.08 99.89 I j O 63. I j 62.S4 99.51

The percentage of organic carbon in most soils is low, and it is usually necessary to add some easily combustible material to the charge in order to start the combustion.

Sulphur used for this purpose, as suggested by Professor Parr. gave sulphur dioxide in the gas resulting from the treatment of the charge with acid after the combustion in the bomb, and this was not prevented by the use of potassium nitrate in the charge. Powdered magnesium was then tried in place of the sulphur. One gram of finely ground magnesium powder, thoroughly mixed with a charge of 2 grams of soil and IO grams of sodium per- oxide, gave a complete and satisfactory combustion with all soils ranging from 0.1 per cent. to 3. j per cent. organic carbon.

The magnesium is not always completely oxidized, however, and that which remains as metallic magnesium will, of course, liberate hydrogen when the charge is treated with acid to set free the carbon dioxide. To avoid error in this way the gas, after meas- uring, is forced into a Rempel pipette filled with potassium hy- droxide ( 2 : I ) and the c a r b n dioxide absorbed, the residue being drawn back into the burette and a second reading taken. The time taken for this absorption and extra rending is compensated by time saved in the previous manipulation by not having to measure out exactly 100 cc. of air. R determination of the inor- ganic carbon is made in the same apparatus by treating a sample of the original soil with acid and measuring the liberated carbon dioxide. The total carbon found hy the combustion less the in- organic carbon gives the organic carbon.

The soils used represent types ranging from a poor silt, lon~ in organic matter, to a drained swamp soil. high in organic matter. The first sample in each set of three is the plowed soil taken to a depth of seven inches The second sample is the soil between the

1 Computed from volume of carbon dioxide nieasiired a t known teniperature and pressure.

Page 3: THE DETERMINATION OF ORGANIC CARBON IN SOILS

\ \- i th this !iietlic;tl. iising Parr 's appamtus , one ma!: c x i easily inalie duplicate tleterminatioiis on i:ight soils i i i ;t tlri!. of eight hours. O n e inaii has made nineteen separate detcriiii!iations in one thy it1 this !aboratory.