1、Standard Method of Test for Determination of Organic Matter in Soils by Wet Combustion AASHTO Designation: T 194-97 (2013) American Association of State Highway and Transportation Officials 444 North Capitol Street N.W., Suite 249 Washington, D.C. 20001 TS-1a T 194-1 AASHTO Standard Method of Test f
2、or Determination of Organic Matter in Soils by Wet Combustion AASHTO Designation: T 194-97 (2013) 1. SCOPE 1.1. The Wet Combustion method for the determination of soil organic matter is most applicable when it is desired to determine the humus-like, easily oxidized organic material to provide inform
3、ation relating to the suitability of the soil for plant growth. The method provides a good estimate of this type of organic matter, and, when the soil contains only this type, it can also provide a good estimate of the total organic content. However, the reagents used react only very slightly with f
4、resh plant materials such as wood, roots, grass, and weed tops, etc., or with hydrocarbons, charcoal, lignite, coal, and the organic remains in ancient sediments. If such materials are present and the total organic matter is to be determined, the Loss on Ignition method found in T 267 must be used.
5、1.2. The following applies to all specified limits in this standard: For the purposes of determining conformance with these specifications, an observed value or a calculated value shall be rounded off “to the nearest unit” in the last right-hand place of figures used in expressing the limiting value
6、, in accordance with ASTM E29, Using Significant Digits in Test Data to Determine Conformance with Specifications. 1.3. The values stated in SI units are to be regarded as the standard. 1.4. Refer to R 16 for regulatory information for chemicals. 2. REFERENCED DOCUMENTS 2.1. AASHTO Standards: R 16,
7、Regulatory Information for Chemicals Used in AASHTO Tests T 267, Determination of Organic Content in Soils by Loss on Ignition 2.2. ASTM Standard: E29, Standard Practice for Using Significant Digits in Test Data to Determine Conformance with Specifications 3. REAGENTS 3.1. 1 Normal Potassium Dichrom
8、ateDissolve 49.04 g of dry reagent K2Cr2O7in distilled or deionized water and dilute to 1 L. 3.2. 0.5 Normal Ferrous SulfateDissolve 140 g of reagent grade FeSO47H2O in distilled water, add 40 mL of concentrated sulfuric acid, cool, and dilute to 1 L. Standardize this reagent each day by a “blank” t
9、itration against exactly 10.5 mL of 1 Normal potassium dichromate, as described in the method given below, but without a soil sample. 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-1a T 194-2 AASHTO
10、3.3. Barium Diphenylamine-SulfonateDissolve 0.16 g of reagent grade barium diphenylamine-sulfonate in 100 mL of distilled water. 3.4. Sulfuric AcidConcentrated (not less than 96 percent), reagent grade. 3.5. Phosphoric Acid85 percent, reagent grade. 4. APPARATUS 4.1. BalanceThe balance shall conform
11、 to M 231, Class B. 4.2. Erlenmeyer flasks, 400 mL. 4.3. Volumetric flasks, 1000 mL. 4.4. Transfer pipettes, 10 mL and 20 mL (or burettes). 4.5. Heat resistant pad. 4.6. Graduated cylinders, 250 mL and 25 mL. 4.7. Dropping bottles for indicator solutions. For barium diphenylamine-sulfonate solution,
12、 add 0.80 mL, which equals 15 drops from a dropping bottle. 4.8. Burettes, 50 mL (two required). 4.9. Burette holder and laboratory stand. 4.10. Washing bottle, for distilled water. 4.11. Glass or polyethylene bottle, minimum capacity, 1 L, for storage of potassium dichromate and ferrous sulfate sol
13、utions (two required). 4.12. Oven, capable of maintaining 110 5C (230 9F). 5. PROCEDURE 5.1. Weigh 0.750 to 1.0 g (Note 1) of oven-dried soil passing the 0.425-mm sieve (No. 40), to the nearest 0.001 g, transfer quantitatively to a 400-mL Erlenmeyer flask, and add from a burette or pipette exactly 1
14、0 mL of the potassium dichromate solution. Immediately add from a graduated cylinder or a pipette 20 mL of concentrated sulfuric acid, directing the stream into the solution. Immediately swirl the flask vigorously to mix thoroughly and then allow it to stand on a sheet of asbestos for about 30 min.
15、(See Note 2.) Add 200 mL of distilled or deionized water from a graduated cylinder, 10 mL of phosphoric acid from a graduated cylinder (Note 3), and 0.80 mL (15 drops) of barium diphenylamine-sulfonate indicator and swirl the flask to ensure thorough mixing. While still swirling the flask, add ferro
16、us sulfate solution quantitatively from a burette. The solution will first turn purple, then from dark blue to light blue, after which the ferrous sulfate should be added slowly until the color changeswith little warningto brilliant green. Add 0.5 mL (10 drops) more of the dichromate solution to res
17、tore an excess of dichromate, wash down the inside walls of the flask with a few mL of water from a wash bottle, and complete the titration to a one-drop, light green end-point by adding the ferrous sulfate drop by drop to the swirling flask. If less than 5 mL of the ferrous sulfate solution has bee
18、n used in the titration, repeat the 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-1a T 194-3 AASHTO determination with less soil. Make a standardizing “blank” run without a soil sample, but using ex
19、actly 10.5 mL of dichromate and otherwise following the procedure given above. Note 1For peat, use the Loss on Ignition method (T 267). It may be necessary to decrease the amount of soil below the minimum specified under Section 4.1 for soils greater than approximately 10-percent organic. The Loss o
20、n Ignition method (T 267) must be used if the soil sample is less than 0.200 g. Note 2Because the action of chromic acid on organic matter is considerably affected by temperature, the flask should be kept in a relatively draft-free place at room temperature, 20 to 30C (68 to 86F) for the 30-min peri
21、od. 6. CALCULATIONS 6.1. Determine the amount of dichromate solution that was reduced by the soil organic matter. This requires (1) standardization of the ferrous sulfate solution so the amount of dichromate equivalent to the ferrous sulfate actually used in the test may be determined and subtracted
22、 from the total dichromate used in the test; and (2) correction for the small amount of dichromate reduced by impurities in the reagents used. Both of these requirements are satisfied by the “blank” determination described in the procedure. For example, in a blank determination, 10.50 mL of the dich
23、romate solution might require 20.75 mL of ferrous sulfate. In the test procedure for a hypothetical 1.000 g soil sample, 10.50 mL of dichromate would have been used; and the subsequent titration might require 6.80 mL of ferrous sulfate. The amount of dichromate actually reduced by the organic matter
24、 of this soil sample would therefore be 10.50 (1 6.80/20.75), or 7.06 mL; and since the dichromate is a 1 Normal solution, this corresponds to 7.06 milliequivalents of dichromate reduced. 6.2. A milliequivalent of carbon in the oxidation reaction involved in this test method is 0.003 g. The amount o
25、f carbon occurring in easily oxidized carbon would therefore be 7.06 0.003, or 0.021 g, and the percentage of easily oxidized carbon would be 0.021 100/1.000= 2.1 percent. 6.3. It is widely accepted that humus-like soil organic matter contains about 58 percent carbon. Therefore, to convert the perce
26、ntage of easily oxidized carbon to the percentage of easily oxidized organic matter, multiply by 100/58or 1.72. The percentage of easily oxidized organic matter in the example soil would accordingly be 2.1 1.72, or 3.6 percent. 6.4. It has been found for many agricultural soils that the amount of or
27、ganic carbon indicated by the Wet Combustion method is about 77 percent of the total organic content, as determined by the Loss on Ignition method (T 267). Therefore, to approximate the total carbon or total organic matter percentage, the percentage of easily oxidized carbon or organic matter is mul
28、tiplied by 100/77or 1.3. In the example soil, the percentage of total carbon would be 2.1 1.3, or 2.7 percent, and the percentage of total organic matter would be 3.6 1.3, or 4.7 percent. 6.5. The information given above in this section may be summarized in the following equations: sample titre, mLE
29、asily oxidized carbon, percent 10.5 1 0.3 mass of soilblank titre, mL= (1) Total carbon, percent = Equation (1) 1.3. (2) Easily oxidized matter, percent = Equation (1) 1.72. (3) Total organic matter, percent = Equation (1) 1.3 1.72, (4) or Equation (1) 2.236. 2015 by the American Association of Stat
30、e Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-1a T 194-4 AASHTO Note 3It is possible that a color change during titration with ferrous sulfate solution may be impossible to observe with certain types and/or colors of soil. If this occurs, filter the sample through No. 54 filter paper prior to the addition of the barium diphenylamine-sulfonate and then continue with the standard method. 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.
