Abstract:
To address the issue of elevated organic matter determination results caused by high chloride content in saline alkali soil samples from Xinjiang when using the potassium dichromate titration directly, the effect of chloride ions on organic matter determination was investigated. A method was proposed, which involved washing the samples to remove chloride prior to organic matter determination using the potassium dichromate titration. Additionally, ion chromatograph and total organic carbon analyzer were employed to determine the chloride ion and organic matter in the supernatant after washing, the effectiveness of the water-soluble chlorine correction factor deduction method was compared, and the effect of water-soluble organic matter on the actual determination was explored. After air-drying and sieving the samples, an aliquot (0.100 0-0.500 0 g) was taken, and approximately 5 mL of deionized water was added. The mixture was oscillated at room temperature for 3 min, and centrifuged for 3 min, and the supernatant was collected in a 25 mL-volumetric flask. This process was repeated once, and the supernatants were combined and their volume was made up with water for determination of the water-soluble organic matter and the water-soluble chloride. The precipitate was dried at 105 °C for 2 h, and 10.00 mL of 50% (volume fraction) sulfuric acid solution containing 0.4 mol·L
-1 potassium dichromate was added. After shaking, the mixture was placed in a temperature-controlled oil bath pot at 185 °C and boiled for 5 min. After cooling, the digested solution and soil residue were transferred to a 150 mL-conical flask, and 3 drops of a 10 g·L
-1 ammonium ferrous sulfate solution containing 14.9 g·L
-1o-phenanthroline were added. The remaining potassium dichromate was titrated with a 0.100 0 mol·L
-1 ferrous sulfate standard solution until the solution turned reddish-brown, at which point the titration was stopped. Non-water-soluble organic matter was then calculated. It was shown that when the mass fraction of water-soluble chloride was not more than 7.00 g·kg
-1, chloride interference was present. Washing the samples 2 times or applying the water-soluble chloride correction factor deduction method could eliminate interference from water-soluble chloride up to 49.05 g·kg
-1. The content of water-soluble organic matter accounted for less than 5.0% of that of the non-water-soluble organic matter, and its effect on detection results was negligible. Repeated determinations of standard samples composed of sodium chloride and the soil effective component analysis reference material GBW07459a were conducted, giving RSDs (
n=7) of the determined values not more than 2.0%, and relative errors between the determined values and certified values in the range of −0.59%-2.3%.