NEW ANALYTICAL METHOD FOR ORGANIC MATTER DETERMINATION

摘要

The determination of organic matter is applied mostly to soils analysis. Literature evidences two volumetric methods for determination, both of them consisting in samples oxidation with dichromate in excess and back titration of the excess with the Mohr salt solution. In addition, two spectrophotometrical analysis are used: one is based on the determination of chromium(III) within the reducing reaction, while the other uses a calibration curve for a sample with an already known organic matter content. The purpose of this study is the elaboration of a new spectrophotometric method, which is more specific, using standard humic acid sodium salt (from Aldrich). Standard solutions of humic acid sodium salt oxidised with dichromate in acidic medium are spectrophoto-metrically analysed. The spectral profile has a maximum at 425 nm and a shoulder at 500-650 ran. When overlaying the spectra of the standard solution of dichromate and humic acid sodium salt, differences between them are being displayed, thus proving the specificity of the method. Moreover, the dichromate excess does not interfere with the oxidated humic acid, due to its different spectral profile. The calibration curve at λ = 645 nm is linear in the range 0.2-2 mg/ml humic acid sodium salt oxidated. The calibration equation, having a very good correlation coefficient (r>0.99), is used for organic matter determination expressed in humic acid sodium salt for mud samples. The mud from the Techirghiol lake -18 km south of Constanta, Romania, traditionally used in therapy of rheumatic diseases, after water extraction and filtration, has been analysed under the same spectrophotometric conditions. Calibration equation at X. = 645 nm was applied for organic matter determination of the mud samples expressed in humic acid sodium salt. The results for most of the samples showed a content of C = 0.5-0.7 (g humic acid Na salt)/(100 g mud). The method for organic matter determination using humic acid sodium salt is specific, reproducible and accurate.