Soil solution osmotic pressure measurement by cryoscopic technique and its metrological characterization

Cryoscopic technique to measure soil solution osmotic pressure is based on the thermodynamic relationship between chemical potential (pressure) and decresasing of the soil solution freezing temperature. The pressure is determined on the basis of the freezing temperature difference beyween distilled water and soil solution. To characterize metrologically soil solution osmotic pressure (OP) measurements by cryoscopic method a laboratory experiment was carried out with several different soil samples, collected from the A horizon of the typicalheavy-clay chernozm (Kursk region, Russia, and light-cloured clay and sandy chestnut soils (Volgograd region, Russia). Sodium chloride solution was added to soil aliquots at 0.05, 0.10, 0.15, 0.20, 0.25, 0.30 eq/L, while Na2SO4 solution was added at 0.50 0.85 eq/L. Distilled water was added to the control variants. Depending on the granulometric composition of soils, soil water content was adjusted to 28, 23 and15%. The samples prepared in such a way were placed in 40-ml Dewar vacuum flasks with 10-4-10-5 mm Hg of vaccuum. The cryoscope measured temperature according to the Beckman thermometer scale. The temperature of the cooling mixture of crushed ice and NaCl (1:5 v/v) in Dewar flasks was maintained in the range of -21º -17º C. The freezing of soil solutions and distilled water went with overcooling. The temperature jump was indicated by clear Hg meniscus stop at the beginning of crystallization. Statistical analysis of the data obtained produced correlations and corresponding scales of the relative standard deviation(V, %) and relative permissible deviation(Drel, %) of the measured OP values. The V was shown to range from 2 to 50%, while the Drel varied from 5 to 100% between 1 and 15 atm. The threshold between the quantitative and semiquantitative esimates when V=30% was found to lie between 1.3-1.7 atm. Based on the correlation between Drel values and the measured OP, a scale for estimating OP is suggested: when Drel is 100% the corresponding OP value is 0.9-1.2 atm, at 80% Drel 1.2-1.7, at 60% Drel 1.7-2.2, at 40% Drel 2.2-3.2, at 30% Drel 3.2-4.2, at 20% Drel 4.2-9.2, at 10% Drel 9.2-10.2 atm, whereas at 9% Drel the OP can be estimated as ranging 10.2-11.2 atm, while at 8% Drel as 11.2-12.5, at 7% Drel as 12.5-14.2 and at 5% Drel as 14.2-14.5 atm. The scale can be used to estimate the reproducibility of experimental measures of soil solution osmotic pressure.