Combined Determination Of Soil Particals Surface Parameters Using An Ion-selective Electrode Method

Soil contains a large number of charged particles,such as various clay minerals,oxides and their hydrates,soil humus,proteins,microorganisms,and so on.Studies have shown that the average surface charge density of soil particles is usually as high as 10¹³-10¹⁴/cm².This charge density means that the electrostatic field on the soil particles surface is as strong as 10⁸ V/m（water medium）or 10¹⁰V/m（vacuum medium）.The surface charge of soil particles affects the most microscopic processes and macroscopic phenomena in soils.For example,the chemical process of interactions between ions,protons and electrons determines the soil pH,buffering performance,redox,adsorption/desorption and other microscopic processes.The electrostatic interaction between soil particles profoundly affects the formation and stability of soil aggregates,which in turn affects soil water,heat,gas,nutrient status and soil erosion resistance,etc.Surface property parameters mainly include surface potential,surface charge density,surface electric field intensity,specific surface area and surface charge number.Among the five surface property parameters,only the specific surface area can achieve instrumental analysis at present.However,the specific surface area instrumental analysis technique uses gas adsorption method with dry samples.While most of the chemical processes in the soil occur under liquid phase conditions.Therefore,the specific surface area instrumental analysis technology cannot determine the inner surface area of swellable substances under liquid phase conditions,which greatly reduces the application value of this technology.On the other hand,although the Zeta potential method can be used for the instrumental determination of the shear surface potential in the soil particle diffusion layer.However,due to the thickness of several layers of water molecules between the shear plane and the true surface of the soil particles,the Zeta potential is often less than 20%of the true surface potential.Based on the discovery of several new mechanisms in the ion interface reaction,Li et al.established an ion-selective electrode method based on the principle of multi-parameter combined analysis,and realized instrumental determination for the above five surface properties in a same condition.The basic principle of this method is:two kinds ions are selected as reference with different interface adsorption energy(such as K⁺and Ca²⁺),the electrode potentials are measured by an ion-selective electrode at adsorption equilibrium for a given condition.And the above five surface properties parameter can be determined.However,some problems exist for the present method,such as long pre-treatment time and unstable results of ion-selective electrode.Therefore,in this study,the determination conditions of ion-selective electrode and the instrumental analysis method of multi-parameter surface properties were studied,which laid an important technical foundation for the development of‘multi-parameter combined analyzer of soil surface properties’.The main results are as follows:1.The determination time,pH and the concentration of background ions can affect the measuration of K⁺and Ca²⁺selective electrodes.The main results are as follows:（1）Controlling the measurement time is critical in this method.If the electrode measurement time is too short,the ions in the solution cannot enter the electrode or the ion concentration inside and outside the electrode fails to reach equilibrium;On the contrary,if the measurement time is too long,the electrode potential is instable.The experiment found that after putting the electrode into the test liquid,stir the test liquid with a constant temperature magnetic stirrer for2 minutes,so that the ions in the test liquid can fully enter the electrode.Then turn off the stirrer and let it stand for 2 minutes to balance the ion concentration inside and outside the electrode.（2）The pH value is greater than 8 or less than 6,which has an impact on the measurement of ion-selective electrodes.It is best to adjust the pH in the range of 6-8 of the tested solution.（3）The presence of higher concentration of Ca²⁺will interfere with the determination of K⁺selective electrode.While the presence of a higher concentration of K⁺will interfere with the determination of the Ca²⁺selective electrode.Experiments have found that adding a background electrolyte with the same concentration as the test solution in the standard solution can effectively eliminate the interference of background ions and obtain accurate measurement results.2.The electrode method greatly shortens the balance time.The equilibrium time of ion exchange in the montmorillonite system is only 30 minutes.In the past,the ion equilibrium batch experiments or adsorption kinetics method was used to determine the surface properties.And the determination time usually needed more than 10 hours.Therefore,this study greatly improves the efficiency of the measurement.3.The pre-treatment method of samples has a great influence on the determination results.Montmorillonite,kaolinite,Ti O₂ and purple soil were made into Na⁺saturated samples and H⁺saturated samples respectively to determin the surface property parameters.Through comparison,it was found that the surface property parameters measured by Na⁺saturated samples of montmorillonite,kaolinite,Ti O₂ and purple soil were more accurate than those obtained by H⁺saturated samples.For example,the specific surface area of Na⁺saturated montmorillonite is 637.4±108.4 m² g^-1,and the specific surface area of H⁺saturated montmorillonite is 1967±93 m² g^-1.While the specific surface area of montmorillonite is generally 600-800 m² g^-1.The surface charge density of Na⁺saturated purple soil was-0.1857±0.0092 C m^-2,and that of H⁺saturated samples was-0.05789±0.00311 C m^-2.The surface charge density of soil is generally in the range of 0.1-0.4 C m^-2.4.Based on the above three research results,the electrode method was used to determine the surface property parameters of five longitude zonal soils in northern China.The five soils are:a phaeozem,a chernozem,a chestnut soil,a sierozem and a gray desert soil.The results showed that:（1）The three parameters of surface potential,surface charge density and surface electric field intensity were as follows:chestnut soil>chernozem>phaeozem>sierozem>gray desert soil;（2）The specific surface area of chernozem>phaeozem>grey desert soil>sierozem>chestnut soil;（3）The number of surface charge in chernozem>phaeozem>chestnut soil>gray desert soil>sierozem.5.The rationality of the surface property parameters of the above soil minerals and soil particles was analyzed through comparison of measurement results by different technical means,the relationship between the correction coefficient and the surface potential,and the relationship between the soil composition and the surface property parameters.It is proved that the electrode method based on the principle of multi-parameter surface property analysis can obtain correct results for determining the surface properties of soil particles.