The Control Mode of Extraction Temperature for Water-soluble Fluorine in Soils Measurement by Selective Electrode Method

BACKGROUND The water-soluble fluoride in soil was pretreated by ultrasonic extraction and determined by ion selective electrode. The thermal effect of ultrasonic cavitation can cause the temperature of the extract to rise, and the test result may be unstable, and it cannot truthfully reflect the water-soluble fluorine content in the soil, which causes problems for the monitoring of fluorine in the soil environment. Therefore, how to effectively control the extraction temperature is the focus of this study. OBJECTIVES To select the optimal temperature control method, verify the accuracy of the results, and preliminarily discuss the inevitability of temperature control. METHODS In order to select the optimal temperature control method, the temperature change of the extraction liquid during the extraction using four temperature control methods was recorded: direct ultrasound, adding ice pack, adding ice water and cooling circulating water. In order to verify its reasonableness and validity, the applicable and representative soil active state component analysis standard material was selected as the experimental sample to determine the soil water-soluble fluorine content. To investigate the effect of temperature on the amount of water-soluble fluoride extracted from soil, the pH value of the extracted liquid was determined during the extraction process. RESULTS The results showed that the cooling circulating water temperature control method could effectively control the extraction temperature within 25±2℃. The relative error (8.9%) was significantly less than that of direct ultrasound method (14.1%). The pH value of the extract liquid did not change significantly, and the determination results were stable and reliable. CONCLUSIONS The temperature control method by cooling circulating water has the advantages of low cost, simple operation, accurate and reliable results, suitable for batch sample treatment, and can be widely used in soil environmental fluorine monitoring.