Soil water-soluble fluoride and total fluoride. Determination of ion-selective electrode method (3)

Seven, calculation and presentation of results

(1) Result calculation

The amount of fluoride in the sample, m (ug/kg), is calculated according to formula (1):

In the formula, E ₁ ——potential response value of the sample, mV;

E——Intercept of the calibration curve, mV;

S——The slope of the calibration curve, mV

The content w (mg/kg) of water-soluble fluoride or total fluoride in the sample is calculated according to formula (2):

(2) where m ₁ ——the amount of fluoride in the sample, ug;

m——weigh the mass of the soil sample, g;

w _{dm ——the} amount of dry matter in the soil sample, %;

V1——Total volume of soil sample extract, mL;

V2——The volume of the supernatant of the sample taken during the measurement, mL

(2) Results presentation

When the measurement result of water-soluble fluoride is less than 10.

When the total fluoride determination result is less than 100mg/kg, the result is reserved for integers; when the determination result is greater than or equal to 100mg/kg, the result is reserved for three significant digits

8.

(1) The correlation coefficient of the calibration curve should not be less than 0.

(2) For each batch of samples or every 20 samples, the standard series of zero mass concentration point and an intermediate mass concentration point should be determined.

(3) At least two blank tests should be done for each batch of samples, and the results should be lower than the detection limit; no less than 10% of the parallel samples should be analyzed for each batch of samples, and the relative deviation of the parallel sample determination results should not exceed 20%

(4) Analyze at least 10% of the spiked samples in each batch of samples, and the spiked recovery rate should be controlled between 70% and 120%

Nine, matters needing attention

(1) When ultrasonically extracting water-soluble fluoride in the soil, the liquid level of the water bath should be higher than the extraction liquid level.

(2) When measuring total fluoride, after the soil is extracted by high-temperature alkali fusion in the muffle furnace, it can be taken out when it is cooled to below 100°C, and it can be dissolved by adding hot water several times with a straw

(3) Pay attention to the cleaning and maintenance of the electrode, and meet the requirements of the electrode's instructions

(4) The samples should be brought to room temperature before the measurement.

(5) Make sure that the electrode reaches equilibrium (the electrode potential change is less than or equal to 1mV/min) before performing the sample measurement

(6) The same stirring speed should be maintained during the measurement.
If a magnetic stirring device is used, it should be prevented that the stirring time is too long and the temperature of the sample fluctuates too much, which affects the measurement result
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The stirring speed should be moderate to prevent the formation of turbulence
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(7) After measuring the high-quality concentration samples, use water to fully clean the electrode until it meets the requirements for electrode use
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Each time the last sample is measured and the next sample is to be measured, rinse the electrode with pure water several times, and then use filter paper to absorb the water
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Do not wipe the electrode to prevent slow reaction of the electrode, and you should rinse the electrode with the next sample before measuring
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(8) Do not touch the surface of the electrode membrane with your hands.
In order to protect the fluorine electrode, the mass concentration of fluorine in the sample should not exceed 40mg/L.
If the surface of the electrode membrane is contaminated by organic matter , it can be cleaned with methanol or acetone , or with detergent
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(9) When the electrode is not in use, it should be placed in a 3mol/L potassium chloride solution, not dry, nor in distilled water or other solutions, to prevent the electrode life from being affected
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(10) The operating temperature of the electrode should be selected within the normal operating temperature range.
If it is often used at the upper limit of the temperature range, the service life of the electrode will be shortened and the electrode will be seriously damaged.
The higher the temperature, the greater the damage
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(11) The waste liquid and waste generated in the experiment should be collected in a centralized manner, properly kept, and entrusted to a qualified unit for disposal
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