Determination of hydrogen cyanide ion selective electrode method

June 26, 2020

Sensitivity: The difference in response value when the concentration of cyanide ion in the test solution changes 10 times is 57 ± 3mV / 25 ℃.

The detection limit is 0.5μg / 10mL, if the sampling volume is 24L, the detection limit concentration is 0.02mg / m3; the measuring range is 1 ~ 100μg / 10mL, if the sampling volume is 24L, the measurable concentration range is 0.04 ~ 4mg / m3.

2 Principle

The hydrogen cyanide in the air is absorbed by the sodium hydroxide solution. The potential value of the absorption liquid is measured by a cyanide ion selective electrode. The change of this potential value is linearly related to the logarithm of the cyanide ion activity in the solution. Based on the potential value, calculate the concentration of hydrogen cyanide.

3 Reagent

3.1 Absorption solution: Add 10g potassium nitrate to 1000mL 0.1mol / L sodium hydroxide solution, dissolve it and store in a plastic bottle.

3.2 Standard solution: Weigh 0.3g of potassium cyanide in 100mL of absorption liquid, which is the standard stock solution, and calibrate its accurate concentration with the following standard silver nitrate solution.

Calibration method: Accurately draw 10.0mL of the stock solution into an Erlenmeyer flask, add 0.1mL test silver spirit indicator (0.02g test silver spirit dissolved in 100mL acetone), and titrate with 0.0200mol / L silver nitrate solution to make the solution yellow The turbid orange-red color is the end point of the titration. The titration operation is repeated twice, and the volume error of the standard silver nitrate solution used in the two titrations does not exceed 0.05 mL. Then, calculate the concentration of hydrogen cyanide as follows: 08.110 × ⋅VM = c

Where: c-the concentration of hydrogen cyanide solution, mg / mL;

M ——concentration of silver nitrate solution, mol / L;

V-volume of silver nitrate solution, mL;

1.08 ——Coefficient of cyanide converted into hydrogen cyanide.

Immediately before use, the stock solution was diluted into 1.00 mL of a standard solution containing 100 μg and 1.00 mL of 10 μg hydrogen cyanide.

4 Instruments

4.1 Bubble absorption tube: ordinary type, with 10mL graduation mark.

4.2 Air sampler: flow range 0.2 ~ 1L / min. The flow rate is stable. When using, use the soap film flowmeter to calibrate the flow rate of the sampling series before and after sampling. The flow error should be less than 5%.

4.3 Ion activity meter or precision acidity meter (accuracy ± 1mV).

4.4 Cyanide ion selective electrode (AgI type): The concentration range of cyanide ion is 1 × 10-6 to 1 × 10-3 mol / L, and its Nernst slope is not less than 54mV at 25 ℃ Response time is less than 3min.

4.5 Double-salt bridge saturated calomel electrode: filled with 0.1mol / L potassium nitrate solution.

4.6 Magnetic stirrer.

4.7 Beaker, 25mL.

5 Sampling

Use a bubble absorption tube containing 10mL of absorption liquid (the intake port of the absorption tube is plugged with a small amount of lead acetate cotton to remove sulfide in the air), with a flow rate of 0.8L / min, and produce 24L of gas. Record the air temperature and atmospheric pressure at the time of sampling.

6 Operation steps

6.1 Drawing of standard curve

Take five 25mL beakers and prepare the standard series as shown in the table below.

0 1 2 3 4 5

10μg / mL standard solution HCN V / mL 0 0.10 0.50 1.00

100μg / mL standard solution HCN V / mL 0.50 1.00

Absorption fluid V / m 10.0 9.9 9.5 9.0 9.5 9.0

Hydrogen cyanide content m / μg 0 1 5 10 50 100

Put a plastic sheathed iron core rod on each, put it on the magnetic stirrer, insert the cyanide ion selective electrode and the double salt bridge saturated calomel electrode, after stirring for 2min, measure the potential value (mV) of the solution. Draw the standard curve with the hydrogen cyanide content (μg) as the abscissa of the logarithmic grid and the measurement potential (mV) as the ordinate of the equidistant grid.

6.2 Determination of sample: After sampling, make up with water to the volume of the absorption solution before sampling. Then follow the steps of drawing a standard curve to measure the potential value (mV) of the sample solution. Find the hydrogen cyanide content (μg) from the standard curve.

Simultaneously with the determination of each batch of samples, 10 mL of unsampled absorption solution was used to determine the reagent blank according to the same procedure.

7 Result calculation 00VAAc− =

Where: c-the concentration of hydrogen cyanide in the air, mg / m3;

A ——Content of hydrogen cyanide in the sample solution, μg;

A0——Content of hydrogen cyanide in reagent blank solution, μg;

V0——Converted to the sampling volume under standard conditions, L.

8 Precision and accuracy

Precision: When the hydrogen cyanide content is 1-50μg / 10mL, the relative standard deviation of repeated measurements is 0.5% -3.2%.

9 Description

9.1 Interference and elimination. The electrode measures free cyanide ions (CN-), and the formed cyanide complex (R-CN) cannot be directly determined. Hydrogen sulfide, sulfide, and iodide in the air have serious interference with the measurement of the cyanide electrode. Lead acetate cotton can be inserted in the air inlet of the absorption tube to remove the interference of these compounds.

9.2 Notes

9.2.1 The cyanide ion selective electrode must be fully activated before use, especially a new electrode must be soaked and activated in 10-5 mol / L potassium cyanide solution for 2 hours, and then soaked in water for 2 days. It should also be immersed in water in the daily work, so that when it is in use, it does not need to be activated again. If the activation time is insufficient, the response curve of the electrode is unstable, and the linear detection lower limit of 5 × 10-7 mol / L cannot be reached.

9.2.2 Determination of hydrogen cyanide by cyanide ion electrode should be carried out in alkaline medium (pH≥12). Do not immerse in a solution with high CN-content (concentration greater than 10-3mol / L) for a long time to avoid electrode performance degradation and sensitivity Membrane failure.

10 References

10.1 Cui Jiusi, Wang Qinyuan, editor-in-chief of Wang Hanping, Air Pollution Monitoring Methods (Second Edition), pp.846 ~ 849, Chemical Industry Press, Beijing, 1997

Appendix Basic Properties of Hydrogen Cyanide

Hydrogen cyanide HCN is a colorless, highly volatile liquid with a molecular weight of 27.03, a bitter almond smell, a relative density of 0.6876 (20 ° C / 4 ° C); a melting point of -14 ° C; a boiling point of 26 ° C; and a vapor pressure of 87.7 kPa at 22 ° C; The relative density of vapor is 0.94 (for air). Hydrogen cyanide is easily soluble in water, alcohol and ether. Cyanide (cyanate) can liberate hydrogen cyanide under high temperature conditions or under the action of acid. Metal cyanide compounds decompose due to sunlight exposure, and can also emit hydrogen cyanide and easily escape from the aqueous solution to the air. It exists as a gas in the air.

2

Hydrogen cyanide is quickly absorbed by the respiratory tract and digestive tract. After entering the body, it is combined with high-speed cytochrome oxidase to become cyanide high-speed cytochrome oxidase, which loses the role of transferring oxygen and causes tissue hypoxia and poisoning. Inhalation of 100 to 200 mg / m3 of hydrogen cyanide within 0.5 to 1 hour can cause death. Suddenly inhaling a high-concentration dose, a drop of liquid hydrocyanic acid can kill you in a few seconds. For the measurement of cyanide, there are colorimetric method for generating azo dye and cyanide ion selective electrode method. The coupling chromogenic agents used in the colorimetric method include pyridine-benzidine, pyridine-p-phenylenediamine, pyridine-barbituric acid, isonicotinic acid-pyrazolone and the like. Among them, due to the low sensitivity of the aniline method, the large amount of pyridine reagent has an odor, and the benzidine reagent has carcinogenic toxicity. This method has not been used for a long time. Isonicotinic acid-barbituric acid is used as the color developing agent, which has the characteristics of low toxicity and high sensitivity, so this reagent is generally used as the colorimetric color developing agent. The ion selective electrode method is accurate, sensitive, fast, simple, and has a wide concentration range for measurement, which can meet the requirements of monitoring and inspection in ambient air. At the same time, this method does not use toxic reagents and other features, and it has been adopted at home and abroad.

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