Indicators of inorganic nonmetals in drinking water - cyanide (isonicotinic-barbiturate spectrophoto
The solution is based on GB/T 5750.5-2023
Introduction
The minimum detection mass of this method is 0.1μg. If 250mL water sample is distilled for determination, the minimum detection mass concentration is 0.002 mg/L.
Method and principle
The cyanide in the water sample is absorbed by the alkaline solution after distillation, reacts with the active chlorine of chloramine T to produce cyanogen chloride, and then reacts with isonicotinic-barbiturate reagent to produce purplish blue compound, which is quantized by colorimetry at 600nm wavelength.
Reagent or material
Unless otherwise specified, only reagents of superior purity and above are used.
Water: In line with GB/ T33087-2016 instrument analysis of high purity water provisions.
The standard solution of impurities required in the test, when no other requirements are indicated, shall be prepared or selected in accordance with the provisions of GB/T 602.
Warning: Potassium cyanide is a highly toxic chemical!
Tartaric acid (H2C4H4O6): Analytically pure.
Zinc acetate solution (100g/L): Weigh 50g zinc acetate dihydrate [Zn(CH3COO)2·2H2O], dissolve in pure water, and dilute to 500 mL.
Sodium hydroxide solution (20g/L): Weigh 2.0g sodium hydroxide solution (NaOH), dissolve in pure water, and dilute to 100mL.
Potassium dihydrogen phosphate solution: Weigh and dissolve 13.6g potassium dihydrogen phosphate (KH2PO4) in pure water and dilute to 100 mL.
Sodium hydroxide solution (12g/L): Weigh 1.2g sodium hydroxide (NaOH), dissolve in pure water, and dilute to 100 mL.
Acetic acid solution (3+97).
Isonicotinic-barbiturate reagent: Weigh 2.0g isonicotinic acid (C6H5O2N) and 1.0g barbituric acid (C4H4N2O3), add to 100mL of sodium hydroxide solution (12g/L) at 60 ℃ ~ 70℃, stir until dissolved, and add pure water to 100mL after cooling. The reagent has a pH of about 12, is colorless or very light yellow, and can be stored in the refrigerator for 30 days.
Chloramine T solution (10g/L): Weigh 1g chloramine T trihydrate (C7H7CINNaO2S·3H2O), dissolve in pure water, and dilute to 100 mL, ready to use. Note: The effective chlorine content of oxamine T has great influence on this method. Chloramine T is easy to decompose and fail. If necessary, use iodometry to determine the effective oxygen content before use.
Potassium cyanide standard solution [0(CN)=100μg/mL]: Weigh 0.25g potassium cyanide (KCN), dissolve in pure water, and set the volume to 1000mL. This solution contains about 0.1mg(CN-) in 1mL. The exact concentration can be calibrated with silver nitrate standard solution [c(AgNO3)=0.019 20mol/L] before use, calculate the content of cyanide in the solution, and then dilute with sodium hydroxide solution (lg/L) to ρ(CN-)=1.00μg/mL standard use solution. Or use certified reference material.
Calibration method of potassium cyanide standard solution; Absorb 10.00mL. Potassium cyanide solution into 100mL conical bottle, add 1mL sodium hydroxide solution (20g/L), so that the pH is above 11, add 0.1mL. Test silver line indicator (0.2g/L). Titrate with silver nitrate standard solution [c(AgNO3)=0.019 20 mol/L] until the solution changes from yellow to orange. The number of milliliters of silver nitrate solution consumed is the mass number of cyanide (measured in CN) in the 10.00mL potassium cyanide standard solution (in mg).
Methyl orange indicator (0.5g/L): Weigh 50mg methyl orange, dissolve in pure water, and dilute to 100mL.
Phenolphthalein solution (1g/L).
Instrument and equipment
All glass still :500mL;
Colorimetric tube with stopper: 25mL and 50mL;
Macylab Analysis V-1300 spectrophotometer
Test procedure
General rule
There are glass and plastic containers for cleaning:
a) The container shall be soaked in nitric acid solution (1+99) for at least 24 h before use;
b) Rinse with water at least three times.
Do not soak polyamide containers with acid.
Preparation of calibration solution
Another 9 25mL colorimetric tubes with stopper were added into the standard solution of potassium cyanide [ρ(CN-)=1.00 μg/mL]0 mL, 0.10 mL, 0.20 mL, 0.40 mL, 0.60 mL, 0.80 mL, 1.00 mL and 1.50, respectively mL and 2.00mL, adding sodium hydroxide solution (12g/L) to 10.0mL. Cyanide quality respectively 0 mu, 0.1 mu g and 0.2 g mu, 0.4 mu g and 0.6 g, 1.0 mu, 0.8 mu mu g g g mu mu, 1.5 g and 2.0 g (in terms of CN -).
Measure
Test of sample
The 10.0mL distilled absorption solution was absorbed and placed in a 25mL stopper colorimetric tube.
Add 1 drop of phenolphthalein solution (1g/L) to each water sample and standard series pipe, and adjust with acetic acid solution (3+97) until the red color just disappears. (Note: The test shows that the pH value of the solution is in the range of 5~8, and the pH value of the color developing solution can be 5.6~6.0 after adding the buffer solution. Maximum and stable absorbance under this condition.)
Add 3.0mL potassium dihydrogen phosphate solution (136g/L) and 0.25mL chloramine T solution (10 g/L) to each tube and mix well.
After placing for 1 min to 2min, 5.0ml isonicotinic-barbiturate reagent was added to each tube, and the solution was colored at 25℃ for 15 min. (Note: The solution can obtain the maximum absorbance at 25℃ for 15min and can be stable for 30min.)
The absorbance was measured at 600nm wavelength in a 3cm colorimetric dish with pure water as the reference.
The cyanide quality in the sample tube can be detected by drawing a standard curve.
The mass concentration of cyanide (in CN) in the water sample is calculated as follows:
ρ(CN-)=m×V1V×v2
Formula:
ρ(CN-)- The mass concentration of cyanide (measured in CN) in a water sample in milligrams per liter (mg/L);
m -- The mass of cyanide (measured in CN-) in the sample tube, measured in micrograms (μg), obtained from the standard curve;
V1 - total volume of distillate, in milliliters (mL);
V -- volume of water sample, in milliliters (mL);
V -- volume of distilled liquid used for color development, in milliliters (mL).