Request for Comments

- Apr 13, 2020-

Normative references:


HJ 91.1 Technical Specifications for Sewage Monitoring, HJ 677 Water Quality Digestion of Total Metals, Nitric Acid Digestion Method.


Method principle


The sample is injected into a graphite furnace atomizer after digestion, and is dried, ashed, and atomized to form an indium ground state atomic vapor, which selectively absorbs characteristic spectral lines emitted by an indium hollow cathode lamp or other light sources. In a certain range, its absorbance is proportional to the mass concentration of indium.


Instruments and equipment


Graphite furnace atomic absorption spectrophotometer: with background correction function.


Indium hollow cathode lamp or other light source.


Temperature control electric heating plate: with temperature control function.


Instruments and equipment commonly used in general laboratories.

Interference and cancellation


Ag, Al, As, B, Ba, Be, Bi, Cd, Co, Cr, Cu, Mn, Mo, Ni, Pb, Se, Sr, Ti, Tl, V, Zn below 10 mg / L, low K, Na, Mg, Ca, Fe at 500 mg / L and chloride ions below 10,000 mg / L have no significant effect on the measurement results.


Preparation of samples


Measure 50.0 ml of the mixed water sample (8.2) into a 150 ml glass beaker, add 2 ml of nitric acid (6.1), place it on a temperature-controlled hot plate (7.3), cover the watch glass, and keep the solution temperature 95 ± 5 ℃ , Reflux without boiling for 30min, remove the watch glass and evaporate to dryness. Remove the cooling and repeat this process until the color of the sample solution becomes light or stable. After cooling, add 1 ml of nitric acid solution (6.5) to dissolve the precipitate. Rinse the inner wall of the beaker and watch glass with experimental water at least 3 times. Transfer the whole amount to a 50 ml volumetric flask. If there are insoluble particles in the sample, it can be left to stand, centrifuged or filtered with a water-based microporous membrane (6.12).


Drawing of standard curve


Pipette 0, 0.50, 1.00, 2.00, 3.00, 4.00, and 5.00 ml of indium standard use solution (6.10) into a 50 ml volumetric flask, dilute to the mark with a nitric acid solution (6.6), and shake well. The standard series concentrations are 0.0, 5.0, 10.0, 20.0, 30.0, 40.0, and 50.0 μg / L. According to the reference measurement conditions (9.1), add 20 μl standard solution and 5 μl matrix improver to the graphite tube in order from low concentration to high concentration, and measure the absorbance. Draw the standard curve with the absorbance as the ordinate and the indium mass concentration as the abscissa.

Calibration curve drawing method


Take 4 equal parts of the sample to be tested (concentration Cx), and prepare 4 parts of the solution with the same total volume. 1 part does not add standard solution, the remaining 3 parts are respectively added to different concentrations of standard solution in proportion, the solution concentrations are usually: Cx, Cx + Co, Cx + 2Co, Cx + 3Co; the concentration of Co added to the standard solution is about 0.5 times the amount The sample concentration is Co≈0.5Cx. Use a blank solution to zero, and sequentially measure the absorbance of 4 solutions under the same conditions. Take the absorbance as the ordinate, add the concentration of the standard solution as the abscissa, and draw a calibration curve. The intersection of the inverse extension of the curve and the abscissa is the test. Sample concentration. The relationship between the concentration of the test sample and the corresponding absorbance is shown in Figure A.1.


Precautions


This method is only applicable to areas where the concentration of the sample to be measured is linear with the absorbance. The volume error caused by adding the standard solution should not exceed 0.5%. This method can only compensate for the effects caused by the matrix effect, and cannot compensate for the effects of background absorption.