GB/T 4325.8-2013Methods for chemical analysis of molybdenum - Part 8: Determination of cobalt content - 5-Cl-PADAB spectrophotometry and flame atomic absorption spectrometry (English PDF)

4.1 Method summary

The sample is decomposed with hydrogen peroxide or sodium hydroxide solution; the main molybdenum is complexed with potassium and sodium tartrate. The cobalt reagent forms an orange-red complex with cobalt in a phosphate buffer solution at a pH of 6 ~ 7; the absorbance is measured in a phosphoric acid medium.

4.2 Reagents

4.2.1 Hydrogen peroxide (rho = 1.10 g/mL), excellent grade pure.

4.2.2 Nitric acid (1 + 1).

4.2.3 Sulfuric acid (1 + 1).

4.2.4 Phosphoric acid (rho = 1.70 g/mL), excellent grade pure.

4.2.5 Sodium hydroxide solution (200 g/L), excellent grade pure.

4.2.6 Potassium sodium tartrate solution (400 g/L), excellent grade pure.

4.2.7 4-[(5-Chloro-2-pyridine)azo]-1,3-diaminobenzene (cobalt reagent) ethanol solution (0.025%), stored in a brown bottle.

4.2.8 Buffer solution (pH 6 ~ 7): Weigh 35.5 g of disodium hydrogen phosphate and dissolve it in 500 mL of water; weigh 34 g of potassium dihydrogen phosphate and dissolve it in 500 mL of water; mix the two; prepare for use.

4.2.9 Phenolphthalein ethanol solution (1 g/L).

4.2.10 pH test paper (pH 1 ~ 14).

4.2.11 Cobalt standard storage solution: Weigh 0.1000 g of metallic cobalt (mass fraction >= 99.99%) in a 250 mL beaker; add 10 mL of nitric acid (4.2.2) to dissolve it; add 10 mL of sulfuric acid (4.2.3); heat until sulfuric acid white smoke appears; remove and cool it; use water to rinse the watch glass and the beaker wall; add 30 mL of water; boil it; cool it; use water to transfer it into a

4.5.4.2 Molybdenum trioxide, ammonium molybdate

Place the sample (4.5.2) in a 150 mL beaker; add 5 mL of sodium hydroxide solution (4.2.5); heat to dissolve until the solution is clear; cool it down. The following shall be carried out according to 4.5.4.1.2 ~ 4.5.4.1.4.

4.5.5 Drawing of working curve

4.5.5.1 Pipette 0 mL, 0.10 mL, 0.20 mL, 0.50 mL, 0.80 mL, 1.00 mL of cobalt standard solution (4.2.12), respectively, into a set of 50 mL colorimetric tubes.

Add 4 mL of potassium sodium tartrate solution (4.2.6); add 1 ~ 2 drops of phenolphthalein ethanol solution (4.2.9); add sodium hydroxide solution (4.2.5) dropwise to neutralize the solution until the solution is light purple; then add sulfuric acid (4.2.3) to make the purple fade (pH 6 ~ 7); use pH test paper to check it. Add water to control the volume within 30 mL. The following shall proceed according to 4.5.4.1.3.

4.5.5.2 Move part of the solution (4.5.5.1) into a 2 cm cuvette; take the reagent blank as a reference; measure its absorbance at a wavelength of 550 nm by a spectrophotometer. Draw a working curve with the content of cobalt as the abscissa and absorbance as the ordinate.

4.6 Calculation of analysis results

The cobalt content is calculated as the mass fraction of cobalt wCo; the value is expressed in % and calculated according to formula (1): […]

5.2 Reagents

5.2.1 Hydrogen peroxide (rho = 1.10 g/mL), excellent grade pure.

5.2.2 Hydrochloric acid (rho = 1.19 g/mL), excellent grade pure.

5.2.3 Nitric acid (1 + 1).

5.2.4 Citric acid solution (500 g/L), excellent grade pure.

5.2.5 Cobalt standard storage solution: Weigh 0.1000 g of metallic cobalt (>= 99.99%) in a 250 mL beaker; use a small amount of water to m; add 20 mL of nitric acid (5.2.3); cover a watch glass; heat it at a low temperature to dissolve it completely; remove and cool to room temperature; transfer to a 1000 mL volumetric flask; use water to dilute to the mark; mix it uniformly. This solution 1 mL contains 100 µg of cobalt.

5.2.6 Cobalt standard solution: Pipette 10.00 mL of cobalt standard stock solution (5.2.5) into a 100 mL volumetric flask; add 10 mL of nitric acid (5.2.3); use water to dilute to the mark; mix well. 1 mL of this solution contains 10 µg of cobalt.

5.2.7 Mo matrix: The mass fraction of cobalt contained is less than 0.0001%.

5.3 Apparatus

Atomic absorption spectrometer, which is equipped with cobalt hollow cathode lamp.

Under the best working conditions of the instrument, those which can achieve the following indicators can be used:

- Characteristic concentration: In a solution consistent with the measurement solution matrix, the characteristic concentration of cobalt shall not be greater than 0.035 µg/mL;

- Precision: Use the highest concentration of standard solution to measure the absorbance 10 times; the standard deviation shall not exceed 1.0% of the average absorbance; use the lowest concentration of standard solution (not the "zero" concentration standard solution) to measure the absorbance 10 times; the standard deviation shall not exceed 0.5% of the average absorbance of the highest concentration;

- The working curve's linearity: Divide the working curve into 5 equal segments according to the concentration, and; the ratio of the absorbance difference between the highest segment and the lowest segment shall not be less than 0.7. […]