GB/T 223.26-2008Iron, steel and alloy - Determination of molybdenum content - The thiocyanate spectrophotometric method (English PDF)

3.2.9 Iron solution, 20 g/L. Weigh

2.0 g of pure iron (the molybdenum content, in mass fraction, must be less than 0.001%). Place it in a 250 mL beaker. Add 40 mL of mixed acid of sulfuric acid-phosphoric acid (3.2.5). Heat to dissolve it. Add nitric acid (3.2.2) dropwise to oxidize it. Heat it until sulfuric acid smoke is produced. Take it off and cool it slightly. Add 40 mL of water. Heat to dissolve the salt. Cool it to room temperature. Transfer it to a 100 mL volumetric flask. Use water to dilute it to the mark. Mix it uniformly. 1 mL of this solution contains 20 mg of iron. min ~ 3 min. Remove to slightly cool it. Add 20 mL of water. Heat to dissolve the salt. Cool it down. Transfer it into a 100 mL volumetric flask. Use water to dilute it to the mark. Mix it uniformly.

3.5.3.3 Pipette two sets of

10.00 mL of test solution into two 50 mL volumetric flasks, respectively [where the iron content is less than 30 mg, add iron solution (3.2.9) to make up].

3.5.3.4 Add 4 mL of sulfuric acid (3.2.3) and 10 mL of perchloric acid (3.2.6) to one set of solution. Mix it uniformly. Add 10 mL of sodium thiocyanate solution (3.2.8). Mix it uniformly. Add 10 mL of stannous chloride solution (3.2.7) whilst shaking it. Use sulfuric acid (3.2.4) to dilute it to the mark. Mix it uniformly. This is the color-developing solution. In the other test solution, except for adding the sodium thiocyanate solution (3.2.8), the rest operations are same as that of the color-developing solution. This is the reference solution.

3.5.3.5 Place it at room temperature for 10 min ~ 15 min. Transfer part of the solution into a 1 cm ~ 2 cm absorption dish. Use the reference solution as a reference. At the wavelength of 470 nm of the spectrophotometer, measure the absorbance. Subtract the absorbance of the blank solution as made along with the sample. From the calibration curve, check the corresponding mass of molybdenum (µg) in the color-developing solution.

3.5.4 Drawing of calibration curve Weigh 9 parts of

0.30 g of pure iron (molybdenum's mass fraction is less than 0.001%). Place them in a 250 mL conical flask. Respectively pipette 0,

0.50 mL,

4.00 mL of molybdenum standard solution (3.2.10). Add 40 mL of the mixed acid of sulfuric acid-phosphoric acid (3.2.5). Heat to dissolve it. Follow the procedures of 3.5.3.2 ~ 3.5.3.5, until measuring the absorbance. Subtract the absorbance of the compensation solution. Use the mass of molybdenum (µg) as the abscissa and the absorbance as the ordinate, to draw the calibration curve.

3.6 Calculation of results The molybdenum content is expressed in mass fraction wMo, the value is expressed in % and calculated according to formula (1): Where: V1 - The volume of the divided test solution, in milliliters (mL); When the tungsten in the specimen is less than 5 mg, use phosphoric acid to mask it. When the tungsten in the specimen is more than 5 mg ~ 20 mg, add 2 ~ 3 g of tartaric acid to mask it. When the molybdenum is less than 0.010%, the ratio of molybdenum to tungsten is not more than 1:80. In the pipetted solution, when copper is more than 5 mg, add thiourea to mask it. When antimony is more than

0.15 mg, it interferes with this method.

4.2 Reagents and materials Unless otherwise stated, only analytically pure reagents and distilled water or water of comparable purity are used in the analysis.

4.2.1 Tartaric acid.

4.2.2 Pure iron (molybdenum's mass fraction shall be less than 0.0003%).

4.2.3 Butyl acetate.

4.2.4 Hydrochloric acid, rho is about

4.2.5 Nitric acid, rho is about

4.2.6 Phosphoric acid, rho is about

4.2.7 Perchloric acid, rho is about

4.2.8 Sulfuric acid, rho is about

4.2.14 Molybdenum standard solution

4.2.14.1 Molybdenum stock solution, 500 µg/mL. Weigh 0.2500 g of pure molybdenum (mass fraction is more than 99.9%). Place it in a 250 mL beaker. Add 10 mL of nitric acid (1 + 3). Heat to dissolve it. Take it off to cool it slightly. Transfer it into a 500 mL volumetric flask. Use water to dilute it to the mark. Mix it uniformly. 1 mL of this solution contains 500 µg of molybdenum. 20 mg of tungsten, produce perchloric acid smoke until the volume is 2 mL ~ 3 mL].

4.5.3.2 Take off the conical flask. Cool it slightly. Add 20 mL of water to dissolve the salt [for the specimen which contains 5 mg ~ 20 mg of tungsten, add another 2 g ~ 3 g tartaric acid (4.2.1); stir to dissolve it; add sodium hydroxide solution (4.2.10) dropwise to dissolve the tungsten; then use sulfuric acid (4.2.8) to neutralize it to acidic]. Cool it to room temperature. Transfer it into a 100 mL volumetric flask. Use water to dilute it to the mark. Mix it uniformly.

4.5.3.3 Pipette

20.00 mL of the test solution. Place it in a 125 mL separatory funnel which contains 10 mL of sulfuric acid (4.2.9) in advance. Mix it uniformly. Add 5 mL of ammonium thiocyanate solution (4.2.12). Mix it uniformly. Add 10 mL of stannous chloride solution (4.2.13) [when the copper content is more than 5 mg, add 10 mL of thiourea (4.2.11)]. Mix it uniformly. When the iron thiocyanate fades to light red, immediately add

20.0 mL of butyl acetate (4.2.3). Shake it for 1 min. Let it stand for stratification. Discard the aqueous phase. Then along the mouth of bottle, add 5 mL of sulfuric acid (4.2.9). Mix it uniformly. Add 5 mL of stannous chloride (4.2.13). Shake it for 30 s. Let it stand for stratification. Discard the aqueous phase.

4.5.3.4 Use absorbent cotton to dry-filter the organic phase (discard the initial filtrate) in a 1 cm or 2 cm absorption dish (consistent with the calibration curve). Use butyl acetate (4.2.3) as a reference. Measure the absorbance at a wavelength of 470 nm of the spectrophotometer (when the specimen contains tungsten, the wavelength uses 500 nm). Subtract the absorbance of the blank solution along with the sample. From the calibration curve, check the corresponding mass of molybdenum (µg) in the color-developing solution.

4.5.4.1 Molybdenum's mass fraction of 0.0020% ~ 0.010% 4.5.4.1.1 Weigh 6 parts of

1.000 g of pure iron (4.2.2). Respectively place them in six 125 mL conical flasks. Add 0,

5.00 mL of molybdenum standard solution (4.2.14.3). The rest operation is carried out based on the tungsten and copper contents of sample according to 4.5.3.1 ~ 4.5.3.3. 4.5.4.1.2 Use absorbent cotton to dry-filter the organic phase (discard the initial filtrate) in a 2 cm absorption dish. Use butyl acetate (4.2.3) as a reference. At the wavelength of 470 nm of the spectrophotometer (when the specimen contains tungsten, use the wavelength of 500 nm), measure the absorbance. Subtract the absorbance of the reagent blank. Use the mass of molybdenum as the abscissa and the absorbance as the ordinate, to draw the calibration curve. m - The mass of the sample, in grams (g).

4.7 Precision The precision test in this part was carried out in 9 laboratories in 1990 for the determination of six levels of molybdenum content; each laboratory made 3 determination of the molybdenum content of each level under the repeatability conditions as specified in GB/T 6379.1. The raw data (measurement results) as reported by each laboratory are as shown in Appendix A (informative). According to GB/T 6379.2, carry out statistical analysis of the obtained measurement results. The precision is as shown in Table 4. Table 4 -- Precision results Mass fraction of molybdenum / % Repeatability limit r Reproducibility limit R 0.0025 ~

0.200 r = 0.0001290 + 0.1078m lgR = -1.1646 + 0.6538 lgm Where: m is the average of two measured values, in % (mass fraction). The repeatability limit r and reproducibility limit R are obtained according to the equation as given in Table 2. Under repeatability conditions, the absolute difference between the two independent test results obtained is not more than the repeatability limit (r). If it is more than the repeatability limit (r), it shall not exceed 5%; Under reproducibility conditions, the absolute difference between the two independent test results obtained is not greater than the reproducibility limit (R). If it is more than the reproducibility limit (R), it shall not exceed 5%.