1 Application Scope
1.1 This standard is applicable to determination of surface water and industrial waste water.
1.2 Determination scope
Volume of the test portion is 50mL. Cuvette with 30mm optical path is used, the minimum detectable amount of this method is 0.2μg Chromium (VI), and the minimum detectable concentration is 0.004mg/L; and the detectable upper concentration limit is 1.0mg/L.
1.3 Interference
When the iron content is greater than 1mg/L, it will show yellow. molybdenum (VI) and mercury (Hg) also react with color reagent to generate colored compound, however, under the color rendering acidity of this method, the reaction is insensitive, and the molybdenum and mercury (Hg) with concentration reaching 200mg/L will not interfere determination; while for vanadium, when its content is greater than 4mg/L, the color rendering will be interfered. 10min after reaction between vanadium and the color reagent, the color may fade automatically.
2 Principle
Chromium (VI) will react with 1, 5 diphenylcarbohydrazide to generate aubergine compound in acid solution, and the spectrophotometric determination will be conducted at 540nm wave length.
3 Reagent
Unless otherwise stated, analytical reagent conforming with national standards or professional standards and distilled water or water with equivalent purity shall be adopted during the determination, and moreover, no reagent shall contain chromium.
3.1 Acetone.
3.2 Sulfuric acid
3.2.1 1+1 sulfuric acid solution.
Sulfuric acid (H2SO4, ρ=1.84g/mL, guaranteed reagent) is added to water of the same volume slowly and mixed uniformly.
3.3 phosphoric acid: 1+1 phosphoric acid solution.
Phosphoric acid (H3PO4, ρ=1.69g/mL, guaranteed reagent) is mixed with water of the same volume.
3.4 Sodium hydroxide: 4g/L sodium hydroxide solution.
1g sodium hydroxide (NaOH) is dissolved into water and diluted to 250mL.
3.5 Zinc hydroxide coprecipitator
3.5.1 Zinc sulfate: 8% (m/V) zinc sulfate solution.
8g zinc sulfate(ZnSO4·7H2O) is weighed to dissolve in 100mL water.
3.5.2 Sodium hydroxide: 2% (m/V) solution
2.4g sodium hydroxide is weighed to dissolve in 120mL water.
The two kinds of solution in Articles 3.5.1 and 3.5.2 are mixed.
3.6 Potassium permanganate: 40g/L solution.
4g potassium permanganate (KMnO4) is weighed and dissolved in water while heated and stirred, and then it is diluted until 100mL.
3.7 Chromium standard stock solution.
0.2829±0.0001g potassium dichromate (K2Cr2O7, guaranteed reagent), dried for 2h at 110℃, is weighed, and it is dissolved with water before transferred into 1000mL volumetric flask, diluted with water to the mark line, and shaken well. 1mL this solution contains 0.10mg Chromium (VI).
3.8 Chromium standard solution.
5.00mL chromium standard stock solution (Section 3.7) is pipeted to 500mL volumetric flask, and is diluted with water to the mark line and shaken well. 1mL this solution contains 1.00μg Chromium (VI), and it is prepared on the using day.
3.9 Chromium standard solution.
25.00mL chromium (Cr) standard stock solution (Section 3.7) is pipeted and placed in a 500mL volumetric flask, and then diluted with water to the scale, and shaken well.1mL this solution contains 5.00μg Chromium (VI), and it is prepared on the using day.
3.10 Urea: 200g/L urea solution.
20g urea [(NH2)2CO] is dissolved into water and diluted until 100mL.
3.11 Sodium nitrite: 20g/L solution.
2g sodium nitrite (NaNO2) is dissolved into water and diluted until 100mL.
3.12 Color reagent (I).
0.2g 1, 5 diphenylcarbohydrazide (C13H14N4O) is weighed and dissolved into 50mL acetone (Section 3.1), and then diluted with water to 100mL and shaken well, later, it is stored in brown bottle and places in refrigerator. It cannot be used when color becomes deep.
3.13 Color reagent (II).
2g 1, 5 diphenylcarbohydrazide is weighed and dissolved into 50mL acetone (Section 3.1) before diluted with water to 100mL and shaken well. It is then stored in brown bottle and placed in refrigerator. It can not be used if its color gets deep.
Note: color reagent (I) may also be prepared according to the following method: 4.0g phthalate anhydride (C6H4O) is weighed and added into 80mL ethanol. It is stirred to dissolve (it may be slightly warmed by water bath where necessary), before 0.5g 1, 5 diphenylcarbohydrazide is added, then, it is diluted with ethanol until 100mL. This solution can be preserved for 6 months at dark place, and it shall be shaken well immediately after color reagent is added at use, so as to avoid reduction of Chromium (VI).
4 Apparatus
General laboratory apparatus and:
4.1 Spectrophotometer.
Note: inner wall of all glass ware must be smooth and clean, so as to avoid adsorption of chromium ion. Potassium dichromate washing liquor shall not be used for the washing, while nitric acid, sulfuric acid mixed solution or synthetic detergent can be used, however, it shall be rinsed clear after washing.
5 Sampling and Sample
Laboratory sample shall be collected with glass bottle, and sodium hydroxide is added to regulate its pH value close to 8 during collecting. The pH shall be determined as soon as possible after collection and in case of storage, it shall not be longer than 24h.
6 Procedures
6.1 Pretreatment of sample
6.1.1 The sample, excluding suspended matter, is clean surface water with low colority, and can be determined directly.
6.1.2 Colority calibration: if the sample is colored and it is not very deep, another portion of sample will be taken according to the procedures in Section 6.3, and 2mL acetone (Section 3.1) will replace color reagent, however, the other procedures will be the same as Section 6.3. It is calculated after the determined absorbance of test portion deducts the calibrated absorbance.
6.1.3 Zinc salt settling separation method: it can be used to pretreat turbid sample with deeper colority.
Proper mass of sample (containing Chromium (VI) less than 100μg) is taken to a 150mL beaker, and then water is added to 50mL. Sodium hydroxide solution (Section 3.4) is dropwise add to regulate the pH value to 7~8. Zinc hydroxide coprecipitator (Section 3.5) is then dropwise add with continuous stirring to regulate the pH value of the solution to 8~9. The solution is transferred to 100mL volumetric flask and diluted with water to the mark line. Slow filter paper is used to dry filter it, and then 10~20mL primary filtrate is discarded; later, 50.0mL filtrate is taken for determination.
Note: after the sample is pretreated by zinc salt settling separation method, if it still contains organic substance which interferes the determination, acidic potassium permanganate oxidization method may be used to destroy the organic substance before the determination. Namely, 50.0mL filtrate will be taken to a 150mL conical flask, and then several glass beads are added before 0.5mL sulfuric acid solution (Article 3.2.1) and 0.5mL phosphoric acid solution (Section 3.3) are added and shaken well.2 drops of potassium permanganate solution (Section 3.6)is added, and the aubergine disappear, Potassium permanganate solution shall be added to maintain the aubergine. Then the solution is heated to boil to left with about 20mL, later it will be filtered with quantitative medium-texture filter paper after taken down and gets cold. Washed with water by several times, the filtrate and washing liquid will be merged with each other to a 50mL colorimetric cylinder. 1mL urea solution (Section 3.10) is added and shaken well, and then sodium nitrite solution (Section 3.11) is dropwise added with a dropper as shaken until aubergine of the potassium permanganate disappear exactly. A moment later, it will be transferred to 50mL and diluted with water to the mark line, after bubbles in the solution are clear, for determination.
6.1.4 Elimination of reducing substances like Ferrum (II), sulfite and thiosulfate: proper mass of sample [containing Chromium (VI) less than 50μg]is taken to a 50mL colorimetric cylinder and diluted with water to the mark line before 4mL color reagent (II) (Section 3.13) is added. It is mixed uniformly and then placed for 5min before 1mL sulfuric acid solution (Section 3.2) is added and shaken well. 5~10min later, the absorbance is determined at 540nm wave length, with cuvette of 10 or 30mm optical path and water as reference. After the absorbance determined in the blank test is deducted, the content of the Chromium (VI) can be examined for the calibration curve, and the calibration curve is drafted by the same method.
6.1.5 Elimination of subtances with oxidizability like hypochlorite: proper mass of sample (containing Chromium (VI) less than 50μg) is taken to a 50mL colorimetric cylinder and diluted with water to the mark line before 0.5mL sulfuric acid solution (Section 3.2), 0.5mL phosphoric acid solution (Section 3.3) and 1.0mL urea solution (Section 3.10) are added and shaken well, and then 1mL sodium nitrite solution (Section 3.11) is added drop by drop ad shaking, so as to remove the bubble generated by the reaction between the excessive sodium nitrite and urea. After the bubbles are removed, the following procedures are the same as those in Section 6.3 (sulfuric acid solution and phosphoric acid solution are added no more).
1 Application Scope
2 Principle
3 Reagent
4 Apparatus
5 Sampling and Sample
6 Procedures
7 Result Expression
Additional Explanation
