ISO-11083-1994.pdf
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1、INTERNATIONAL STANDARD IS0 11083 First edition 1994-08-l 5 Water quality - Determination of chromium(VI1 - Spectrometric method using 1,5-diphenylcarbazide Qualit de I eau - Dosage du chrome(VI) - MBthode par spectrom it shall be between 7.5 and 8,0. If the pH is outside this range, adjust with sodi
2、um hydroxide solution (3.2) or phosphoric acid solution A (3.3) accordingly. Add 1 ml of aluminium sulfate solution (3.5) and mix. Check the pH; it shall be between 7,0 and 7.2. If the pH is outside this range, adjust with phosphoric acid solution A (3.3). Allow the precipitate to settle for at leas
3、t 2 h. Decant the supernatant and filter 200 ml through a membrane filter (4.2), discarding the first 50 ml of filtrate. 5.2 Samples in the presence of oxidizing or reducing substances Collect a 1 000 ml sample in a glass bottle, add 10 ml of buffer solution (3.1) and mix. Measure the pH using the e
4、quipment (4.3); it shall be between 7,5 and 8.0. If the pH is outside this range, adjust with sodium hydroxide solution (3.2) or phosphoric acid solution A (3.3), accordingly. Add 1 ml of aluminium sulfate solution (3.5) and mix. Check the pH; it shall be between 7.0 and 7.2. If the pH is outside th
5、is range, adjust with phosphoric acid solution A (3.3). Add 1 ml of sulfite solution (3.6), checking for an excess of sulfite using sulfite test paper (3.7). If there is no excess of sulfite, add more sulfite solution until an excess is obtained. Copyright International Organization for Standardizat
6、ion Provided by IHS under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/26/2007 01:36:11 MDTNo reproduction or networking permitted without license from IHS -,-,- 0 IS0 IS0 11083:1994(E) Allow the precipitate to settle for at least 2 h. Decant the supernatant and
7、 filter 200 ml through a membrane filter (4.2), discarding the first 50 ml of filtrate. 7 Procedure 7.1 Procedure in the absence of oxidizing or reducing substances 6 Interferences In the presence of lead, barium and silver ions (salts), chromates of low solubility may be formed and the chromium(W)
8、contained in them will not be determined. Hexavalent molybdenum and mercury salts also form a yellow or blue colour respectively with the reagent, but the intensities are much lower than for chromium(Vl). Iron(lll) forms a yellow colour at concentrations above 1 mg/l and vanadium forms a yellow colo
9、ur that fades. Chromium(lll) and other interfering metal ions are precipitated in a phosphate-buffered solution using aluminium sulfate as a precipitation aid and are removed by filtration. Transfer 50 ml (volume V) of the filtrate (5.1) to a 100 ml volumetric flask. Add 2 ml of phosphoric acid solu
10、tion B (3.4) and 2 ml of diphenylcarbazide solution (3.8) and dilute to 100 ml with water. Measure the absorbance after 5 min to 15 min at a wavelength between 540 nm and 550 nm using water in the reference cell (absorbance A,), and cells with an optical path length of 40 mm or 50 mm for concentrati
11、ons below 0.5 mg/l, and of 10 mm for concentrations between 0,5 mg/l and 3 mg/l. If the concentration is higher than 3 mg/l, repeat the determination, using a smaller aliquot of the filtrate (volume v). For the blank value, carry out a blank test in parallel with the determination using water in pla
12、ce of the sample (absorbance At,). Valency changes of chromium due to the presence of oxidizing or reducing substances can be avoided by using the following pretreatment steps. Oxidizing substances are reduced by the addition of sulfite to the neutralized sample; chromium(Vl) will not react under th
13、ese conditions. Excess sulfite and other reducing substances are then oxidized with hypochlorite. Excess hypochlorite and any chloro- amines formed are destroyed in the acid solution by the addition of sodium chloride and the chlorine formed is purged with air. NOTE 2 This blank does not take into a
14、ccount the chromium content of the precipitation reagents, which has been found to be negligible. If the measured blank disagrees significantly with the blank from the calibration function (7.3), check the latter. If the filtrate IS coloured or turbid, take another aliquot and treat it as described
15、in this subclause, omitting the 1,5-diphenylcarbazide solution. Use the absorb- ante measured as a colour correction (absorbance At). In spite of this pretreatment of the sample, slow reduction of chromium(VI) may occur in some waters. Leachate from landfills, raw domestic waste water and certain wa
16、ste waters from chemical plants will show losses of chromium(W) after several hours. It is therefore essential that these samples be analysed as soon as possible after collection. Ammonia nitrogen does not interfere in concen- trations below 500 mg/l, but amine compounds may be transformed by hypoch
17、lorite into chloroamines, which are not always decomposed by the addition of chloride. This interference is indicated by the appearance of a yellow or brownish colour on addition of 1,5-diphenylcarbazide. 7.2 Procedure in the presence of oxidizing or reducing substances Transfer 50 ml (volume VI of
18、the filtrate (5.2) to a 100 ml volumetric flask. Add 1 ml of sodium hypochlorite solution (3.91, after checking for 1 min for excess chlorine using the potassium iodide starch test paper (3.10). If there IS no excess chlorine, add more sodium hypochlorite solution (3.9) until an excess is obtained.
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