⚗️ Total chromium in water and waste water — atomic absorption spectrometry: the nitrous oxide/acetylene flame (method A) and electrothermal atomisation (method B), 357.9 nm

Physicochemistry PN-EN 1233

Overview

The standard PN-EN 1233 implements EN 1233:1996 and is non-equivalent to ISO 9174:1990, which describes the same pair of methods. Chromium occurs in water in the oxidation states III and VI; both methods determine total chromium — the sum of both forms — as acid-soluble chromium or as dissolved chromium, depending on the preparation of the sample. The choice of the method depends on the expected concentration.

Method A (the flame method) is intended for water and waste water in the range 0.5–20 mg/l of chromium. Below 0.5 mg/l the determination may be carried out after careful evaporation of the acidified sample to a small volume, taking care not to bring about the precipitation of a residue — the standard warns however that concentrating intensifies the influence of interfering substances and below 0.1 mg/l it recommends method B.

Method B (the electrothermal method) covers the range 5–100 µg/l with an injection of 20 µl; higher concentrations are determined with a smaller injection volume. The chromium is determined directly in the acidified sample introduced into an electrically heated graphite tube; where necessary the standard addition technique is used. The tubes are to be pyrolytically coated and conditioned according to the manufacturer’s instructions; the L’vov platform is permitted. Because of the very small contents determined, the standard lays particular stress on the risk of contamination at every stage of sampling and analysis.

Interferences in the flame method: the ions tested do not interfere below the following concentrations: sulfates 10 000 mg/l, chlorides 12 000 mg/l, sodium 9000 mg/l, potassium 9000 mg/l, magnesium 2000 mg/l, calcium 2000 mg/l, iron 500 mg/l, nickel 100 mg/l, copper 100 mg/l, cobalt 100 mg/l, aluminium 100 mg/l, zinc 100 mg/l. The dry residue of the sample should not exceed 15 g/l and the conductivity 20 mS/cm. The influence of potential interferences is checked by analysing samples enriched with chromium with the addition of the interfering substance by the standard addition method.

In the Polish legal order the regulation on the conditions for discharging waste water into waters or into the ground (Dz.U. 2019 item 1311) names for total chromium the reference methods: atomic absorption spectrometry with flameless atomisation according to PN-EN ISO 15586, AAS with flame atomisation according to PN-EN 1233, atomic emission spectrometry ICP according to PN-EN ISO 11885 and ICP-MS according to PN-EN ISO 17294-2.

Method principle

Method A: measurement of the atomic absorption of chromium in the acidified sample introduced into a nitrous oxide/acetylene flame at a wavelength of 357.9 nm, with the addition of lanthanum in order to limit the influence of the matrix. The sample is prepared in two ways: for acid-soluble chromium — immediately after sampling it is acidified with nitric acid to pH 1–2, then to 90 ml of sample 1 ml of 30 % hydrogen peroxide and 1 ml of nitric acid are added, it is boiled and evaporated to about 50 ml (never to dryness), 10 ml of nitric acid 1.5 mol/l are added, it is transferred into a 100 ml flask, 10 ml of lanthanum chloride solution (20 g La/l) are added and it is made up with water; for dissolved chromium — the sample is filtered through a 0.45 µm membrane filter as soon as possible after sampling, the filtrate is acidified to pH 1–2, and a 100 ml flask with 10 ml of lanthanum solution is made up with the filtrate. The calibration is prepared from a standard solution of 50 µg Cr/ml — at least five solutions (0.5; 1; 2.5; 5 and 10 mg/l) with the addition of 2 ml of nitric acid and 10 ml of lanthanum solution per 100 ml. Method B: the chromium is determined directly in the acidified sample injected into an electrically heated graphite tube; the absorbance is measured at 357.9 nm, with background correction (non-specific absorption) and — where necessary — with the use of the standard addition technique. The calibration solutions (at least five, for example 5, 10, 15, 20 and 25 µg/l) are prepared from a solution of 0.5 µg Cr/ml immediately before use; the linearity of the curve has to be checked and, where there is a deviation, the range limited to the linear part. The stock chromium solution of 1.000 g/l is obtained from 2.825 ± 0.001 g of potassium dichromate dried for 2 h at 105 °C ± 2 °C, with the addition of 5 ml of nitric acid, made up to 1000 ml; it is stable for a year in the dark at pH 1–2. The glassware is soaked before use for 24 h in nitric acid 1.5 mol/l and rinsed with water — glassware cleaned with chromic mixture must not be used.

Applications

• Waste water from tanneries, electroplating shops and the metal industry — control of total chromium in water permits
• Surface water and groundwater in areas affected by industry — monitoring of contamination
• Drinking water — control of the parameter chromium (method B because of the low admissible concentrations)
• Leachate from landfills and process waters — determinations after mineralisation with hydrogen peroxide
• Laboratories carrying out determinations by the reference method according to Dz.U. 2019 item 1311 (PN-EN 1233 for flame AAS)

Key parameters

Standard

Standard number

PN-EN 1233:2000 (EN 1233:1996)

Title (PL)

Jakość wody — Oznaczanie chromu — Metody absorpcyjnej spektrometrii atomowej

Title (EN)

Water quality — Determination of chromium — Atomic absorption spectrometric methods

Step-by-step procedure

1. Sampling

   Take the samples into polyethylene or borosilicate glass bottles previously washed with nitric acid 1.5 mol/l and rinsed with water. When determining very small concentrations (method B) take particular care not to contaminate the sample at any stage.

2. Acid-soluble chromium

   Immediately after sampling acidify the sample with nitric acid to pH 1–2. To 90 ml of the acidified sample add 1 ml of 30 % hydrogen peroxide and 1 ml of concentrated nitric acid, bring to the boil and evaporate to about 50 ml — do not evaporate to dryness.

   🌡 Temperature: boiling

3. Making up with lanthanum (method A)

   To the evaporated solution add 10 ml of nitric acid 1.5 mol/l, transfer into a 100 ml volumetric flask, add with a pipette 10 ml of the lanthanum chloride solution, make up with water to the mark and mix. For dissolved chromium: make up a 100 ml flask containing 10 ml of the lanthanum solution with the filtrate from the 0.45 µm filter acidified to pH 1–2.

4. Calibration

   Before every series prepare at least five calibration solutions from the solution of 50 µg Cr/ml (method A: 0.5; 1; 2.5; 5 and 10 mg/l — 2 ml of HNO₃ and 10 ml of lanthanum per 100 ml each; method B: for example 5, 10, 15, 20 and 25 µg/l from the solution of 0.5 µg/ml, 1 ml of HNO₃ per 100 ml each, prepared immediately before use). Carry out a blank by the same route as the samples.

5. Measurement — method A

   Set the instrument according to the manufacturer’s instructions to 357.9 nm in the nitrous oxide/acetylene flame, optimise the aspiration, the height of the burner and the conditions of the flame on a calibration solution, zero the absorbance with water. Measure the series of standards and the blank, plot the graph, and then measure the samples — after each measurement aspirate water and where necessary correct the zero.

6. Measurement — method B

   Inject into the graphite tube a volume of the calibration solution giving a satisfactory response (typically 20 µl), measure the blank and the standards, check the linearity of the curve. Measure the samples with background correction; where an influence of the matrix is suspected apply the standard addition.

7. Calculation and check of the interferences

   Read the concentrations off from the curve for the sample and for the blank, calculate the result in mg/l (method A) or µg/l (method B) taking into account the volume of the sample and the dilutions. Check the influence of suspected interferences by analysing samples enriched with chromium with the addition of the interfering substance by the standard addition method.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• The nitrous oxide/acetylene flame requires absolute observance of the safety instructions of the manufacturer of the spectrometer — an incorrect order of ignition and extinguishing risks an explosion.
• Potassium dichromate (the standard) contains chromium(VI) — a carcinogenic and sensitising substance; weigh it out in gloves, under a fume hood, collect the solutions as hazardous waste.
• The dissolution of lanthanum oxide in hydrochloric acid is strongly exothermic — carry it out carefully, in a vessel resistant to the temperature.
• Hydrogen peroxide 30 % is strongly oxidising and corrosive; do not mix it with organic materials.
• Do not use glassware cleaned with chromic mixture — the residues of chromium falsify the result and are hazardous waste.
• Carry out the evaporation with nitric acid under a fume hood; never bring the sample to dryness.

Frequently asked questions