WATER QUALITY: Analytical Methods - chromium method (1236-73) July 23, l973 QUALITY OF WATER BRANCH TECHNICAL MEMORANDUM NO. 74.02 Subject: WATER QUALITY: Analytical Methods - chromium method (1236-73) Enclosed are copies of a new direct atomic absorption spectrophotometric (AAS) method for chromium in water. This method (1236-73) will determine as little as 10 ug/l of chromium, and it is to be preferred when analyzing samples containing significant amounts of iron which has been found to interfere in the present AAS methods for chromium (TWRI Bk. 5, Ch. Al, pp. 76-77, and pp. 78-80). Iron interference, plus that of nickel, cobalt and magnesium which also interfere, can be eliminated or greatly minimized by adding amonium chloride solution to the sample before analysis by AAS. This proposed method provides for that addition . This method is hereby approved as a provisional method for adoption by all WRD laboratories; please initiate immediate use in your laboratory. You will note that the method is provisional; it will remain in this category until such time as reliable precision data can be obtained by round-robin testing of a standard reference water sample. R. J. Pickering Chief, Quality of Water Branch Enclosure WRD Distribution: A, FO-L, PO Water Chromium (1236-73) Atomic-absorption method--direct 1. Summary of method 1.1 Chromium is determined by atomic-absorption spectrophotometry by direct aspiration of the sample into an air-acetylene flame without preconcentration or pretreatment of the sample other than the addition of ammonium chloride to mask interferences. 2. Application 2 1 Samples containing at least l0 ug of chromium per liter may be analyzed by this method. Samples containing more than 400 ug/l must be diluted or less scale expansion used. 3. Interferences 3.1 Iron, nickel, and cobalt at 100 ug/l, and magnesium at 30 mg/l interfere by depressing the absorption of the chromium. These interferences are eliminated in solutions containing about 18,000 mg/l of ammonium chloride (Barnes, 1966, and Giammarise, 1966). Samples adjusted to this concentration of ammonium chloride show no interference from 700 mg/l of iron and 10 mg/l each of nickel, and cobalt, or from 1,000 mg/l of magnesium 3.2 Potassium concentrations greater than 500 mg/l enhance the chromium absorption. At 1,000 mg potassium per liter, the enhancement is approximately 15 percent. 3.3 Sodium (8,000 mg/l), calcium (4,000 mg/l), nitrate (100 mg/l), sulfate (8,000 mg/l), and chloride (10,000 mg/l) do not interfere 4. Apparatus 4.1 Atomic-absorption spectrophotometer, Perkin-Elmer Model 303, or equivalent. Chromium (1236-73) 4.2 With this instrument, the following operating conditions have been used: Grating. . . . . . . . . . Ultraviolet Wavelength counter . . . . . . . . . 357.9 (3579A) Slit 4 Source (hollow-cathode lamp) , . . . Cr Lamp current . . . . . . . . . . . , As specified on lamp Recorder readout Scale expansion. . . . . . . . . . . lOX Response time control. . . . . . . . 3 Burner . . . . . . . . . . . . . . . Boling Burner height. . . . . . . . . . . . Adjust for maximum sensitivity while aspirating a chromium standard solution Air pressure . . . . . . . . . . . . 28 psi, 6 5 on flowmeter Acetylene pressure . . . . . . . . . 8 psi, 8.5 on flowmeter 4.3 With these operating conditions, the following readings have been observed: Chromium concention (ug/l) Scale reading 1O 0 8 50 5.0 100 9.8 200 19.3 300 29.0 400 38 6 5 Reagents 5.1 Ammonium chloride solution: Dissolve 200 g NH4Cl in demineralized water and dilute to 1 liter. 5.2 Chromium standard solution I, 1.00 ml=100 ug Cr: Dissolve 0.2829 g reagent-grade K2Cr207 dried for 1 hr. at 150!C in demineralized water and dilute to 1,000 ml 5.3 Chromium standard solution II, 1.00 ml=l ug Cr: Dilute 10.0 ml Chromium standard solution I to 1,000 ml with demineralized water. 5.4 Chromium standard working solutions: Prepare a series of standard working solutions containing from O to 400 ug/l of chromium by diluting Chromium standard solution II. To each standard working solution, add 1.0 ml of NH4Cl solution for each 10 ml of standard. Chromium (1236-73) 6 Procedure 6 1 Filter thc sample (0 45-um membrane filter) when necessary to avoid clogging the atomizer-burner 6 2 Add 1.0 ml ammonium chloride solution to 10.0 ml sample and mix thoroughly 6,3 Aspirate and record the scale reading of each sample and standard. 7. Calculations 7.1 Determine the Cr concentration (ug/l) in the sample from a plot of scale readings of standards. Because a scale expansion of lOX is used, scale readings need not be converted to absorbances. Exact reproducibility is not obtained, and an analytical Curve must be prepared with each set of samples. 8. Report 8.1 Report Cr concentrations as follows: Less than 1,000 ug/l, nearest 10 ug/l; 1,000 ug and above, 2 significant figures. 9. Precision 9.1 Analysis of 3 test samples 6 times each, by one operator, resulted in mean values of 14, 21, and 56 ug/l, and standard deviations of 3, 5, and 5 ug/l, respectively References Barnes, L , Jr., 1966, Determination of chromium in low alloy steels by atomic absorption spectrometry: Anal. Chem., v. 38, p. 1083-1085. Giammarise, A., 1966, The use of ammonium chloride in analyses of chromium samples containing iron: Atomic Absorption Newsletter, v. 5, p. 113-115.