WATER QUALITY: Analytical Methods - Determination of Chromium (VI) (1232-73) and Chromium (III+VI) (1238-73) October 3, 1973 QUALITY OF WATER BRANCH TECHNICAL MEMORANDUM NO. 74.03 Subject: WATER QUALITY: Analytical Methods - Determination of Chromium (VI) (1232-73) and Chromium (III+VI) (1238-73) Enclosed are copies of revisions of the Chromium (VI) and Chromium (III+VI) methods as given in TWRI, Bk. 5, Ch. Al, Atomic- absorption spectrophotometric method--chelation-extraction, pp. 76-77, and pp. 78-80, respectively. The revisions include only: (l) Addition of paragraph 2.2 to both methods--this paragraph cautions against use of the methods to analyze samples containing iron at concentrations exceeding 5 mg/l, and (2) changing paragraph "3. Interferences" in both methods to point out that iron at concentrations greater than 5 mg/l interfers by suppressing the chromium absorption. All Division laboratories are hereby requested to adopt the subject revised methods for immediate use. R. J. Pickering Chief, Quality of Water Branch Enclosure WRD Distribution: A, FO-L, PO Water Chromium (Vl) (1232-73) Atomic-absorption spectrophotometric method--chelation-extraction 1. Summary of method 1.1 Chromium is determined by atomic-absorption spectrophotometry. The element is chelated with ammonium pyrrolidine dithiocarbamate (APDC) and extracted with methyl isobutyl ketone (MIBK). The extract is aspirated into the flame of the spectrophotometer (Midgett and Fishman, 1967). 2. Application 2.1 The method may be used to analyze samples containing from 1.0 to 25 ug of chromium per liter. Samples containing more than 25 ug/l may be either diluted or aspirated directly into the spectrophotometer without chelation and extraction (1236-73). 2.2 If the iron concentration of the sample exceeds 5 mg/l, determine chromium (III&VI) by direct atomic absorption (1236-73). 3. Interferences 3.1 Concentrations of iron greater than 5 mg/l interfere by Suppressing the chromium absorption. 4. Apparatus 4.1 Atomic-absorption spectrophotometer, Perkin-Elmer Model 303, or equivalent. 4.2 With this instrument the following operating conditions have been used: Grating Ultraviolet. Wavelength counter 357.9 (3579A). Slit 3. Source (hollow-cathode lamp) Cr. Lamp current As specified on lamp. Scale expansion lOX. Response time control 1. Burner Boling. Air pressure 28 psi; 7.0 on flowmeter. Acetylene pressure 8 psi; 5.0 on flowmeter. 4.3 With these operating conditions the following readings have been observed: Chromium (VI) (1232-73) Chromium concentration (ug/l) Scale reading 5.0 18.8 10 36.8 15 54.2 20 73.8 25 86.5 5. Reagents 5.1 Ammonium pyrrolidine dithiocarbamate (APDC) solution, 1.0 g/100 ml: Dissolve 1.0 g APDC in demineralized water and dilute to 100 ml. Prepare fresh daily. 5.2 Bromphenol blue indicator solution, 0.1 g/100 ml: Dissolve 0.1 g bromphenol blue in 100 ml 5O-percent ethanol. 5.3 Chromium standard solution I, 1.00 ml=100 ug Cr: Dissolve 0.2829 g pure dried K2Cr207 in demineralized water and dilute to 1,000 ml. 5.4 Chromium standard solution II, 1.00 ml=10.0 ug Cr: Dilute 100 ml chromium standard solution I to 1,000 ml with demineralized water. 5.5 Chromium standard solution III, 1.00 ml=O.10 ug Cr: Dilute 10.0 ml chromium standard solution II to 1,000 ml with demineralized water. 5.6 Methyl isobutyl ketone (MIBK). 5.7 Sodium hydroxide solution, lM: Dissolve 40 g NaOH in demineralized water and dilute to 1 liter. 5.8 Sulfuric acid, 0.12M: Slowly add 6.5 ml concentrated H2S04 (sp gr 1.84) to demineralized water and dilute to 1 liter. 6. Procedure Samples should be collected according to instructions given in "Part I, Sample Collection and Treatment." 6.1 Pipet a volume of sample containing less than 2.5 ug chromium (100 ml maximum) into a 200-ml volumetric flask, and adjust the volume to approximately 100 ml. 6.2 Prepare a blank and sufficient standards, and adjust the volume of each to approximately 100 ml. Chromium (VI) (1232-73) 6.3 Add 2 drops bromphenol blue indicator solution. 6.4 Adjust the pH by addition of lM NaOH solution by drops until a blue color persists. Add 0.12M H2S04 by drops until the blue color just disappears in both the standards and sample. Then add 2.0 ml 0.12M H2S04 in excess. The pH at this point should be 2.4 (NOTE 1). NOTE 1. The pH adjustment in steps 6.3 and 6.4 may be made with a pH meter instead of using indicator. 6.5 Add 5.0 ml APDC solution and mix. The pH at this point should be 2.8. 6.6 Add 10.0 ml MIBK and shake vigorously for 3 min. 6.7 Allow the layers to separate and add demineralized water until the ketone layer is completely in the neck of the flask. The Cr-APDC complex is stable for at least 36 hr. 6.8 Aspirate the ketone layer and record the scale reading for each sample and standard against the blank. Repeat, and average the duplicate results. 7. Calculations 7.1 Determine the ug/l Cr+6 in each sample from a plot of scale readings of standards. Because a scale expansion of lOX is used, it is not necessary to convert scale readings to absorbance. Exact reproducibility is not obtained, and an analytical curve must be prepared with each set of samples. 8. Report 8.1 Report Cr(VI) concentrations as follows: Less than 1,000 ug/l, nearest ug/l. 9. Precision 9.1 Precision obtainable with this method should be equal to that for "Chromium, Hexavalent and Tervalent, Atomic-Absorption Method- -Chelation-Extraction (1238-73)." References Midgett, M. R., and Fishman, M. J., 1967l Determination of total chromium in fresh waters by atomic absorption: Atomic Absorption Newsletter, v. 6, p. 128-131. Water Chromium (III+VI) (1238-73) Atomic-absorption spectrophotometric method--chelation-extraction 1. Summary of method 1.1 The procedure used #or determination of chromium is the same as that for hexavalent chromium, except for an additional step to oxidize any tervalent chromium present to the hexavalent state. A modification of a method described by Saltzman (1952), whereby chromium i8 oxidized by potassium permanganate, has been found satisfactory. Excess permanganate is reduced with sodium azide, which must not be present in excess as it interferes with subsequent pH adjustment and chelation. 2. Application 2.1 Water containing from 1.0 to 25 ug of chromium per liter may be analyzed by this method, whereas concentrations greater than 25 ug/l must be determined either by dilution or directly on the aqueous sample. 2.2 If the iron concentration of the sample exceeds 5 mg/l, determine chromium by direct atomic absorption (1236-73) 3. Interferences 3.1 The optimum pH for the extraction of the Cr-APDC complex by MIBK is 3.1. At this pH, however, manganese is also partially extracted. The Mn-APDC complex is unstable and decomposes to a fine suspension of manganese oxides which clog the atomizer- burner. If the pH of the sample is adjusted to 2.4 prior to chelation and extraction, less manganese is extracted, and there is only a slight loss in extraction efficiency for chromium. If the extract is not clear after overnight standing, it must be centrifuged. 3.2 Concentrations of iron greater than 5 mg/l interfere by suppressing the chromium absorption. 4. Apparatus 4.1 See "Chromium, Hexavalent, Atomic-Absorption Method-- Chelation-Extraction (1232-73)." 5. Reagents 5.1 Ammonium pyrrolidine dithiocarbamate (APDC) solution, 1.0 g/100 ml: Dissolve 1.0 g APDC in demineralized water and dilute to 100 ml. Prepare fresh.daily. Chromium (III+VI) (1238-73) 5.2 Bromphenol blue indicator solution, 0.1 g/100 ml: Dissolve 0.1 g bromphenol blue in 100 ml 50-percent ethanol. 5.3 Chromium standard solution I, 1.00 ml=80 ug Cr+6: Dissolve 0.2263 g pure dried KzCr207 in demineralized water and dilute to 1,000 ml, 5.4 Chromium standard solution II, 1.00 ml=2.00 ug Cr+3: Pipet 5.0 ml chromium standard solution I into an erlenmeyer flask. Add approximately 15 mg Na2S03 and 0.5 ml concentrated HN03 (sp gr 1.42). Gently evaporate just to dryness; strong heating reoxidizes the Cr. Add 0.5 ml concentrated HN03 and again evaporate to dryness to destroy any excess sulfite. Dissolve the residue in 1 ml concentrated HN03 with warming and dilute to 200 ml with demineralized water. 5.5 Chromium standard solution III, 1.00 ml=0.50 ug Cr+3: Dilute 25.0 ml chromium standard solution II to 100 ml with demineralized water. Prepare immediately before use. 5.6 Methyl isobutyl ketone (MIBK). 5.7 Potassium permanganate solution, 0.32 g/100 ml: Dissolve 0.32 g KMnO4, in demineralized water and dilute to 100 ml. Allow to stand several days and decant if necessary. 5.8 Sodium azide solution, 0.10 g/100 ml: Dissolve 0.10 g NaN3 in 100 ml demineralized water. 5.9 sodium hydroxide solution, lM: Dissolve 40 g NaOH in demineralized water and dilute to 1 liter. 5.10 Sulfuric acid, 0.12M: Slowly add 6.5 ml concentrated H2S04 (sp gr 1.84) to demineralized water and dilute to 1 liter. 6. Procedure Samples should be collected according to instructions given in Part I, Sample Collection and Treatment." 6.1 Pipet a volume of sample containing less than 2.5 ug chromium (100 ml maximum) into a 200-ml volumetric flask, and adjust the volume to approximately 100 ml. The pH must be 2.0 or less. Add concentrated HN03 if necessary. 6.2 Acidify a liter of demineralized water with 1.5 ml concentrated HN03. Prepare a blank and sufficient standards, and adjust volumes to approximately 100 ml with the acidified demineralized water. Chromium (III+VI) (1238-73) 6.3 Add KMnO4 solution by drops to blank, standards, and samples until a faint pink color persists. 6.4 Heat on a steam bath for 20 min. If the color disappears, add KMnO4 by drops to maintain a slight excess. 6.5 While still on the steam bath, add sodium azide solution by drops until the KMnO4 color just disappears. Heat for about 2 min between each addition and avoid adding any excess. Continue heating for 5 min after adding the last drop of sodium azide solution. 6.6 Transfer to a water bath and cool to room temperature. 6.7 Remove from the water bath and filter through Whatman No. 40 filter paper any sample which has a brownish precipitate or coloration which may interfere with the pH adjustment. 6.8 Add 2.0 ml lM NaOH and 2 drops bromphenol blue indicator solution. Continue the addition of lM NaOH by drops to all samples and standards in which the indicator change from yellow to blue has not occurred. Add 0.12M H2S04 by drops until the blue color just disappears, then add 2.0 ml in excess. The pH at this point should be 2.4 (NOTE 1). NOTE 1. The pH adjustment in step 6.8 may be made with a pH meter instead of using indicator. 6.9 Add 5.0 ml APDC solution and mix. The pH at this point should be 2.8. 6.10 Add 10.0 ml MIBK and shake vigorously for 3 min. 6.11 Allow the layers to separate and add demineralized water until the ketone layer is completely in the neck of the flask. 6.12 Stopper and allow to stand overnight. The Cr-APDC complex is stable for at least 36 hrs. 6.13 Aspirate the ketone layer and record the scale reading for each standard and sample against the blank. Repeat, and average the duplicate results. 7. Calculations 7.1 Determine the ug/l Cr in each sample from a plot of scale readings of standards. Because a scale expansion of lOX is used, it is not necessary to convert scale readings to absorbance. Exact reproducibility is not obtained, and a 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 ug/l. 9. Precision 9.1 Analysis of two test samples by 10 laboratories using this method resulted in means of 47.8 ug/l and 9.0 ug/l and standard deviations of 19.6 ug/l and 7.4 ug/l, respectively. References Saltzman, B., 1952, Microdetermination of chromium with diphenylcarbazide permanganate oxidation: Anal. Chemistry, v. 24, p. 1016.