## 3500-Cr CHROMIUM*

3500-Cr A. Introduction

1. Occurrence and Significance

Chromium (Cr) is the first element in Group VIB in the periodic table; it has an atomic number of 24, an atomic weight of 51.99, and valences of 0 and 2 through 6. The average abundance of Cr in the earth’s crust is 122 ppm; in soils, Cr ranges from 11 to 22 ppm; in streams, it averages about 1 g/L, and in groundwaters, it is generally 100  g/L. Chromium is found chiefly in chrome–iron ore (FeO  Cr2O 3 ). Chromium is used in alloys, in electroplating, and in pigments. Chromate compounds frequently are added to cooling water for corrosion control.

In natural waters, trivalent chromium exists as Cr 3 

, Cr(OH) 2  , Cr(OH) 2  , and Cr(OH) 4 ; in the hexavalent form, chromium exists as CrO 4 2

and as Cr2 O 7 2

. Cr 3  would be expected to form strong complexes with amines, and would be adsorbed by clay minerals.

Chromium is considered nonessential for plants, but an essential trace element for animals. Hexavalent compounds have been shown to be carcinogenic by inhalation and are corrosive to tissue. The chromium guidelines for natural water are linked to the hardness or alkalinity of the water (i.e., the softer the water, the lower the permitted level for chromium). The United Nations Food and Agriculture Organization-recommended maximum level for irrigation waters is 100  g/L. The U.S. EPA primary drinking water standard MCL is 100  g/L for total chromium.

2. Selection of Method

The colorimetric method (B) is useful for the determination of hexavalent chromium in a natural or treated water in the range from 100 to 1000 g/L. This range can be extended by appropriate sample dilution or concentration and/or use of longer cellpaths. The ion chromatographic method with photometric detection (C) is suitable for determining dissolved hexavalent chromium in drinking water, groundwater, and industrial wastewater effluents from 0.5 to 5000  g/L. The electrothermal atomicabsorption spectrometric method (3113B) is suitable for determining low levels of total chromium (  50  g/L) in water andwastewater, and the flame atomic absorption spectrometric methods (3111B and C) and the inductively coupled plasma methods (3120 and 3125) are appropriate for measuring total chromium concentrations up to milligram-per-liter levels.

3. Sample Handling

If dissolved chromium content is desired, then immediately after collection, filter sample through a 0.45-  m membrane filter, and acidify filtrate with conc HNO 3 to pH  2. Analyze within 6 months of collection.

If dissolved hexavalent chromium content is desired, adjust filtrate pH to between 9.3 and 9.7 with buffer solution plus 600


L 5 N NaOH per 100 mL of sample (3500-Cr.B.4 b ). Some samples may require more NaOH (5 N or 1 N ). Alternatively, use buffer without 5 N NaOH, and add 1 N NaOH as necessary within the first 24 h to bring sample pH within range. Never dilute sample volume by more than 10%. Refrigerate to  6°C. Analyze within 28 d of collection. If pH is not within proper range, then either analyze sample or adjust its pH within the 24 h.

If total chromium content is desired, then immediately after collection, acidify unfiltered samples with conc HNO 3 to pH  2. Analyze within 6 months of collection.

If total hexavalent chromium content is desired, adjust pH of unfiltered samples to 9 with buffer solution plus 600  L 5 N NaOH per 100 mL of sample (3500Cr.B.4 b ). Some samples may require more NaOH (5 N or 1 N ). Alternatively, use buffer with 5 N NaOH, and add 1 N NaOH as necessary within the first 24 h to bring sample pH within range. Never dilute sample volume by more than 10%. Refrigerate to  6°C. Analyze within 28 d of collection. If pH is not within proper range, then either analyze sample or adjust its pH within the first 24 h.



## 3500-Cr B. Colorimetric Method

1. General Discussion

a. Principle: This procedure measures only hexavalent chromium [Cr(VI)]. For total chromium determination, acid-digestthe sample (see Section 3030) and follow with a suitable instrumental analysis technique. Hexavalent chromium is determinedcolorimetrically by reaction with diphenylcarbazide in acid solution. A red-violet colored complex of unknown composition is produced. The reaction is very sensitive, the molar absorptivity based on chromium being about 40 000 L g 1 cm 1 at 530 or 540 nm. (N OTE : Validation data for Method 3500-Cr.B were developed at 540 nm. Validation data for Method 3500-Cr.C were developed at 530 nm.)

b. Interferences: The reaction with diphenylcarbazide is nearly specific for chromium. Hexavalent molybdenum and mercury salts will react to form color with the reagent, but the intensities are much lower than that for chromium at the specified pH. Concentrations of Mo or Hg as high as 200 mg/L can be tolerated. Vanadium interferes strongly, but concentrations up to 10 times that of chromium will not result in significant analytical error. Iron in concentrations greater than 1 mg/L may produce a yellow color, but the ferric ion (Fe 3  ) color is not strong and no * Approved by Standard Methods Committee, 2009. Joint Task Group: Rock J. Vitale (chair), C. Ellen Gonter, Timothy S. Oostdyk; 20th Edition—See 3500-Al.

1difficulty is encountered normally if the absorbance is measured photometrically at the appropriate wavelength.

2. Apparatus

a. Colorimetric equipment: One of the following is required: 1) Spectrophotometer, for use at 530 or 540 nm, with a light path of 1 cm or longer.

2) Filter photometer, providing a light path of 1 cm or longer and equipped with a greenish yellow filter having maximum transmittance at 530 or 540 nm.

b. Laboratory ware: Soak all reusable items (glass, plastic, etc.), including sample containers, overnight in laboratory-grade detergent, rinse, and soak for 4 h in a mixture of nitric acid (1 part), hydrochloric acid (2 parts), and reagent water (9 parts). Rinse with tap water and reagent water. (N OTE : Never use chromic acid cleaning solution.)

c. pH meter: Use any commerical pH meter. Standardize and calibrate according to manufacturer’s instructions. Pay special attention to temperature compensation and electrode care.

3. Reagents

Use reagent water (see Section 1080) for reagent preparation and analytical procedure.

a. Stock chromium solution: Dissolve 141.4 mg K2 Cr2 O 7 in water and dilute to 100 mL; 1.00 mL  500  g Cr. C AUTION :Hexavalent chromium is toxic and a suspected carcinogen. Handle with care.

b. Standard chromium solution: Dilute 1.00 mL stock chromium solution to 100 mL; 1.00 mL  5.00  g Cr. Prepare calibration standards at time of analysis.

c. Nitric acid (HNO 3 ) conc. d. Sulfuric acid (H 2 SO 4 ) conc, 18N , and 6N

. e. Sulfuric acid (H 2 SO 4 ) 0.2N: Dilute 17 mL 6N H 2 SO 4 to 500 mL with water.

f. Phosphoric acid (H 3 PO 4 ) conc. g. Diphenylcarbazide solution: Dissolve 250 mg 1,5-diphenylcarbazide (1,5-diphenylcarbohydrazide) in 50 mL acetone. Store in a brown bottle. Prepare weekly. Discard if the solution becomes discolored.

h. Sodium hydroxide , 1 N: Dissolve 40 g low-level metal impurity NaOH in 1 L water. Store in plastic bottle. Smaller volumes may be prepared.

i. Sodium hydroxide , 5 N: Dissolve 200 g low-level metal impurity NaOH in 1 L water. Store in plastic bottle. Smaller volumes may be prepared.

j. Buffer solution: Dissolve 33 g ammonium sulfate in 75 mL water. Add 6.5 mL ammonium hydroxide, and dilute to 100 mL with water. Smaller volumes may be prepared.

4. Procedure

a. Preparation of calibration curve: To compensate for possible slight losses of chromium during analytical operations, treat standards and samples with the same procedure. Accordingly, pipet measured volumes of standard chromium solution (5  g/ mL) ranging from 2.00 to 20.0 mL, to give standards for 10 to 100  g Cr, into 250-mL beakers or conical flasks. Depending on pretreatment used in ¶ b below, proceed with subsequent treatment of standards as if they were samples.

Develop color as for samples, transfer a suitable portion of each colored solution to a 1-cm absorption cell, and measure absorbance at 540 nm, using reagent water as reference. Correct absorbance readings of standards by subtracting absorbance of a reagent blank carried through the method.

Construct a calibration curve by plotting corrected absorbance values against micrograms chromium in 102 mL final volume.

b. Treatment of sample: Filter sample through a 0.45- m filter. Use a portion of sample to rinse syringe filter unit and filter, then collect the required volume of filtrate. Adjust pH to between 9.3 and 9.7 by adding 1 mL buffer plus 600  L 5 N NaOH per 100 mL sample. More NaOH (5 N or 1 N ) may be required to bring sample pH into range. Alternatively, use the buffer without the 5 N NaOH, and add 1 N NaOH as necessary within the first 24 hours to bring sample pH into range. Never dilute sample volume by more than 10%. Ship and store samples at  6°C. Bring to room temperature before analysis. Analyze samples within 28 d of collection. If pH is not within the proper range, either analyze the sample or adjust its pH within the first 24 hours. Drinking water samples do not require field filtration.

c. Color development and measurement: Bring samples to room temperature before analysis. Add 0.25 mL (5 drops) H 3 PO 4 . Use 0.2N H 2 SO 4 and a pH meter to adjust solution to pH 2.0  0.5. Transfer solution to a 100-mL volumetric flask, dilute to 100 mL, and mix. Add 2.0 mL diphenylcarbazide solution, mix, and let stand 5 to 10 min for full color development. Transfer an appropriate portion to a 1-cm absorption cell and measure its absorbance at 530 or 540 nm, using reagent water as a reference. Correct absorbance reading of sample by subtracting absorbance of a blank carried through the method (see also note below). From the corrected absorbance, determine micrograms chromium present by reference to the calibration curve.

(N OTE : If the solution is turbid after dilution to 100 mL in ¶ c )above, take an absorbance reading before adding carbazide reagent and correct absorbance reading of final colored solution by subtracting the absorbance measured previously.)

5. Calculation

mg Cr/L   g Cr (in 102 mL final volume)
A

where:

A  mL original sample.

6. Precision and Bias

Collaborative test data from 16 laboratories were obtained on reagent water, tap water, 10% NaCl solution, treated water from synthetic organic industrial waste, EPA extraction leachate, process water, lake water, and effluent from a steel pickle liquor treatment plant. 3 The test data yielded the following relationships:

Reagent water:

S t 0.037 x  0.006 S
o 0.022 x  0.004 CHROMIUM (3500-Cr)/Colorimetric Method

2Drinking or wastewater:

S t 0.067 x  0.004 S
o 0.037 x  0.002

Leachate:

S t 0.032 x  0.007 S
o 0.017 x  0.004

where:

St  overall precision, S
o  single-operator precision, and

x  chromium concentration, mg/L.

7. References

1. U.S. ENVIRONMENTAL PROTECTION A GENCY . 1996. Determination of

hexavalent chromium by ion chromatography. Method 1636. EPA 821-R-96-003, U.S. Environmental Protection Agency, Washington, D.C. 2. EATON , A., A. H AGHANI & L. RAMIREZ . 2001. The Erin Brockovich factor. In Proc. Water Quality Technology Conf. (Nashville, Tenn., Nov. 11–15, 2001). American Water Works Assoc., Denver, Colo. 3. AMERICAN S OCIETY FOR TESTING AND M ATERIALS . 1996. Chromium, total. D1687-92, Annual Book of ASTM Standards, Vol. 11.01. American Soc. Testing & Materials, Philadelphia, Pa. 8. Bibliography

R OWLAND , G.P., J R . 1939. Photoelectric colorimetry—Optical study of permanganate ion and of chromium-diphenylcarbazide system. Anal. Chem. 11:442. URONE , P.F. 1955. Stability of colorimetric reagent for chromium, 5-diphenylcarbazide, in various solvents. Anal. Chem. 27:1354. ALLEN , T.L. 1958. Microdetermination of chromium with 1,5-diphenylcarbohydrazide. Anal. Chem. 30:447. O NISHI , H. 1979. Colorimetric Determination of Traces of Metals, 4th ed. Interscience Publishers, New York, N.Y.



## 3500-Cr C. Ion Chromatographic Method

1. General Discussion

a. Principle: This method is applicable to determination of dissolved hexavalent chromium in drinking water, groundwater, and industrial wastewater effluents. An aqueous sample is filtered and its pH adjusted to 9 to 9.5 with a concentrated buffer. This pHadjustment reduces the solubility of trivalent chromium and preserves the hexavalent chromium oxidation state. The sample is introduced into the instrument’s eluent stream of ammonium sulfateand ammonium hydroxide. Trivalent chromium in solution is separated from the hexavalent chromium by the column. After separation, hexavalent chromium reacts with an azide dye to produce a chromogen that is measured at 530 or 540 nm. (NOTE : Validationdata for 3500-Cr.C were developed at 530 nm. Hexavalent chromium is identified on the basis of retention time.)

Although this method was developed using specific commercial equipment, use of another manufacturer’s equipment should be acceptable if appropriate adjustments are made.

b. Interferences: Interferences may come from several sources. Use analytical grade salts for the buffer because trace amounts of chromium may be included.

Several soluble species of trivalent chromium in the sample may be oxidized to the hexavalent form in an alkaline medium in the presence of such oxidants as hydrogen peroxide, ozone, and manganese dioxide. The hexavalent form can be reduced to the trivalent in the presence of reducing species in an acid medium.

High ionic concentration may cause column overload. Samples high in chloride and/or sulfate might show this phenomenon, which is characterized by a change in peak geometry.

Interfering organic compounds are removed by the guard column.

c. Minimum detectable concentrations: The method detection levels obtained in a single laboratory with a 250-  L loop were as follows: Reagent water 0.4  g/L Drinking water 0.3  g/L Groundwater 0.3  g/L Primary wastewater effluent 0.3  g/L Electroplating waste 0.3  g/L

d. Sample preservation and holding time: Filter sample through a 0.45-  m filter. Use a portion of sample to rinse syringe filter unit and filter, then collect the required volume of filtrate. Adjust pH to between 9.3 and 9.7 by adding 1 mL buffer plus 600  L 5 N NaOH per 100 mL of sample. More NaOH (5N or 1 N

) may be required to bring sample pH into range. Alternatively, use the buffer without the 5 N NaOH, and add 1 N NaOH as necessary within the first 24 h to bring sample pH into range. Never dilute sample volume by more than 10%. Ship and store samples at  6°C. Bring to room temperature before analysis. Analyze samples within 28 d of collection. If pH is not within the proper range, either analyze the sample or adjust its pH via one of the above methods within the first 24 h. Drinking water samples do not require field filtration.

2. Apparatus

a. Ion chromatograph equipped with a pump capable of precisely delivering a flow of 1 to 5 mL/min. The metallic parts of the pump must not contact sample, eluent, or reagent. Sample loops should be available, or the instrument should be capable of delivering from 50- to 250-  L injections of sample. The visible absorption cell should not contain metallic parts that contact the eluent-sample flow. The cell must be usable at 530 nm. Use plastic pressurized containers to deliver eluent and post-column reagent. Use high-purity helium (99.995%) to pressurize the eluent and post-column reagent vessel.

b. Guard column , to be placed before the separator column,containing an adsorbent capable of adsorbing organic comCHROMIUM (3500-Cr)/Ion Chromatographic Method

3pounds and particulates that would damage or interfere with the analysis or equipment.*

c. Separator column , packed with a high-capacity anionexchange resin capable of resolving chromate from other sample constituents.†

d. Recorder, integrator, or computer for receiving signals from the detector as a function of time.

e. Laboratory ware: Soak all reusable items (glass, plastic, etc.), including sample containers, overnight in laboratory-grade detergent, rinse, and soak for 4 h in a mixture of nitric acid (1 part), hydrochloric acid (2 parts), and reagent water (9 parts). Rinse with tap water and reagent water. (N OTE : Never use chromic acid cleaning solution.)

f. Syringe , equipped with male luer-type fitting and a capacity of at least 3 mL.

g. pH meter: Use any commercial pH meter. Standardize and calibrate according to manufacturer’s instructions. Pay special attention to temperature compensation and electrode care.

3. Reagents

a. Reagent water: Deionized or distilled water free from interferences at the minimum detection level of each constituent, filtered through a 0.2- m membrane filter and having a conductance of less than 0.1 S/cm. Use for preparing all reagents (see Section 1080).

b. Cr(VI) stock solution , 100 mg Cr6  /L: Prepare from pri

mary standard grade potassium dichromate. Dissolve 0.1414 g K2 Cr2 O 7 in water and dilute to 500 mL in a volumetric flask. pH adjustment is not required. Store in plastic. C AUTION : Hexavalent chromium is toxic and a suspected carcinogen; handle with care.

c. Eluent: Dissolve 33 g ammonium sulfate [(NH 4 ) 2 SO 4 ] in 500 mL water and add 6.5 mL conc ammonium hydroxide (NH 4 OH). Dilute to 1 L with water.

d. Post-column reagent: Dissolve 0.5 g 1,5-diphenylcarbazide in 100 mL HPLC-grade methanol. Add with stirring to 500 mL water containing 28 mL conc H 2 SO 4 . Dilute to 1 L with water. Reagent is stable for 4 or 5 d; prepare only as needed.

e. Buffer solution: Dissolve 33 g ammonium sulfate [(NH 4 ) 2 SO 4 ], in 75 mL water and add 6.5 mL conc ammonium hydroxide (NH 4 OH). Dilute to 100 mL with water.

f. Sodium hydroxide, 5N: Dissolve 200 g NaOH in 1 L water. Store in plastic bottle. Smaller volumes may be prepared.

4. Procedure

a. Instrument setup: Establish ion chromatograph operating conditions as indicated in Table 3500-Cr:I. Set flow rate of the eluent pump at 1.5 mL/min and adjust pressure of reagent delivery module so the system flow rate, measured after the detector, is 2.0 mL/min. Measure system flow rate using a graduated cylinder and stopwatch. Allow approximately 30 min after adjustment before measuring flow.

Use an injection loop size based on required sensitivity. A 50-  L loop is sufficient, although a 250-  L loop was used to determine the method detection level. b. Calibration: Before sample analysis, construct a calibration curve using a minimum of a blank and three standards thatbracket the expected sample concentration range. Prepare calibration standards from the stock standard (3 b ) by appropriate dilution with reagent water in volumetric flasks. Adjust to pH 9 to 9.5 with buffer solution (3 e ) before final dilution. Injection volumes of standards should be about 10 times the injection loop volume to ensure complete loop flushing.

c. Sample analysis: Bring chilled, pH-adjusted sample to ambient temperature. Fill a clean syringe with sample, attach a 0.45-  m syringe filter, and inject 10 times the sample loopvolume into the instrument. Dilute any sample that has a concentration greater than the highest calibration standard.

5. Calculation

Determine area or height of the Cr(VI) peak in the calibrationstandard chromatograms. Establish a calibration curve by performing a regression analysis of peak height or peak area against standard concentration in mg/L. If correlation coefficient is less than 0.995, do not use these data. Check analytical process.

For samples, measure area (or height) of Cr(VI) peak insample chromatogram, as determined by retention time. Calculate Cr(VI) concentration by interpolating from the calibration line. Correct data for any dilutions made.

Currently available instrumentation automates the entire measurement process (peak measurement, calibration, and sample measurements and calculations). Ensure that enough quality control samples are analyzed to monitor the instrumental processes.

6. Quality Control

a. Initial demonstration of performance: Before sample analysis, set up instrument and analyze enough known samples to determine estimates for the method detection level and linear calibration range. Use the initial demonstration of performance to characterize instrument performance [i.e., method detection levels (MDLs) and linear calibration range].

b. Initial and continuing calibration performance: Initially, after every 10 samples and after the final sample, analyze anindependent check sample and a calibration blank. The concentration of the calibration check sample should be near the mid-calibration range; prepare from a source independent of the calibration standards. Use acceptance criteria for check standard recovery and calibration standard concentration based on project goals for preci-

* IonPac NG1, Dionex, 4700 Lakeside Drive, Sunnyvale, CA 94086, or equivalent. † IonPac AS7, Dionex, or equivalent. TABLE 3500-Cr:I. I ON C HROMATOGRAPHIC C ONDITIONS

Variable Value

Guard column Dionex IonPac NG1 Separator column Dionex IonPac AS7 Eluent 250 m M (NH4) 2 SO 4 100 m M NH 4 OH Eluent flow rate 1.5 mL/min Post-column reagent 2 m M diphenylcarbohydrazide 10% v/v CH 3 OH 1 N H 2 SO 4 Post-column reagent flow rate 0.5 mL/min Detector Visible @ 530 nm Retention time 3.8 min CHROMIUM (3500-Cr)/Ion Chromatographic Method

4sion and accuracy. Typical values for recovery of the check standard range from 90 to 110%. The acceptance criteria for the calibration blank are typically set at  the nominal MDL.

c. Reagent blank analysis: Analyze one laboratory reagent blank with each batch of samples. Significant Cr(VI) detected in the reagent blank is a sign of contamination. Identify source and eliminate contamination.

d. Laboratory-fortified matrix (also known as matrix spike) analysis: To a portion of a sample, add a known quantity of Cr(VI). After analysis, calculate percent recovery of the known addition. If the recovery falls outside the control limits (typically 75 to 125%),the matrix may be interfering with the analysis. If matrix interferences are thought to be present, use the method of standard additions, if appropriate, to minimize the effect of interferences. (NOTE : There are situations in which low recoveries of analyte additions can be overcome, or the nondetect analyte results can be further validated by the application of three-point method of standard addition. The decision to apply method of standard additions should be a function of achieving desired data quality objectives.) Analyze fortified matrix samples as frequently as dictated by project goals and anticipated similarity of matrices in the sample set.

e. Laboratory control sample: Analyze a laboratory control sample (LCS) from an external source with every sample batch. Process LCS and samples identically, including filtering and pH adjustment. Base acceptance criteria for LCS recovery on projectgoals for precision and bias. Typical values for acceptable recovery range from 90 to 110%.

7. Precision and Bias

The instrument operating conditions and data from a singlelaboratory test of the method are shown in Tables 3500-Cr:I and 3500-Cr:II, respectively.

Multilaboratory test data are shown in Table 3500-Cr:III.‡ Fifteen laboratories analyzed samples ranging from 1.2 to 960


g Cr/L. For reagent water matrix:

S
o 0.033 x  0.106 S t 0.050 x  0.559 Mean recovery  1.04x  0.183

where:

S
o  single-operator precision, St  overall precision, and

x  added amount.

For wastewater matrix:

S
o 0.041 x  0.039 S t 0.059 x  1.05 Mean recovery  0.989x
 0.41

The eleven water samples consisted of a reagent water blank and five Youden pairs. The nine wastewater samples consisted of a wastewater blank and four Youden pairs.

8. Bibliography

D IONEX . 1990. Technical Note No. 26. Dionex, Sunnyvale, Calif. U.S. ENVIRONMENTAL PROTECTION AGENCY . 1991. Methods for the Determination of Metals in Environmental Samples. Method 218.6, EPA-600/ 4-91-010, Environmental Monitoring Systems Lab., Cincinnati, Ohio. EDGEL , K.W., J.A. L ONGBOTTOM & R.A. J OYCE . 1994. Determination of dissolved hexavalent chromium in drinking water, ground water,and industrial wastewater effluents by ion chromatography: Collaborative study. J. Assoc. Offic. Anal. Chem. 77:994. ‡ The multilaboratory precision and bias data cited in this method were the result of a collaborative study carried out jointly between U.S. EPA Environmental Monitoring Systems Laboratory (Cincinnati) and Committee D-19 of ASTM. TABLE 3500-Cr:II. S INGLE -LABORATORY PRECISION AND B IAS

Sample Type Concentration*


g/

L Mean Recovery % RPD†
Reagent water 100 100 0.8 1000 100 0.0 Drinking water 100 105 6.7 1000 98 1.5 Groundwater 100 98 0.0 1000 96 0.8 Primary wastewater

effluent 100 100 0.7 1000 104 2.7 Electroplating

effluent 100 99 0.4 1000 101 0.4

* Sample fortified at this concentration level. † RPD  relative percent difference between fortified duplicates. TABLE 3500-Cr:III. M ULTILABORATORY D ETERMINATION OF B IAS FOR HEXAVALENT C HROMIUM *

Water Amount Added


g/

L Amount Found


g/

L St S
o Bias %
Reagent 6.00 6.68 1.03 0.53  11.3 8.00 8.64 1.10  8.0 16.0 17.4 2.25 0.77  8.8 20.0 21.4 2.31  7.0 100 101 1.91 3.76  1.0 140 143 5.52  2.1 800 819 24.3 12.7  2.4 960 966 18.5  7.3

Waste 6.0 5.63 1.17 0.55

6.2 8.0 7.31 1.91

8.6 16.0 15.1 2.70 1.85

5.6 20.0 19.8 1.01

1.0 100 98.9 4.36 3.31

1.1 140 138 8.39

1.4 800 796 60.6 27.1

0.5 960 944 72.1

1.7
* Each Youden pair was used to calculate one laboratory data point ( S
o ). CHROMIUM (3500-Cr)/Ion Chromatographic Method

5