HEXAVALENT CHROMIUM

Commercial manufacturers and products mentioned in this method are for descriptive use only and do not constitute endorsements by USDOL-OSHA.
Similar products from other sources can be substituted.

1. Introduction

This method describes the air sampling and subsequent analysis of workplace exposures to chromic acid and chromate compounds. Analysis is conducted by differential pulse polarography (DPP).

1.1. History

This method has been developed at the OSHA Salt Lake City Technical Center (OSHA-SLTC) to improve the determination of chromic acid and chromates [as total Cr(VI)] by minimizing interferences and offering increased sensitivity.

The classical method of Cr(VI) analysis is colorimetry using s-diphenylcarbazide (DPC) after acid extraction of the Cr(VI) from the sample (8.1., 8.2.). This method is unsatisfactory for determining certain insoluble chromate compounds (8.3.) and also has interferences from many heavy metals (8.2.). Reducing agents, such as Fe(II), could convert the Cr(VI) to Cr(III) in the acidic extraction medium used (8.4.).

The extraction of Cr(VI) in basic solution and subsequent analysis by colorimetry using DPC has been reported (8.3.). The extraction technique used in this method is a modification of that suggested in reference 8.3.

In this method the analytical technique for Cr(VI) is DPP. Polarographic techniques have been previously reported for the analysis of chromium species (8.5., 8.6.).

1.2. Principle

1.2.1. An air sample is collected on a 37-mm polyvinyl chloride (PVC) filter [Note: Cellulose ester filters are unacceptable because they may react with and reduce the hexavalent chromium [Cr(VI)] species (8.7.-8.9.)]. The filter is treated with a hot 10% sodium carbonate / 2% sodium bicarbonate buffer solution to extract the Cr(VI) from the sample and to protect against reduction to Cr(III). The Cr(VI) in the extract is analyzed by DPP using a dropping mercury electrode.

1.2.2. The reaction between chromates and carbonate is illustrated by the following equation (8.3.):

MCrO₄ + CO₃^{(2 »)}< ----> MCO₃ + CrO₄^{(2 »)}

  where M   =   metals (i.e. lead, zinc, cadmium, ...)

In the presence of a large excess of carbonate, equilibrium is quantitatively shifted to the right. The chromate compounds (soluble and insoluble) are converted to their corresponding carbonates.

1.3. Advantages and Disadvantages

1.3.1. The analysis is specific for Cr(VI) in the presence of Cr(III). Other reducing substances, such as magnetite (Fe₃O₄) do not appear to significantly interfere (8.8.).

1.3.2. In addition to the Cr(VI) analysis, it is possible to determine other soluble compounds such as lead and zinc salts in the same solution.

1.3.3. By using alkaline extraction conditions (pH 10 to 11), sample recovery is improved by preventing Cr(VI) losses which may occur in a more acidic extraction media. Both water soluble and insoluble Cr(VI) compounds are soluble in this alkaline extraction medium.

1.3.4. The sensitivity is adequate for measuring workplace atmospheric concentrations of Cr(VI) and is less sensitive to interferences noted when using the colorimetric/DPC procedure (8.2., 8.7.). Potential interferences with the polarographic determination may be rendered insignificant by altering analytical conditions such as changing the supporting electrolyte solution.

1.3.5. Polarographic instruments have a wide analytical range. This diminishes the need for withdrawing aliquots or diluting the samples in order to be within the linear analytical range of the instrument.

1.3.6. A disadvantage is the polarographic instrument may not be available in some analytical laboratories; however, the extracted samples may be acidified and then analyzed using a modified colorimetric/DPC method (please see Section 7 of reference 8.8. for further information). Spiked samples using compounds known to be present in the sample matrix should be taken through this alternate procedure first to determine if any loss of Cr(VI) occurs during acidification. Detection limits should also be determined.

1.4. Uses

Occupations having a potential exposure to compounds containing Cr(VI) as well as a list of different chromate compounds are listed in reference 8.10.

2. Range and Detection Limit (8.8.)

2.1. This method was validated using insoluble and soluble chromate compounds. The compounds used were lead, zinc, calcium, and potassium chromates. Filter samples were spiked with about 2 to 9 µg [as Cr(VI)], prepared, and then diluted to 10-mL sample solution volumes. Using 30- or 840-L air volumes, these spiked samples would give an approximate concentration range of:

30-L air sample		0.1 to 0.6 mg/m3 as CrO3
840-L air sample		0.005 to 0.02 mg/m3 as CrO3

This method has the sensitivity necessary to determine compliance with either the OSHA Transitional or the Final Rule PEL. Samples for Final Rule determinations should be taken with at least 30-L air volumes.

2.2. The qualitative and quantitative detection limits for 10-mL sample solution volumes were 0.19 µg and 0.58 µg as CrO₃, respectively.

3. Method Performance (8.8.)

The DPP analytical method has been evaluated using a time weighted average concentration of 0.009 mg/m³ as CrO₃ (840-L air sample).

3.1. The pooled analytical coefficients of variation (CV₁) and recoveries at 0.5, 1, and 2 times this concentration for specific chromate compounds were:

Compound		CV1		Recovery
Lead chromate (PbCrO4)		0.012		100.3%
Zinc chromate (4ZnO+ CrO3.3H2O)*		0.017		105.3%
Calcium chromate (CaCrO4)		0.015		101.9%
Potassium chromate (K2CrO4)		0.019		101.2%
*   Molecular formula was confirmed by X-ray diffraction (8.8.)

3.2. A comparison of methods using spiked samples containing PbCrO₄ showed that results obtained by a modified colorimetric/DPC method were duplicated for the DPP method. There was no significant bias between the two methods (8.8.).

3.3. A collection efficiency of 0.945 ¦ 0.035 has been previously determined for chromic acid mist collected on PVC filters (8.11.).

3.4. Quality control samples were prepared by spiking aqueous solutions of potassium dichromate on PVC filters. These samples were analyzed along with survey samples at OSHA-SLTC from 1982 to 1989. The following results were obtained (8.12.):

Samples (N)		282
Average recovery		94.1%
CV1		0.10

4. Interferences

4.1. Reducing species such as Cr(III) or magnetite (Fe₃O₄) in an excess of 10:1 or 50:1, respectively, over Cr(VI) did not produce a significant interference with this method (8.8.).

4.2. The effect of many interferences can be minimized by changing the operating conditions of the polarograph. Additional polarographic confirmation of a cation in a sample may be performed in a second electrolyte and observing if the new half-wave potential is consistent with the determination made using the first electrolyte.

5. Sampling

5.1. Sampling Equipment (Note: Bulk samples can be collected and analyzed. Filter or wipe samples collected on cellulose or cellulose esters are unacceptable due to chromate species instability on these media.)

5.1.1. Sample assembly:

Filter holder consisting of a two- or three-piece cassette, 37-mm diameter.
Backup pad, 37-mm, cellulose.
Membrane filter, PVC, 37-mm, 5-µm pore size [part no. 625413, Mine Safety Appliances (MSA), Pittsburgh, PA or cat. no. P-503700, Omega Specialty Instrument Co., Chelmsford, MA).

5.1.2. Gel bands (Omega Specialty Instrument Co., Chelmsford, MA) for sealing cassettes.

5.1.3. Sampling pumps capable of sampling at 2 L/min.

5.1.4. Assorted flexible tubing.

5.1.5. Stopwatch and bubble tube or meter for pump calibration.

5.1.6. Scintillation vials (for bulk samples), 20-mL, part no. 74515 or 58515, (Kimble, Div. of Owens-Illinois Inc., Toledo, OH) with polypropylene or Teflon cap liners.

5.2. Sampling Procedure

5.2.1. Place a PVC filter and a cellulose backup pad in each two- or three-piece cassette. Seal each cassette with a gel band.

5.2.2. Calibrate each personal sampling pump with a prepared cassette in-line to approximately 2 L/min.

5.2.3. Attach prepared cassettes to calibrated sampling pumps (the backup pad should face the pump) and place in appropriate positions on the employee or workplace area. Collect the samples using a total air volume of at least 30-L.

5.2.4. For Time Weighted Average samples: If the filter becomes overloaded while sampling, consecutive samples using shorter sampling periods should be taken.

5.2.5. Wipe samples can be taken using PVC filters as the wipe media. Wear clean, impervious, disposable gloves when taking each wipe sample. If possible, wipe a surface area covering 100 cm¦. Fold the wipe sample with the exposed side in and then transfer into a 20-mL scintillation vial.

5.2.6. If bulk samples are necessary, collect the bulk samples using a grab sampling technique suitable for the particular material(s) in use. If possible, transfer any bulk samples into 20-mL scintillation vials.

5.3. Shipment

5.3.1. Place plastic end caps on each cassette after sampling. Submit at least one blank sample with each set of air samples. Blank filter samples should be handled in the same manner as other samples, except no air is drawn through the blank. Attach an OSHA-21 seal around each cassette in such a way as to secure the end caps. Send the samples to the laboratory with the OSHA 91A paperwork requesting chromate analysis.

5.3.2. Seal scintillation vials with vinyl or electrical tape. Securely wrap an OSHA-21
seal length-wise from vial top to bottom.

5.3.3. Bulk samples should be shipped separately from air samples. They should be accompanied by Material Safety Data Sheets if available. Check current shipping restrictions and ship to the laboratory by the appropriate method.

6. Analysis

6.1. Safety Precautions

6.1.1. Certain chromate compounds have been identified as suspected carcinogens (8.10.). Care should be exercised when handling these compounds.

6.1.2. When handling any chemicals, a labcoat, safety glasses or goggles, and gloves should be worn.

6.1.3. The buffer/extraction/electrolyte (BEE) solution is basic and somewhat corrosive. Clean up any spills immediately. This solution should be stored in polyethylene bottles since precipitated salts form readily during evaporation and will cause glass stoppers to seize. Samples prepared in glassware should be analyzed and properly disposed of as soon as possible.

6.1.4. Mercury is used as the working electrode in DPP. Always exercise caution to prevent any potential spills of mercury. Containment vessels should surround the polarograph and spill control devices should be available when handling or working with mercury.

6.1.5. Refer to the Standard Operating Procedure (SOP) (8.13.) and instrument manuals for proper operation of the polarographic instrument and safety precautions.

6.1.6. Extra care should be used when handling perchloric acid (HClO₄). Perchloric acid should only be used in a hood that has been approved for HClO₄ use. In this hood:

1. Organic reagents should not be used.
2. A water washdown system for the ducts and work surface is installed and periodically used.
3. Precautions should be taken to ensure that explosions or spontaneous ignition of sample material from HClO₄ is prevented.

Working with HClO₄ is very hazardous. Be sure to wear safety glasses, a labcoat, and gloves. Always add nitric acid (HNO₃) with HClO₄ when digesting samples. Watch the samples during HClO₄ digestion carefully since there is a chance they could ignite. Always keep HNO₃ nearby when using HClO₄. In the event of sample media ignition, quickly douse the sample with a small portion of HNO₃.

6.2. Equipment

6.2.1. Polarographic Analyzer or Controller, Model 384 or 384B, (Princeton Applied Research, Princeton, NJ), with a Model 303 or 303A dropping mercury electrode.

6.2.2. Glass polarographic cells, 15-mL.

6.2.3. Nitrogen purification system: Gas purifier for deoxygenating nitrogen, [(Oxiclear, part no. DGP-250, Labclear, Oakland, CA). As an alternative, an oxygen scrubber can be constructed using a vanadous chloride solution as described in reference 8.14.].

6.2.4. Hot plate and exhaust hood.

6.2.5. Phillips beakers, borosilicate, 125-mL, with watch glass covers.

6.2.6. Filtration apparatus: Vacuum, vacuum flask, and PVC filters, 5-µm pore size, 37 mm diameter.

6.2.7. Teflon-coated magnetic stirring bar and stirrer.

6.2.8. Micro-analytical balance (0.01 mg).

6.2.9. Polyethylene bottles, 100-mL to 1-L size.

6.2.10. Volumetric and micropipettes, volumetric flasks, beakers, and general laboratory glassware. Do not use glassware for sample analysis of chromate compounds if it was:

1. previously cleaned with chromic acid cleaning solution
2. previously used for storage of chromium (VI) standards
3. previously used for storage of bulks containing high concentrations of chromium (VI)

6.3. Reagents - All chemicals should be reagent grade or better.

6.3.1. Nitrogen gas.

6.3.2. Deionized water (DI H₂O) with a specific conductance of less than 10 µS.

6.3.3. Sodium carbonate (Na₂CO₃), anhydrous.

6.3.4. Sodium bicarbonate (NaHCO₃).

6.3.5. Buffer/extraction/electrolyte (BEE) solution (pH approximately 10.5): Dissolve 100 g of Na₂CO₃ and 20 g of NaHCO₃ in about 500 mL DI H₂O contained in a 1-L volumetric flask. A Teflon-coated magnetic stirring bar and stirrer will facilitate dissolution. Rinse and remove the stirring bar and then dilute to the mark with DI H₂O. Transfer and store this solution in a tightly capped polyethylene bottle. Prepare monthly.

6.3.6. Potassium dichromate (K₂Cr₂O₇), or potassium chromate (K₂CrO₄).

6.3.7. Cr(VI) Stock Standard (1,000 µg/mL): Dissolve 2.829 g K₂Cr₂O₇ or 3.735 g K₂CrO₄ in DI H₂O and dilute to the mark in a 1-L volumetric flask. Prepare this solution every six months.

6.3.8. Cr(VI) standard (100 µg/mL): Dilute 10 mL of the Cr(VI) stock standard to 100 mL with DI H₂O. Prepare this solution every three months.

6.3.9. Cr(VI) working standard (10 µg/mL): Dilute 10 mL of the Cr(VI) 100 µg/mL standard to 100 mL with the BEE solution. Transfer to a polyethylene bottle. Prepare this solution daily.

6.3.10. Cr(VI) working standard (1 µg/mL): Dilute 10 mL of the Cr(VI) 10 µg/mL working standard to 100 mL with the BEE solution. Transfer to a polyethylene bottle. Prepare this solution daily.

6.3.11. Nitric acid (HNO₃), concentrated (69 to 71%).

6.3.12. Nitric acid 6 M: Carefully dilute 384 mL of concentrated (conc.) HNO₃ to 1 L using DI H₂O.

6.3.13. Nitric acid, 10% (v/v): Carefully dilute 100 mL of conc. HNO₃ to 1 L using DI H₂O.

6.3.14. Perchloric acid (HClO₄), conc. (69 to 71%).

6.3.15. Hydrogen peroxide (H₂O₂), 30%.

6.3.16. Mercury, triple distilled, for the working electrode.

6.4. Sample Preparation

6.4.1. Wash all glassware in hot water with detergent and rinse with tap water, 10% HNO₃, and DI H₂O (in that order). Under no circumstances should chromic acid cleaning solutions be used.

6.4.2. Adjust the hot plate to a temperature below the boiling point of the BEE solution.

6.4.3. If bulk samples are submitted, weigh out a representative aliquot of each bulk on separate blank PVC filters.

6.4.4. Carefully remove the PVC filter from the cassette or balance, place it face-down in a 125-mL Phillips beaker, and add 5 mL of BEE solution. Cover the beaker with a watch glass and heat the solution on the hot plate, with occasional swirling for 30 to 60 min. Allow extra extraction time for heavily loaded samples taken from spray paint operations. Do not allow any solutions to boil or evaporate to dryness. Conversion of Cr(VI) to Cr(III) can occur from excess heat (8.4.).

6.4.5. Allow the solutions to cool to room temperature. Quantitatively transfer each solution to a 10- or 25-mL volumetric flask using BEE solution rinses. Dilute to volume with the BEE solution. Use 10-mL sample volumes for samples taken to determine if exposures exceed the OSHA Ceiling Permissible Exposure Limit for chromate.

6.4.6. If the solution is cloudy and/or other metal analyses are desired, filter the solution through a PVC filter in a vacuum filtration apparatus. If necessary, prepare and analyze samples for other metals using the appropriate techniques. An example would be to determine the total metal content of the sample residue by atomic absorption or inductively coupled plasma spectroscopy.

6.4.7. For samples taken from spray painting operations, digest the extracted filters containing the paint residue according to the following procedure:

---

Note:

Evidence indicates base extractions are capable of recovering Cr(VI) in specific paint matrices (8.4.). Due to the resistant properties of some industrial paints, an additional digestion is used for samples collected during spray painting to assure complete recovery of all Cr(VI).

---

1. After the extraction solutions are transferred to volumetric flasks and diluted to volume, place the sample beakers containing the remaining paint residue and any blanks in an exhaust hood. Carefully add 5 to 10 mL of conc. HNO₃ to each beaker. Place the beakers on a hot plate and heat the samples until about 1 mL remains.
2. Add 2 mL of conc. HClO₄ along with a second portion of 2 mL HNO₃, heat each sample, and then remove when about 1 mL remains. (Note: Please see Section 6.1.6. before using HClO₄.)
3. Add 1 or 2 mL of 30% H₂O₂ to the cooled solution to reduce any remaining Cr(VI). Let the sample sit for several min and then heat for approximately 5 min to boil off the H₂O₂. Allow the samples to cool to room temperature.
4. Dilute each digested sample to a 25 mL final volume with DI H₂O. Analyze these samples for chromium by atomic absorption using the procedure mentioned in reference 8.15.

6.5. Standard Preparation

Prepare a series of Cr(VI) standards in the analytical range of 0.050 to 10 µg/mL. Make appropriate serial dilutions of the Cr(VI) working standards with the BEE solution.

6.6. Analytical Procedure

6.6.1. Cleaning equipment:

Soak polarographic cells in 6 M HNO₃ (preferably overnight), rinse thoroughly with DI H₂O, and air-dry. Errors occurring from the adsorption of chromium on the walls of glassware and analytical reagent contamination with chromium have been reported (8.16., 8.17.). Therefore, take special precautions and also analyze a reagent blank using identical treatment as the samples.

6.6.2. Set the operating conditions for the instrument as follows (Note: If other types of instruments are used, refer to their operating and service manuals for comparable settings):

Analytical technique:	DPP
Initial potential:	-0.100 V
Final potential:	-0.450 V
Peak potential:	-0.300 V*
Nitrogen purge:	30 to 240 s
Scan increment:	2 mV
Pulse height:	0.050 to 0.080 V
Drop time:	1 s
Drop size:	medium or large
* Varies slightly - Dependent on instrument and sample conditions

6.6.3. Refer to reference 8.13. or other instrument manuals for operating procedures.

6.6.4. Transfer a sample or blank to a polarographic cup. If the final solution volume was 10-mL, transfer the entire sample; if 25-mL, transfer a 10-mL aliquot. Transfer 10 mL of each standard into separate polarographic cups.

6.6.5. Purge each standard, blank, or sample between 30 to 240 s with purified nitrogen.

6.6.6. Analyze the reagent blank (10 mL of BEE solution), the standards, and the samples by measuring the peak current (nA). A standard should be analyzed after every five or six samples.

6.6.7. Wash the polarographic electrodes thoroughly with DI H₂O after each sample is analyzed.

6.6.8. Record the peak current (nA) and potential for each determination. A differential pulse polarogram of Cr(VI) in the BEE solution should display a peak at approximately -0.300 V when using the conditions described. A polarogram of a 1 µg/mL standard is shown in Figure 1.

6.6.9. If the peak current from a sample is above the largest standard used, an aliquot should be taken from the sample and diluted to 10 mL with BEE solution and analyzed. A dilution factor for this sample is applied when calculating results (Section 7.2.).

6.6.10. Other metals such as lead and zinc may be determined in the same solution if required. Approximate peak potentials of -0.630 V for Pb and -1.350 V for Zn were found when these species were present in the BEE solution.

7. Calculations

7.1. Use a least-squares regression program to plot a concentration-response curve of peak current vs. concentration (µg/mL of standards). Determine the concentration (µg/mL) of each sample and blank from the curve.

7.2. Determine the air concentration of CrO₃ in each extraction sample according to the following equation:

C =	(A × SA × D × GF) - (B × SB × GF) air volume

Where:
	C	=	mg/m3 CrO3
	A	=	amount of Cr(VI) in the sample solution (µg/mL)
	B	=	amount of Cr(VI) in the blank solution (µg/mL)
	SA	=	sample solution (mL)
	SB	=	blank solution (mL)
	D	=	dilution factor (if any)
	GF	=	gravimetric factor used to convert the amount of Cr(VI) to CrO3, GF = 1.923
Air Vol		=	air volume sampled (L)

7.3. For digested spray paint samples analyzed according to OSHA method no. ID-121, the calculations above may be used without the gravimetric factor or use calculations mentioned in that method to determine the amount of total chromium.

7.4. For bulk samples, calculate the total composition (in %) of CrO₃ in each sample using:

CrO3%(w/w) =

(A × SA)(100%) × D × GF (sample weight)(1000 µg/mg)

(Bulk Samples)

Where:

Sample wt   =   aliquot (in mg) of bulk taken in Section 6.4.3.

7.5. Report air sample results (from base extractions) to the industrial hygienist as mg/m¦ CrO₃.

7.6. For spray paint samples, also report results obtained from the digestion of the residue. Report each result from digested samples as mg/m¦ chromium metal and insoluble salts. Each result can be combined with the result in Section 7.5. by the industrial hygienist if the paint used during sampling does not contain other chromium compounds. Before combining results, the industrial hygienist has to perform the following calculation:

CrO₃(residual) mg/m³   =   Cr metal (mg/m³) + 1.923

Then:

Total CrO₃ mg/m³   =   CrO₃(residual) + CrO₃(extraction)

7.7. Report bulk sample results to the industrial hygienist as approximate per cent CrO₃.

8. References

8.1. National Institute for Occupational Safety and Health: NIOSH Manual of Analytical Methods. 2nd ed., Vol. 1 (DHEW/NIOSH Pub. No. 77-157-A). Cincinnati, OH: National Institute for Occupational Safety and Health, 1977. P&CAM 169, pp. 169-1-169-6.

8.2. National Institute for Occupational Safety and Health: NIOSH Manual of Analytical Methods. 2nd ed., Vol. 3 (DHEW/NIOSH Pub. No. 77-157-C). Cincinnati, OH: National Institute for Occupational Safety and Health, 1977. pp. S317-1-S317-6.

8.3. Thomsen, E. and R.M. Stern: A Simple Analytical Technique for the Determination of Hexavalent Chromium in Welding Fumes and Other Complex Matrices. Scand. J. of Work, Environ. and Health 5: 386-403 (1979).

8.4. Molina, D. and M.T. Abell: An Ion Chromatographic Method for Insoluble Chromates in Paint Aerosol. Am. Ind. Hyg. Assoc. J. 48: 830-835 (1987).

8.5. Dubois, L. and J.L. Monkman: Polarographic Determination of Heavy Metals in Air Samples. Am. Ind. Hyg. Assoc. J. 25: 485-491 (1964).

8.6. Urone, P.F., M.L. Druschel and H.K. Anders: Polarographic Microdetermination of Chromium in Dusts and Mists. Anal. Chem. 22: 472-476 (1950).

8.7. Abell, M.T. and J.R. Carlberg: A Simple Reliable Method for the Determination of Airborne Hexavalent Chromium. Am. Ind. Hyg. Assoc. J. 35: 229-233 (1974).

8.8. Occupational Safety and Health Administration Technical Center: Hexavalent Chromium Backup Data Report (ID-103) by J. Ku. Salt Lake City, UT. Revised 1991.

8.9. Dutkiewicz, R., J. Konczalik and M. Przechera: Assessment of the Colorimetric Methods of Determination of Chromium in Air and Urine by Means of Radioisotope Techniques. Acta Pol. Pharm. 26: 168-176 (1969).

8.10. National Institute for Occupational Safety and Health: Criteria for a Recommended Standard -- Occupational Exposure to Cr(VI) (DHEW/NIOSH Pub. No. 76-129). Cincinnati, OH: National Institute for Occupational Safety and Health, 1975.

8.11. National Institute for Occupational Safety and Health: Documentation of the NIOSH Validation Tests. Backup Data Report, Chromic Acid & Chromates, No. S317 (Contract No. CDC-99-74-45). Cincinnati, OH: National Institute for Occupational Safety and Health, 1977.

8.12. Occupational Safety and Health Administration Analytical Laboratory: Quality Control Data - Chromate Analysis by B. Babcock. Salt Lake City, UT. 1987 (unpublished).

8.13. Occupational Safety and Health Administration Technical Center: Standard Operating Procedure for Polarography. Salt Lake City, UT. In progress (unpublished).

8.14. Princeton Applied Research: Application note 108, Why Dearation... and How. Princeton, NJ: Princeton Applied Research, 1974.

8.15. Occupational Safety and Health Administration Technical Center: Metal and Metalloid Particulates in Workplace Atmospheres (Atomic Absorption) (OSHA-SLTC Method No. ID-121). Salt Lake City, UT. Revised 1990.

8.16. Beyerman, K.: The Analytical Behavior of Minutest Chromium Quantities, Part I. Z. Anal. Chem. 190: 4-33 (1962).

8.17. Beyerman, K.: The Analytical Behavior of Minutest Chromium Quantities, Part II. Z. Anal. Chem. 190: 346-369 (1962).

Polarogram of a 1 µg/mL Cr(VI) Standard


Figure 1

NA = nanoamperes