For problems with accessibility in using figures and illustrations, please contact the Salt Lake Technical Center at 801-233-4900.
These procedures were designed and tested for internal use by OSHA personnel. Mention of any company name or commercial product does not constitute endorsement by OSHA.
|OSHA Permissible Exposure Limits
Chromic acid and chromates as
CrO3 (Final Rule Limit):
0.1 mg/m3 (Ceiling)
|Chromic acid and chromates as
CrO3 (Transitional Limit):
0.1 mg/m3 (Time Weighted Average)
||An air sample is collected on a 37-mm diameter polyvinyl chloride filter (5-µm pore size) using a calibrated personal sampling pump.
|Recommended Sampling Rate:
|Recommended Air Volume Range:
||30 to 960 L
||The chromium (VI) is extracted from the filter using a carbonate/bicarbonate buffer solution and then analyzed by differential pulse polarography.
0.006 mg/m3 as CrO3 (30-L air sample)
||0.019 mg/m3 as CrO3 (30-L air sample)
|Precision and Accuracy
0.1 to 0.6 mg/m3 as CrO3 (30-L sample)
||0.012 to 0.019
||+0.012 to +0.053
|Overall Error Range:
||¦2.7 to ¦8.7%
|Date: 1982, Revised February 1990
Chemist: James Ku
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.
Branch of Inorganic Methods Development
OSHA Technical Center
Salt Lake City, Utah
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)
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,
- 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.
- The reaction between chromates and carbonate is illustrated by the following
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.
- Advantages and Disadvantages
- 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).
- 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.
- 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.
- 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.
- 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.
- 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.
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.
- Range and Detection Limit (8.8)
- 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.
- The qualitative and quantitative detection limits for 10-mL sample solution volumes
were 0.19 µg and 0.58 µg as CrO3, respectively.
- 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).
- The pooled analytical coefficients of variation (CV1) and recoveries at 0.5, 1, and 2 times this concentration for specific chromate compounds were:
|Lead chromate (PbCrO4)
|Zinc chromate (4ZnO+ CrO3.3H2O)*
|Calcium chromate (CaCrO4)
|Potassium chromate (K2CrO4)
|* Molecular formula was confirmed by X-ray diffraction (8.8)
- 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).
- A collection efficiency of 0.945 ¦ 0.035 has been previously determined for chromic
acid mist collected on PVC filters (8.11).
- 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):
- 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
- 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.
- 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.)
- 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).
- Gel bands (Omega Specialty Instrument Co., Chelmsford, MA) for sealing
- Sampling pumps capable of sampling at 2 L/min.
- Assorted flexible tubing.
- Stopwatch and bubble tube or meter for pump calibration.
- 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.
- Sampling Procedure
- Place a PVC filter and a cellulose backup pad in each two- or three-piece
cassette. Seal each cassette with a gel band.
- Calibrate each personal sampling pump with a prepared cassette in-line to
approximately 2 L/min.
- 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.
- For Time Weighted Average samples: If the filter becomes overloaded while
sampling, consecutive samples using shorter sampling periods should be taken.
- 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.
- 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.
- 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.
- Seal scintillation vials with vinyl or electrical tape. Securely wrap an
OSHA-21 seal length-wise from vial top to bottom.
- 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.
- Safety Precautions
- Certain chromate compounds have been identified as suspected carcinogens
(8.10). Care should be exercised when handling these compounds.
- When handling any chemicals, a labcoat, safety glasses or goggles, and gloves
should be worn.
- 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.
- 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.
- Refer to the Standard Operating Procedure (SOP) (8.13) and instrument
manuals for proper operation of the polarographic instrument and safety
- 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:
a. Organic reagents should not be used.
b. A water washdown system for the ducts and work surface is installed and
c. 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.
- Polarographic Analyzer or Controller, Model 384 or 384B, (Princeton Applied
Research, Princeton, NJ), with a Model 303 or 303A dropping mercury
- Glass polarographic cells, 15-mL.
- 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].
- Hot plate and exhaust hood.
- Phillips beakers, borosilicate, 125-mL, with watch glass covers.
- Filtration apparatus: Vacuum, vacuum flask, and PVC filters, 5-µm pore size, 37 mm diameter.
- Teflon-coated magnetic stirring bar and stirrer.
- Micro-analytical balance (0.01 mg).
- Polyethylene bottles, 100-mL to 1-L size.
- 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
- Reagents - All chemicals should be reagent grade or better.
- Nitrogen gas.
- Deionized water (DI H2O) with a specific conductance of less than 10 µS.
- Sodium carbonate (Na2CO3), anhydrous.
- Sodium bicarbonate (NaHCO3).
- 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.
- Potassium dichromate (K2Cr2O7), or potassium chromate (K2CrO4).
- 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.
- 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.
- 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.
- 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.
- Nitric acid (HNO3), concentrated (69 to 71%).
- Nitric acid 6 M: Carefully dilute 384 mL of concentrated (conc.) HNO3 to 1 L using DI H2O.
- Nitric acid, 10% (v/v): Carefully dilute 100 mL of conc. HNO3 to 1 L using DI H2O.
- Perchloric acid (HClO4), conc. (69 to 71%).
- Hydrogen peroxide (H2O2), 30%.
- Mercury, triple distilled, for the working electrode.
- Sample Preparation
- 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.
- Adjust the hot plate to a temperature below the boiling point of the BEE
- If bulk samples are submitted, weigh out a representative aliquot of each bulk
on separate blank PVC filters.
- 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).
- 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.
- 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.
- For samples taken from spray painting operations, digest the extracted filters
containing the paint residue according to the following procedure:
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
- 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
- Analytical Procedure
- 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.
- 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):
||30 to 240 s
||0.050 to 0.080 V
||medium or large
| * Varies slightly - Dependent on instrument and sample conditions
- Refer to reference 8.13 or other instrument manuals for operating procedures.
- 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.
- Purge each standard, blank, or sample between 30 to 240 s with purified
- 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.
- Wash the polarographic electrodes thoroughly with DI H2O
after each sample is analyzed.
- 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.
- 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 ¦ating results
- 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.
- 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.
- Determine the air concentration of CrO3 in each extraction sample according to the following equation:
(A × SA × D × GF) - (B × SB × GF)
||amount of Cr(VI) in the sample solution (µg/mL)
||amount of Cr(VI) in the blank solution (µg/mL)
||sample solution (mL)
||blank solution (mL)
||dilution factor (if any)
||gravimetric factor used to convert the amount of Cr(VI) to CrO3, GF = 1.923
||air volume sampled (L)
- 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.
- For bulk samples, calculate the total composition (in %) of CrO3 in each sample using:
(A × SA)(100%) × D × GF
(sample weight)(1000 µg/mg)
Sample wt = aliquot (in mg) of bulk taken in Section 6.4.3.
- Report air sample results (from base extractions) to the industrial hygienist as mg/m¦
- 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
Total CrO3 mg/m3 = CrO3(residual) + CrO3(extraction)
- Report bulk sample results to the industrial hygienist as approximate per cent CrO3.
- 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.
- 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.
- 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).
- 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).
- Dubois, L. and J.L. Monkman: Polarographic Determination of Heavy Metals in Air
Samples. Am. Ind. Hyg. Assoc. J. 25: 485-491 (1964).
- Urone, P.F., M.L. Druschel and H.K. Anders: Polarographic Microdetermination
of Chromium in Dusts and Mists. Anal. Chem. 22: 472-476 (1950).
- 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).
- Occupational Safety and Health Administration Technical Center: Hexavalent
Chromium Backup Data Report (ID-103) by J. Ku. Salt Lake City, UT. Revised
- 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).
- 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.
- 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.
- Occupational Safety and Health Administration Analytical Laboratory: Quality
Control Data - Chromate Analysis by B. Babcock. Salt Lake City, UT. 1987
- Occupational Safety and Health Administration Technical Center: Standard
Operating Procedure for Polarography. Salt Lake City, UT. In progress
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Metalloid Particulates in Workplace Atmospheres (Atomic Absorption) (OSHA-SLTC
Method Number ID-121). Salt Lake City, UT. Revised 1990.
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Polarogram of a 1 µg/mL Cr(VI) Standard
NA = nanoamperes