FORMALDEHYDE by VIS 3500

H2C=O MW: 30.03 CAS: 50-00-0 RTECS: LP8925000

METHOD: 3500, Issue 2 EVALUATION: FULL Issue 1: 15 May 1989

Issue 2: 15 August 1994

OSHA : 0.75 ppm; 2ppm STEL

NIOSH: 0.016 ppm; C 0.1 ppm; carcinogen;

ACGIH: C 0.3 ppm; suspected human carcinogen

(1 ppm = 1.23 mg/m 3 @ NTP)

PROPERTIES: gas; BP -19.5 °C; vapor density 1.067 (air

 = 1.00); explosive range 7 to 73 % v/v in

 air

SYNONYMS: methanal; formalin (aqueous 30 to 60 % w/v HCHO); methylene oxide

SAMPLING

SAMPLER: FILTER + IMPINGERS

(1-μm PTFE membrane and 2 impingers,

each with 20 mL 1% sodium bisulfite

solution)

FLOW RATE: 0.2 to 1 L/min

VOL-MIN: 1 L @ 3 ppm

 -MAX: 100 L

SHIPMENT: transfer samples to low-density

polyethylene bottles before shipping

SAMPLE 

STABILITY: 30 day @ 25 °C [1]

BLANKS: 2 to 10 field blanks per set

MEASUREMENT

TECHNIQUE: VISIBL E ABSORPTIO N

SPECTROMETRY

ANALYTE: formaldehyde

SAMPLE

WORKUP: note liquid volume; remove 4-mL aliquot

COLOR

DEVELOPMENT: chromotropic acid + sulfuric acid;

absorbance @ 580 nm

CALIBRATION: standard solutions of formaldehyde in

distilled water

RANGE: 2 to 40 μg per sample [1,2]

ESTIMATED LOD: 0.5 μg per sample [2,3]

PRECISION (Sr): 0.03 @ 1 to 20 μg per sample [3]

ACCURACY

RANGE STUDIED: 1.25 to 7.5 mg/m3 [2]

(80-L samples)

BIAS: none identified

OVERALL PRECISION (Sˆ

rT): 0.09 [2]

ACCURACY: ± 18%

APPLICABILITY: The working range is 0.02 to 4 ppm (0.025 to 4.6 mg/m 3) for an 80-L air sample. This is the most sensitive

formaldehyde method in the NIOSH Manual of Analytical Methods and is able to measure ceiling levels as low as 0.1 ppm (1 5-L

sample). It is best suited for the determination of formaldehyde in area samples.

INTERFERENCES: Oxidizable organic materials may give a positive interference [2]. Phenol to formaldehyde ratios as low as

0.3 produce a - 15% bias [4]. A method for the removal of the phenol interference has been reported by Hakes et. al. [5] . Ethanol

and higher M.W. alcohols, olefins, aromatic hydrocarbons [6] and cyclohexanone also produce small negative interferences [7].

Little interference is seen from other aldehydes [7].

OTHER METHODS: This method was originally adapted from the Intersociety Committee [8] and designated P&CAM 125 [4].

For personal samples or where interferences to this method are present, use Method 2541. Ref [9] is a recent review of

formaldehyde methods.

NIOSH Manual of Analytical Methods (NMAM), Fourth Edition, 8/15/94
FORMALDEHYDE: METHOD 3500, Issue 2, dated 15 August 1994 - Page 2 of 5

REAGENTS:

 1. Chromotropic acid, 1%. Dilute 0.10 g 4,5-

dihydroxy-2,7-naphthalene disulfonic acid

disodium salt to 10 mL with distilled water.

Filter. Store in brown bottle. Prepare fresh

weekly. 

 2. Sulfuric acid (H 2SO4), 96%.*

 3. Formaldehyde stock solution, 1 mg/mL (See

APPENDIX).

 4. Formalin solution, 37%.*

 5. Distilled, deionized water.

 6. Sulfuric acid, 0.02 N, aqueous.

 7. Sodium hydroxide, 0.01 N, aqueous.

 8. Sodium sulfite, 1.13 M, aqueous. Prepare

fresh immediately before use.

 9. Sodium bisulfite (NaHSO 3), 1%. Dissolve 1 g

in distilled water. Dilute to 100 mL. Prepare

fresh weekly.

10. Magnesium sulfate.

* See SPECIAL PRECAUTIONS.

EQUIPMENT:

1. Sampler: 37-mm filter cassette with 37-mm

polytetrafluoroethylene (PTFE) membrane

filter, 1- to 3-μm pore size supported by

stainless steel screen or O-ring followed by

two midget impingers; inert, flexible tubing for

cassette-to-impinger connection.

 2. Personal sampling pump, 0.2 to 1 L/min, with

flexible connecting tubing.

 3. Bottles, screw-cap low-density polyethylene

(Nalgene CPE or equivalent), 50-mL.

NOTE: Do not use bottles with "polycone"

liners (see step 4)

 4. Spectrophotometer, visible, 580 nm, with

cuvettes, 1-cm.

 5. Volumetric pipettes, 0.1-, 0.5-, 1-, 4-, 5-, 6-

and 10-mL; 1-, 2- and 5-m, graduated in 0.1-

mL units, with pipet bulb.

 6. Volumetric flasks, 10- and 100-mL, and 1-L.

 7. Burets, 50-mL.

 8. pH meter.

 9. Flasks, glass-stoppered, 25-mL.

10. Graduated cylinder, 25-mL.

11. Waterbath at 95 °C.

12. Magnetic stirrer.

13. Beaker, 50-mL.

SPECIAL PRECAUTIONS: Sulfuric acid is extremely corrosive; handle while wearing acid-resistant

gloves, apron and full face shield with goggles. Formaldehyde is viewed as a potential carcinogen [9,10]

and should be handled in a hood.

SAMPLING:

1. Calibrate each personal sampling pump with a representative sampler in line.

2. Fill the two impingers for each sample with 20 mL, 1% sodium bisulfite solution. Make

cassette-to-impinger and impinger-to-sampling pump connections with flexible, inert tubing. 

Insert a second filter/cassette assembly in line between the sampler and sampling pump to trap

any liquid which might splash over from the impingers during sampling.

NOTE: The PTFE filter is necessary when sampling is to be conducted in a dusty environment,

which could contribute either a positive or negative interferences to the method [1]. The

use of dual impingers in series is recommended to ensure efficient collection of

formaldehyde.

3. Sample at an accurately known flow rate between 0.2 and 1 L/min for a total sample size of 1 to

100 L.

4. Transfer the contents of the impingers to separate low-density polyethylene bottles for shipping.

Note: Sample contamination may occur if glass scintillation vials with "polycone" plastic lined

caps are used [11].

SAMPLE PREPARATION:

5. Transfer each impinger solution to a clean, dry 25-mL graduated cylinder. Record volume of

solution from front impinger, V f (mL) and backup impinger, V b (mL).

NIOSH Manual of Analytical Methods (NMAM), Fourth Edition, 8/15/94
FORMALDEHYDE: METHOD 3500, Issue 2, dated 15 August 1994 - Page 3 of 5

6. Pipette 4-mL aliquots from each sample solution into 25-mL glass-stoppered flasks.

NOTE: Adjust aliquot size to contain between 2 and 20 μg formaldehyde for optimum

absorbance. The calibration graph between becomes non-linear above an absorbance

of ca. 1.0. A.U.

7. Add 0.1 mL 1% chromotropic acid to the flask and mix.

NOTE: This amount of chromotropic acid can react with ca. 40 μg of formaldehyde. Due to this

fact, the range of the calibration curve should not exceed 36 μg (90% of theoretical).

8. Add 6 mL conc. H 2SO4 slowly to the flask. Replace the stopper gently. Gently swirl the solution

to mix.

CAUTION: Mixing of the sample solution with concentrated sulfuric acid is highly exothermic.

9. Heat the solution to 95 °C for 15 min. Allow the solution to cool at room temperature for 2 to 3

hours.

NOTE: Use caution due to the corrosive nature of hot sulfuric acid and the possible pressure

buildup within the flask.

CALIBRATION AND QUALITY CONTROL:

10. Calibrate daily with at least six working standards.

a. Prepare a calibration stock solution by dilution of 1 mL of 1 mg/mL formaldehyde stock

solution to 100 mL 1% sodium bisulfite solution.

b. Pipet, e.g., 0.1, 0.3, 0.5, 0.7, 1.0, and 2.0 mL calibration stock solution into 25-mL glassstoppered flasks.

c. Add 1% sodium bisulfite solution to bring the volume of each working standard to 4 mL. 

Also prepare a reagent blank.

d. Analyze together with samples and blanks (steps 5 through 9, and steps 11 and 12).

e. Prepare calibration graph (absorbance vs. μg formaldehyde/mL).

MEASUREMENT:

11. Set spectrophotometer according to manufacturer's recommendations and to conditions given on

p. 3500-1. Fill 1-cm cuvette with sample.

12. Read sample absorbance at 580 nm.

NOTE: If absorbance is greater than the highest standard, take a smaller aliquot of the

remaining unreacted sample solution, dilute to 4 mL with 1% sodium bisulfite solution,

and analyze. (Steps 11 and 12). For optimum results, all samples containing over

20 μg formaldehyde should be diluted and reanalyzed.

CALCULATIONS:

13. Calculate the mass, μg, of formaldehyde in each front impinger (M f), back impinger (M b) and

average reagent blank (M B). Use the appropriate aliquot factor (e.g., 4 mL aliquot from step 6)

and the total sample volume noted in step 5.

NOTE: Discard the sample if the mass found in the backup impinger exceeds 1/3 the mass

found in the front impinger. Collection efficiency is ca. 95% for each impinger [15], but

may be reduced due to chipped or mismatched impinger parts, or by carryover from the

front impinger.

14. Calculate the concentration, C, of formaldehyde in the air volume sampled, V (L):

NIOSH Manual of Analytical Methods (NMAM), Fourth Edition, 8/15/94
FORMALDEHYDE: METHOD 3500, Issue 2, dated 15 August 1994 - Page 4 of 5

EVALUATION OF METHOD:

The method was checked for reproducibility by having three different analysts in three different

laboratories analyze standard samples containing between 1 and 20 μg formaldehyde. The results

agreed within ± 5% [8]. This method was independently compared with the

2,4,-dinitrophenylhydrazine-coated silica gel method of Beasley et al. [12] over the range of 0.8 to 2.2

ppm formaldehyde and was found to give approximately 25% lower concentrations.

In another study comparing this method and the method of Beasley, et al., all three methods were found

to be statistically equivalent for loadings from 8.2 to 22.4 μg per sample [13]. The method of Septon

and Ku [14] was compared with this method. The slope of the correlation line was 1.09. The

concentration range of the comparison was not given.

In a study comparing the pararosaniline method [4] and NIOSH P&CAM 125 over the range of 0.021 to

0.5 ppm, the NIOSH method gave better collection efficiency and accuracy [15]. Precision for both

methods was equivalent.

REFERENCES:

 [1] Kennedy, E. R., D. L. Smith, and C. L. Geraci, Jr. "Field Evaluation of Sampling and Analytical

Methods for Formaldehyde," Chapter 11 in Advances in Chemistry Series No. 210 (Formaldehyde:

Analytical Chemistry and Toxicology ), American Chemical Society (Washington, D.C., 1985).

 [2] Formaldehyde, NIOSH Method No. S327 Failure Report, NIOSH/OSHA Standards Completion

Program Contract Report, Cincinnati, Ohio (1976).

 [3] User check, Southern Research Institute, NIOSH Sequence #3500 (unpublished,

November 10, 1983).

 [4] Miksch, R. R., D. W. Anthon, L. Z. Fanning, C. D. Hollowell, K. Revzan and J. G. Glanville, Anal.

Chem., 53, 2118-2123 (1981)

 [5] Hakes, D.C., C. D. Johnson and J. S. Marheva. Am. Ind. Hyg. Assoc. J. 45, 274-277 (1984).

 [6] Sleva, S .F. Determination of Formaldehyde: Chromotropic Acid Method, PHS Publication

999-AP-11, H-1 (1965).

 [7] NIOSH Manual of Analytical Methods, 2nd ed., V.1, P&CAM 125, U.S. Department of Health,

Education, and Welfare, Publ. (NIOSH) 77-157-A (1977).

 [8] "Methods of Air Sampling and Analysis," Method 111, Intersociety Committee for a Manual of

Methods of Air Sampling and Analysis, American Public Health Association, Washington, DC,

194-198 (1972) and M.Katz, ed., "Methods of Air Sampling and Analysis," 2nd ed., Intersociety

Committee on Methods of Air Sampling and Analysis, American Public Health Association,

Washington, DC, Method 116, 303-307 (1977).

 [9] Kennedy, E. R., A. W. Teass, and Y. T. Gagnon. Industrial Hygiene Sampling and Analytical

Methods for Formaldehyde. ACS Advances in Chemistry Series 210: Analyt. Chem. Toxicol. (1985).

[10] NIOSH/OSHA Occupational Health Guidelines for Occupational Hazards, U.S. Department of

Health and Human Services, Publ. (NIOSH) 81-123 (1981), available as GPO Stock #017-033-

00337-8 from Superintendent of Documents, Washington, DC 20402.

[11] Dollberg, D. D., D. L. Smith and D. G. Tharr. App. Ind. Hyg., 1, F44-F46 (1986).

[12] Beasley, R. K., C. E. Hoffmann, M. L. Reuppel and J. W. Worley. Anal. Chem., 52, 1110-1114

(1980).

[13] Kennedy, E. R., D. L. Smith, M. Bolyard and R. Hornung. Further Adventures in Formaldehyde

Sampling and Analysis, Poster Session at 1982 American Industrial Hygiene Conference,

Cincinnati, OH (1982).

[14] Septon J. C. and J. C. Ku. Am. Ind. Hyg. Assoc. J., 43, 845-852 (1982) 

[15] Petreas, M., S. Twiss, D. Pon and M. Imada. Am. Ind. Hyg. Assoc., J., 47, 276-280 (1986)

NIOSH Manual of Analytical Methods (NMAM), Fourth Edition, 8/15/94
FORMALDEHYDE: METHOD 3500, Issue 2, dated 15 August 1994 - Page 5 of 5

METHOD WRITTEN BY:

Eugene R. Kennedy, Ph.D., NIOSH/DPSE.

APPENDIX:

PREPARATION AND STANDARDIZATION OF FORMALDEHYDE STOCK SOLUTION (ca. 6.6

mg/mL)

Dilute 2.7 mL 37% formalin solution to 1 L with distilled, deionized water. This solution is stable for at

least three months. Standardize by placing 5.0 mL of freshly prepared 1.13 M sodium sulfite solution in

a 50-mL beaker and stir magnetically. Adjust pH to between 8.5 and 10 with base or acid. Record the

pH. Add 10.0 mL formaldehyde solution. The pH should now be about 11. Titrate the solution back to

its original pH with 0.02 N sulfuric acid (1 mL acid = 0.600 mg HCHO; about 17 mL acid needed). If the

endpoint pH is overrun, back-titrate to the endpoint with 0.01 N sulfuric hydroxide. Calculate the

concentration, C s (mg/mL), of the formaldehyde stock solution:

where: 30.0 = 30.0 g/equivalent of formaldehyde

 Na = normality of sulfuric acid (0.02 N)

 Va = volume of sulfuric acid (mL) used for titration

 Nb = normality of NaOH (0.01 N)

 Vb = volume of NaOH (mL) for back-titration

 Vs = volume of formaldehyde stock solution (10.0 mL).

NIOSH Manual of Analytical Methods (NMAM), Fourth Edition, 8/15/94