4500-SO3

2 SULFITE*

4500-SO3

2 A. Introduction

1. Occurrence

Sulfite ions (SO3

2) may occur in boilers and boiler feedwaters treated with sulfite for dissolved oxygen control, in natural

waters or wastewaters as a result of industrial pollution, and in

treatment plant effluents dechlorinated with sulfur dioxide (SO2).

Excess sulfite ion in boiler waters is deleterious because it lowers

the pH and promotes corrosion. Control of sulfite ion in wastewater treatment and discharge may be important environmentally, principally because of its toxicity to fish and other aquatic

life and its rapid oxygen demand.

2. Selection of Method

The iodometric titration method is suitable for relatively clean

waters with concentrations above 2 mg SO3

2/L. The phenanthroline colorimetric determination, following evolution of sulfite from the sample matrix as SO2, is preferred for low levels of

sulfite.

4500-SO3

2 B. Iodometric Method

1. General Discussion

a. Principle: An acidified sample containing sulfite (SO3

2)

is titrated with a standardized potassium iodide–iodate titrant.

Free iodine, liberated by the iodide–iodate reagent, reacts

with SO3

2. The titration endpoint is signalled by the blue

color resulting from the first excess of iodine reacting with a

starch indicator.

b. Interferences: The presence of other oxidizable materials, such as sulfide, thiosulfate, and Fe2ions, can cause

apparently high results for sulfite. Some metal ions, such as

Cu2, may catalyze the oxidation of SO3

2 to SO42 when

the sample is exposed to air, thus leading to low results. NO2



will react with SO3

2 in the acidic reaction medium and lead

to low sulfite results unless sulfamic acid is added to destroy

nitrite. Addition of EDTA as a complexing agent at the time

of sample collection inhibits Cu2catalysis and promotes

oxidation of ferrous to ferric iron before analysis. Sulfide and

thiosulfate ions normally would be expected only in samples

containing certain industrial discharges, but must be accounted for if present. Sulfide may be removed by adding

about 0.5 g zinc acetate and analyzing the supernatant of the

settled sample. However, thiosulfate may have to be determined by an independent method (e.g., the formaldehyde/

iodometric method1

), and then the sulfite determined by difference.

c. Minimum detectable concentration: 2 mg SO3

2/L.

d. Quality control (QC): The QC practices considered to be

an integral part of each method are summarized in Table

4020:I.

2. Reagents

a. Sulfuric acid: H2SO4, 1 1.

b. Standard potassium iodide–iodate titrant, 0.002083M: Dissolve 0.4458 g primary-grade anhydrous KIO3 (dried for 4 h at

120°C), 4.35 g KI, and 310 mg sodium bicarbonate (NaHCO3) in

distilled water and dilute to 1000 mL; 1.00 mL  500 g SO3

2.

c. Sulfamic acid, NH2SO3H, crystalline.

d. EDTA reagent: Dissolve 2.5 g disodium EDTA in 100 mL

distilled water.

e. Starch indicator: To 5 g starch (potato, arrowroot, or

soluble) in a mortar, add a little cold distilled water and grind to

a paste. Add mixture to 1 L boiling distilled water, stir, and let

settle overnight. Use clear supernatant. Preserve by adding either

1.3 g salicylic acid, 4 g ZnCl2, or a combination of 4 g sodium

propionate and 2 g sodium azide to 1 L starch solution.

3. Procedure

a. Sample collection: Collect a fresh sample, taking care to

minimize contact with air. Fix cooled samples (50°C) immediately by adding 1 mL EDTA solution/100 mL sample. Cool hot

samples to 50°C or below. Do not filter.

b. Titration: Add 1 mL H2SO4 and 0.1 g NH2SO3H crystals to a

250-mL erlenmeyer flask or other suitable titration vessel. Accurately measure 50 to 100 mL EDTA-stabilized sample into flask,

keeping pipet tip below liquid surface. Add 1 mL starch indicator

solution. Titrate immediately with standard KI–KIO3 titrant, while

swirling flask, until a faint permanent blue color develops. Analyze

a reagent blank using distilled water instead of sample.

*Approved by Standard Methods Committee, 2000. Editorial revisions, 2011.

1
4. Calculation

mg SO3

2/L  (A B)  M  6  40 000

mL sample

where:

A  mL titrant for sample,

B  mL titrant for blank, and

M  molarity of KI-KIO3 titrant.

5. Precision and Bias

Three laboratories analyzed five replicate portions of a standard sulfite solution and of secondary treated wastewater effluent

to which sulfite was added. The data are summarized below.

Individual analyst’s precision ranged from 0.7 to 3.6% standard

deviation (N  45).

Sample

X

mg/L

Standard

Deviation, 

mg/L

Relative

Standard

Deviation

%

Standard,

6.3 mg SO3

2/L 4.5 0. 25 5.5

Secondary

effluent with

2.0 mg SO3

2/L

2.1 0.28 13.4

Secondary effluent with

4.0 mg SO3

2/L 3.6 0.17 4.8

6. Reference

1. KURTENACKER, A. 1924. The aldehyde-bisulfite reaction in mass analysis. Z. Anal. Chem. 64:56.

4500-SO3

2 C. Phenanthroline Method

1. General Discussion

a. Principle: An acidified sample is purged with nitrogen

gas, and the liberated SO2 is trapped in an absorbing solution

containing ferric ion and 1,10-phenanthroline. Ferric iron is

reduced to the ferrous state by SO2, producing the orange

tris(1,10-phenanthroline) iron(II) complex. After excess ferric

iron is removed with ammonium bifluoride, the phenanthroline complex is measured colorimetrically at 510 nm.1

b. Interferences: See Section 4500-SO3

2.B.1b.

c. Minimum detectable concentration: 0.01 mg SO3

2/L.

d. Quality control (QC): The QC practices considered to be an

integral part of each method are summarized in Table 4020:I.

2. Apparatus

a. Apparatus for evolution of SO2: Figure 4500-SO3

2:1

shows the following components:

1) Gas flow meter, with a capacity to measure 2 L/min of pure

nitrogen gas.

2) Gas washing bottle, 250-mL, with coarse-porosity, 12- mmdiam fritted cylinder gas dispersion tube.

3) Tubing connectors, quick-disconnect, polypropylene.

4) Tubing, flexible PVC, for use in all connections.

5) Nessler tube, 100-mL.

b. Colorimetric equipment: One of the following is required:

1) Spectrophotometer, for use at 510 nm, providing a light

path of 1 cm or longer.

2) Filter photometer, providing a light path of 1 cm or longer

and equipped with a green filter having maximum transmittance

near 510 nm.

3. Reagents

a. 1,10-phenanthroline solution, 0.03M: Dissolve 5.95 g

1,10-phenanthroline in 100 mL 95% ethanol. Dilute to 1 L with

distilled water. Discard if solution becomes colored.

b. Ferric ammonium sulfate solution, 0.01M: Dissolve 4.82 g

NH4Fe(SO4)2  12H2O in 1 L distilled water to which has been

added 1 mL conc H2SO4 to suppress ferric hydrolysis. Filter

through a glass fiber filter if insoluble matter is visible. If

necessary, adjust volume of acid so a mixture of 10 parts of

phenanthroline solution and one part of ferric ammonium sulfate

solution will have a pH between 5 and 6.

c. Ammonium bifluoride, 5%: Dissolve 25 g NH4HF2 in

500 mL distilled water. Store in a polyethylene bottle and dispense with a plastic pipet.

d. Potassium tetrachloromercurate (TCM), K2HgCl4, 0.04M:

Dissolve 10.86 g HgCl2, 5.96 g KCl, and 0.066 g disodium EDTA

in distilled water and dilute to 1 L. Adjust pH to 5.2. This reagent

normally is stable for 6 months, but discard if a precipitate forms.2

e. Dilute TCM-stabilized sulfite standard: Dissolve 0.5 g Na2SO3

in 500 mL distilled water. Standardize on the day of preparation, but

wait at least 30 min to allow the rate of oxidation to slow. Determine

molarity by titrating with standard 0.0125M potassium iodide–

Figure 4500-SO3

2:1. Apparatus for evolution of SO2 from samples for

colorimetric analysis.

SULFITE (4500-SO3

2)/Phenanthroline Method

2

SULFITE (4500-SO3

2)/Phenanthroline Method
iodate titrant using starch indicator (see 4500-SO3

2.B). Calculate

molarity of working standard as follows:

Molarity of SO3

2standard  (A B)M

mL sample

where:

A  titrant for sample, mL,

B  titrant for blank, mL,

M  molarity of potassium iodide–iodate titrant.

Because stock Na2SO3 solution is unstable, immediately after

standardization, pipet 10 mL into a 500-mL volumetric flask

partially filled with TCM and dilute to mark with TCM. Calculate the concentration of this dilute sulfite solution by multiplying the stock solution concentration by 0.02. This TCM-stabilized standard is stable for 30 d if stored at 5°C. Discard as soon

as any precipitate is noticed at the bottom.

f. Standard potassium iodide-iodate titrant, 0.002083M: See

4500-SO3

2.B.2b.

g. Hydrochloric acid, 1 1.

h. Octyl alcohol, reagent-grade.

i. Sulfamic acid, 10%: Dissolve 10 g NH2SO3H in 100 mL

distilled water. This reagent can be kept for a few days if

protected from air.

j. EDTA reagent: See 4500-SO3

2.B.2d.

4. Procedure

a. Sample collection: Collect a fresh sample, taking care to

minimize contact with air. Fix cooled samples (50°C) immediately by adding 1 mL EDTA solution for each 100 mL sample.

b. SO2 evolution: Prepare the absorbing solution by adding

5 mL 1,10-phenanthroline solution, 0.5 mL ferric ammonium

sulfate solution, 25 mL distilled water, and 5 drops octyl alcohol

(to act as defoamer) to a 100-mL nessler tube; insert a gas

dispersion tube. Add 1 mL sulfamic acid solution to the gas

washing bottle and 100 mL of sample or a portion containing less

than 100 g SO3

2 diluted to 100 mL. Add 10 mL 1 1 HCl and

immediately connect the gas washing bottle to the gas train as

shown in Figure 4500-SO3

2:1. Place a spring or rubber band on

the gas washing bottle to keep the top securely closed during gas

flow. Adjust nitrogen flow to 2.0 L/min and purge for 60 min.

c. Colorimetric measurement: After exactly 60 min, turn off

nitrogen flow, disconnect nessler tube, and immediately add 1 mL

ammonium bifluoride solution. Remove gas dispersion tube after

rinsing it with distilled water into the tube and forcing the rinse

water into the nessler tube with a rubber bulb. Dilute to 50 mL in the

nessler tube and mix by rapidly moving the tube in a circular

motion. Do not let rubber stoppers or PVC tubing come in contact

with the absorbing solution. After at least 5 min from the time of

adding ammonium bifluoride, read the absorbance versus distilled

water at 510 nm using either a 5-cm cell for a range of 0 to 30 g

SO3

2 per portion or a 1-cm cell for a range of 0 to 100 g SO32.

Avoid transferring octyl alcohol into the cell by letting it rise to the

surface of the absorbing solution and transferring the clear lower

solution to the cell with a pipet. Make a calibration curve by

analyzing a procedure blank and at least three standards. Run at

least one standard with each set of samples. For maximum accuracy

hold samples and standards at the same temperature and keep the

time interval from start of purging to the addition of ammonium

bifluoride constant. This is easier to do if several gas trains are used

simultaneously in parallel. If ambient temperatures are subject to

frequent fluctuation, a water bath may be used to control color

development at a fixed temperature.

5. Calculation

mg SO3

2/L  g SO3

2 from calibration curve

mL sample

6. Precision and Bias

Three laboratories analyzed five replicate portions of a standard

sulfite solution and of secondary treated wastewater effluent to which

sulfite was added. The data are summarized below. Individual analyst’s

precision ranged from 4.1 to 10.5% standard deviation (N  45).

Sample

X

mg/L

Standard

Deviation,



mg/L

Relative

Standard

Deviation

%

Standard,

4.7 mg SO3

2/L 3.7 0.78 21

Secondary effluent with

0.12 mg SO3

2/L 0.12 0.03 25

Secondary effluent with

4.0 mg SO3

2/L 3.7 0.30 8.0

7. References

1. STEPHENS, B.G. & F. LINDSTROM. 1964. Spectrophotometric determination of sulfur dioxide suitable for atmospheric analysis. Anal.

Chem. 36:1308.

2. WEST, P.W. & G.C. GAEKE. 1956. Fixation of sulfur dioxide as

sulfitomercurate and subsequent colorimetric determination. Anal.

Chem. 28:1816.

3. HASKINS, J.E., H. KENDALL & R.B. BAIRD. 1984. A low-level spectrophotometric method for the determination of sulfite in water.

Water Res. 18(6):751.

SULFITE (4500-SO3

2)/Phenanthroline Method

3

SULFITE (4500-SO3

2)/Phenanthroline Method