Sulfur dioxide determination by absorption in tetrachloromercurate (TCM) solution, followed by pararosaniline spectrophotometry, is a classical wet-chemical reference method for measuring trace-level SO2 in ambient and indoor air, codified in GB 8970-88 and equivalent to the internationally recognized West-Gaeke procedure. This article explains the underlying chemistry, reagents, sampling procedures, calibration, interference control, quality assurance, and typical applications of the method, together with practical guidance on mercury waste handling and mercury-free alternatives.
Technical Overview
The determination of sulfur dioxide by absorption in tetrachloromercurate (TCM) solution is a well-established manual reference technique for quantifying SO2 at the low concentrations typical of ambient and indoor air. The method is specified in the Chinese national standard GB 8970-88, "Air quality — Determination of sulfur dioxide — Tetrachloromercurate (TCM) salt solution absorption followed by pararosaniline spectrophotometry," and is chemically equivalent to the classical West-Gaeke method that served for decades as a regulatory reference procedure in other jurisdictions. Although automated ultraviolet fluorescence analyzers now dominate routine network monitoring, the TCM-pararosaniline method remains valuable as a verification, arbitration, and field-deployable manual technique.
The analytical principle relies on two sequential chemical steps. First, sampled air is bubbled through an aqueous potassium tetrachloromercurate absorbing solution, in which sulfur dioxide is captured as a stable, non-volatile dichlorosulfitomercurate complex. The complexation reaction may be represented as: SO2 + [HgCl4]2- + H2O producing [HgCl2SO3]2- + 2H+ + 2Cl-. This fixation prevents both the oxidation of dissolved sulfite and its loss by volatilization, which makes the mercury-based absorbent uniquely reliable for sample transport and storage.
In the second step, the stabilized complex is reacted with formaldehyde and pararosaniline (a rosaniline-class dye) under controlled acidic conditions to form a purple-red pararosaniline methyl sulfonic acid dye. The dye concentration, measured spectrophotometrically at a wavelength of approximately 548 nm in a 1 cm cuvette, follows the Beer-Lambert relationship with the amount of SO2 collected. With 10 mL of absorbing solution and a sampled air volume of about 30 L, typical method detection limits are on the order of 0.02 mg/m3, and the working range extends to approximately 0.7 mg/m3 without dilution.
Testing Samples
Ambient Air
The primary sample matrix for this method is ambient air collected at urban, industrial, suburban, rural, and background monitoring sites. The technique is particularly suited to verifying compliance with 1-hour and 24-hour average sulfur dioxide limit values specified in ambient air quality standards such as GB 3095-2012, for example 500 µg/m3 as a 1-hour Class II limit and 150 µg/m3 as a 24-hour Class II limit.
Indoor Air
Indoor environments such as offices, schools, residences, and underground or poorly ventilated spaces can be monitored, especially where combustion appliances, kerosene heaters, or coal stoves are potential SO2 sources and where instrumental analyzers are not available on site.
Workplace Atmospheres
The method can support occupational exposure assessments at low-to-moderate sulfur dioxide levels. For high-concentration process atmospheres, partial sampling or sample dilution is required to keep the collected mass within the linear calibration range.
Chamber Test Atmospheres and Pilot Studies
Accredited third-party laboratories and research institutions use the method in environmental chambers, desulfurization pilot units, and scrubber performance studies, where a robust, integrative, low-cost reference measurement of generated or residual SO2 is needed.
Diluted Source Emissions
Flue gas and combustion exhaust may be measured after validated dilution or sample splitting. For concentrated source streams, complementary techniques such as iodometric titration or instrumental analyzers are generally more appropriate.
Testing Items
The core deliverable of the test is the sulfur dioxide concentration of the sampled air, reported in mg/m3 (with optional conversion to ppbv at standard conditions). Typical reported items include:
- Short-term (30 to 60 minute) average SO2 concentration obtained at a nominal sampling flow of 0.5 L/min.
- 1-hour average concentration for direct comparison against short-term regulatory limit values.
- 24-hour average concentration obtained by low-flow extended sampling, typically around 0.2 L/min, with an increased absorbing solution volume.
- Blank-corrected SO2 mass collected, sample volume converted to standard-state conditions (e.g., 101.325 kPa, 25 °C or 0 °C as defined by the governing standard), and the corresponding calibrated result.
- Supporting quality information such as field blank values, duplicate sample agreement, and calibration curve parameters.
Testing Methods
Reagents
All reagents should be of analytical grade or better and prepared with distilled or deionized water free of oxidants:
- Tetrachloromercurate absorbing solution, approximately 0.04 mol/L, prepared from accurately weighed mercuric chloride and potassium chloride, commonly stabilized with disodium EDTA and, in some formulations, a small fraction of glycerol to inhibit oxidation of the retained sulfite.
- Sulfamic acid solution, typically 0.6 g/100 mL, added before color development to destroy nitrite and eliminate nitrogen oxide interference.
- Formaldehyde solution, typically 0.2 g/100 mL, freshly prepared and standardized by iodimetric or sulfite-based back-titration as required by the standard.
- Pararosaniline stock solution and its working dilution, purified as necessary to reduce blank coloration, with concentration verified by spectrophotometric assay.
- Sodium metabisulfite stock solution, standardized against iodine standard solution with starch indicator, used to prepare the calibration series.
- Iodine standard solution, starch indicator, and phosphate or acetate buffer components as specified.
Apparatus
- Multi-hole glass impingers or bubblers of specified design with matched frits for efficient absorption.
- Constant-flow air samplers with flow meters or electronic mass-flow control, calibrated with a soap-film meter or certified dry gas meter before and after each survey.
- Ultraviolet-visible spectrophotometer equipped with 1 cm matched cuvettes and wavelength accuracy verified at approximately 548 nm.
- Thermometer, barometer, ice box or thermostatted bath for absorbent temperature control, and light-tight transport containers.
- Absorbent-treated prefilter holder where particulate matter must be excluded, using inert (non-alkaline) filter media only.
Sampling Procedure
- Pipette 10 mL of TCM absorbing solution into a clean impinger; connect the impinger to the sampler using short inert tubing, with the frit submerged in the absorbent.
- Sample at 0.5 L/min for 40 to 60 minutes for short-term determinations, or at approximately 0.2 L/min over 24 hours with a correspondingly larger absorbent volume for daily averages.
- Keep the absorbing solution below approximately 25 °C during sampling (using an ice bath in hot weather) and shield the impinger from direct sunlight to prevent photodecomposition.
- Record ambient temperature and pressure at the site, together with elapsed time and cumulative volume, to allow conversion of the sample volume to standard conditions.
- Transport field samples and field blanks promptly, refrigerated and in the dark, and complete analysis within the holding time defined by the standard.
Color Development and Measurement
- Transfer a measured aliquot of the absorbing solution to a color-development tube and add sulfamic acid solution, allowing the specified reaction time to destroy nitrite.
- Add pararosaniline working solution and formaldehyde solution in the prescribed sequence and volumes, mixing thoroughly after each addition.
- Allow color development at 20 ± 2 °C for the time specified in the standard, ensuring that calibrants and samples are developed under identical temperature and timing conditions.
- Measure the absorbance at approximately 548 nm against a reagent blank, subtract the blank, and read the SO2 mass from the calibration curve.
- Calculate the concentration as the blank-corrected SO2 mass divided by the standard-state sample volume, expressing the result in mg/m3 and rounding according to the reporting rules of the standard.
Calibration
A sulfite calibration stock, standardized iodimetrically immediately before use, is diluted to prepare at least five calibration levels spanning the expected sample range, commonly 0 to 20 µg of SO2 in the final development volume. Each level is carried through the complete color-development procedure, and the calibration curve of blank-corrected absorbance versus SO2 mass is constructed. The linear correlation coefficient should typically be 0.999 or better, and a continuing calibration check must accompany each analytical batch.
Interference Control
- Nitrogen oxides, chiefly nitrite, are eliminated by sulfamic acid addition prior to color development.
- Ozone interference is removed by allowing the absorbing solution to stand for approximately 20 minutes after sampling so that residual ozone decomposes.
- Heavy metal ions such as copper, iron, and manganese are masked by the EDTA included in the absorbing solution or added during analysis.
- Ammonia, hydrogen sulfide, and formaldehyde at normal ambient levels cause negligible bias; significant point-source interference should be assessed case by case.
- Alkaline particulate matter and alkaline filter media must be avoided because they convert dissolved SO2 to non-reactive sulfite salts that can bias results low.
Quality Assurance and Quality Control
- Field blanks accompany every sampling campaign, and reagent blanks accompany every analytical batch.
- Parallel duplicate samples are collected at a defined frequency, with relative deviations meeting the acceptance criteria of the standard.
- Matrix spike recoveries are verified periodically, typically requiring 90 to 110 percent recovery.
- Sampler flow is verified before and after sampling, with deviations normally limited to within 5 percent.
- Temperature logs for sampling and color development are retained, and control charts track blank values, calibration slope, and check-standard recovery over time.
Applications
- Manual reference and verification monitoring of ambient SO2, including co-location studies that validate continuous ultraviolet fluorescence analyzers against a classical method.
- Arbitration and dispute resolution where a legally recognized, integrative manual measurement is required.
- Grid and background surveys in regions without continuous monitoring infrastructure, providing 1-hour and 24-hour average data for regulatory reporting.
- Indoor air quality investigations associated with solid-fuel combustion, heating appliances, and ventilation deficiency.
- Occupational and community exposure screening around industrial facilities such as thermal power plants, smelters, sulfuric acid plants, and refineries.
- Research applications including atmospheric chemistry studies, smog chamber experiments, desulfurization efficiency testing, and evaluation of sorbent or filter media performance.
- Proficiency testing and method-comparison programs organized by standards bodies and accredited third-party laboratories.
FAQ
Why is a mercury-based absorbing solution used?
The tetrachloromercurate anion binds absorbed sulfur dioxide as a stable dichlorosulfitomercurate complex. This fixation prevents oxidation of sulfite to sulfate and prevents volatilization losses, giving the method excellent retention efficiency, sample stability during transport, and long-proven reproducibility at trace concentrations.
Is the method safe, and how is mercury waste handled?
Mercuric salts are highly toxic, so all TCM solutions, impinger rinsings, and colored reaction mixtures must be collected in dedicated, clearly labeled containers and never discharged to drains. Laboratories typically pre-treat the waste chemically, for example by sulfide precipitation of mercury sulfide under controlled conditions, and transfer it through licensed hazardous-waste channels in accordance with applicable environmental regulations.
Are there mercury-free alternatives with comparable performance?
Yes. Formaldehyde-buffer absorption followed by pararosaniline spectrophotometry, as specified in GB/T 15262, eliminates mercury from the absorbent. For continuous monitoring, automated ultraviolet fluorescence analyzers covered by ISO 10498, as well as electrochemical sensor systems and ion chromatographic determination after peroxide absorption, are widely used mercury-free options.
What are the detection limit and working range?
With 10 mL of absorbing solution and about 30 L of sampled air, the detection limit is approximately 0.02 mg/m3, and the linear working range extends to roughly 0.7 mg/m3. Higher concentrations can be measured by reducing the sampled volume or by validated dilution of the absorbing solution.
How is a 24-hour average obtained?
Sampling is performed continuously at a low flow of approximately 0.2 L/min over a full 24-hour period, generally into a larger volume of absorbing solution held cool and protected from light, and the total collected SO2 mass is divided by the standard-state total volume.
How sensitive is the method to temperature?
Both sampling and color development are temperature dependent. The absorbing solution should be kept below about 25 °C during sampling, and color development must be carried out at 20 ± 2 °C with calibrants and samples handled identically; temperature and timing records form part of the quality documentation.
How does the method compare with automatic SO2 analyzers?
The TCM method provides a time-integrated concentration over the sampling period rather than real-time readings. When performed under strict quality control, results agree with calibrated ultraviolet fluorescence analyzers typically within about 10 percent, which is why the method remains valuable for analyzer validation and arbitration.
Can it be used for stack emission testing?
It is not the preferred technique for concentrated source streams. For flue gas, validated dilution sampling can be applied, but iodometric titration, instrumental analyzers, or ion chromatographic procedures are generally better suited to the higher SO2 levels found in exhaust gases.