Photo by KOMUnews, March 13, 2014: Fulton Officials Discuss Improvements to Wastewater Treatment Plant.
In many industries, monitoring the levels of hydrogen sulfide (H2S) is an important task for air quality, environmental and safety professionals to help Companies prevent complaints, penalties, citations, fires, illness and even fatality.
A Number of Methods have been Developed to Measure Trace Levels of H2S
Gold film analyzers, lead acetate cassette tape gas detectors, electrochemical detectors, colorimetric gas detection tubes and SO2 conversion are some of the commonly used measurement methods. This article discusses the pros and cons of each method to help air quality, environmental and safety Professionals alike in choosing the H2S analyzer ideal for their unique requirements.
Key criteria in the selection of an H2S analyzer include potential interferences, accuracy of measurements, level of maintenance and desired detection level needed to ensure proper functioning of the equipment.
Hydrogen Sulfide Analyzer Comparison Chart.
Gold Film Sensor Analyzers Offer Portable and Fixed-Point Solutions that are Both Accurate and Precise for Low-Level H2S Analysis
Gold film sensor technology for H2S detection was introduced in the mid-1980s. The Jerome® brand of hydrogen sulfide analyzers from Arizona Instrument LLC detects ultra-low levels of H2S in air by utilizing gold’s sensitivity to H2S. Sampling is performed by pulling ambient air over the gold film sensor using an internal pump. The H2S present in the sample is absorbed by the sensor, causing an increase in its electrical resistance proportional to the mass of hydrogen sulfide present in the sample. This action allows the device to measure and display the concentration of H2S in ppm or ppb. J605 is the latest Jerome gold film hydrogen sulfide analyzer capable of detecting as low as 3 ppb with a resolution of 20 ppt.
Jerome J605 Portable H2S Analyzer
Jerome gold film sensor analyzers are rugged, user-friendly and reliable. They are offered in fixed-point and portable variants, making them suitable for various testing situations, including scrubber efficiency testing, odor control monitoring at landfill and wastewater treatment facilities, and regulatory compliance and permitting. Other applications include paper production, geothermal emissions monitoring, agriculture and livestock production, control room corrosion monitoring and semiconductor manufacturing.
Unlike other devices, gold film sensors do not show any response to water vapor, sulfur dioxide (SO2), carbon monoxide (CO), carbon dioxide (CO2) and hydrocarbons. These robust instruments deliver repeatable and highly accurate results both in the lab and in the field. Based on the variant, they are capable of storing up to 50,000 samples that can then be exported for further analysis. These instruments are also available for purchase or rent, making the Jerome® brand of hydrogen sulfide analyzers an instrument of choice for short- and long-term H2S monitoring situations.
Jerome 651 Fixed-Point H2S Monitoring Solution.
Potential interferences with gold film hydrogen sulfide analyzers are rare and most of them can be avoided with proper maintenance. Nitrogen dioxide (NO2), ammonia, chlorine and most mercaptans can affect the results. However, the concentration of ammonia or chlorine gas in the sample can be reduced before it reaches the sensor using specially designed filters, thereby significantly decreasing such interferences.
On the maintenance front, quarterly or yearly calibration is recommended based on the amount of testing and environment wherein testing is performed. If ammonia or chlorine filters are used, they should be changed at regular intervals based on the frequency of use and the environment wherein they are being used.
SO2 Converters are Accurate but only Available as Fixed-Point Solutions
The EPA recommended method is SO2 conversion, which is useful for low level detection of H2S between the range of 0.5 ppb and 10 ppm but is an indirect method of determining the concentration of H2S in air.. A collected sample is directed into the SO2 converter to remove any SO2 present in the initial gas stream and H2S is converted into SO2 through a catalytic reaction. The resultant SO2 sample is drawn into a fluorescence chamber where the molecules are excited by ultraviolet (UV) light. The amount of fluorescence is then detected and converted into the SO2 concentration by a photomultiplier tube. The SO2 concentration is directly proportional to the concentration of hydrogen sulfide in the original sample.
SO2 converters are widely used for ambient fence-line monitoring at wastewater treatment plants, oil refineries and landfill facilities. They are only available as benchtop analyzer, not portable, and are usually housed in an enclosure of some sort due to power requirements. Potential interferences include water vapor, nitric oxide (NO) and various hydrocarbons, and the lack of portability largely limits the detection capabilities of SO2 converters to strictly stationary monitoring.
Colorimetric Gas Detection Tubes are Easy to Use but Results are Subjective
Colorimetric gas detection tubes are a well-established technique for measuring H2S concentration in air. They are relatively inexpensive for short-term applications, simple to operate, deliver results rapidly and can be used in almost all industries. A specific volume of air is drawn into the glass detection tube to measure its concentration. Various chemicals within the tube then react with their target gases by changing colors. The target gas concentration in the sample is determined using the depth and length of the color change.
Colorimetric Gas Detection Tubes 
Although this testing method can be easily executed, it has disadvantages. Since results are based on color change as perceived by the human eye, this method is generally considered to be subjective and semiquantitative. Although gas detection tubes are portable, they cannot be set to automatically sample. They are sensitive to humidity and temperature during use and storage, have a limited shelf life, and their results can be interfered by a number of chemicals, including hydrogen peroxyl (HO2), isobutylene, hydrogen chloride (HCl) and other acids and bases, various mercaptans, and high concentrations of ammonia. Nitrogen dioxide (NO2), methyl mercaptan, and SO2 can also alter the color change, making it difficult to read the result of the test.
It is also important to note that each detection tube is specially developed to determine a specific level of the gas of interest and will not have the same interferences. Unless users have a fair idea of how much of the target gas they expect to find as well as which interferences may be present and in what proportions, tube selection may become a challenging task. Moreover, users may have to bring multiple gas detection tubes, which can get expensive, to ensure they are obtaining a correct reading.
Electrochemical Detectors are Inexpensive but Require Frequent Calibration
Electrochemical detectors for H2S measurement are predominantly used for monitoring H2S levels for scrubber efficiency testing as well as at landfill and wastewater facilities. They are relatively inexpensive and simple to operate, and can be employed as portable or fixed point monitoring solutions for a variety of gases, including H2S down to ppm level. In electrochemical cells, a permeable membrane surrounding the electrodes allows a sample of air to travel through and diffuse into the cell. In the presence of a target chemical, a reduction or oxidation reaction takes place and the corresponding change in the current is measured and converted to concentration of target gas present in the original air sample.
Portable Electrochemical Cell Multi Gas Detector 
This kind of instrument can be effective in H2S measurement and is supplied in both portable and fixed-point solutions, but it has its own drawbacks. Calibration needs to be performed frequently for electrochemical cell based sensors. Their sensitivity to humidity, heat and low oxygen environments can cause drift and cell deterioration. Other interferences include methyl and ethyl mercaptan, SO2, phosphine (PH3), NO2 and other light hydrocarbons.
Lead Acetate Tape Gas Detectors have been around for Decades but are Susceptible to a Number of Interferences
Lead acetate tape gas detectors have been in use for the past several years and still play a key role in various industries. They are widely used for fixed-point H2S monitoring and to monitor scrubber efficiency. The color of the lead acetate tape changes when H2S is present. The instrument is equipped with specially calibrated optics to measure H2S concentration by determining slight variations in the depth of the color change.
Lead Acetate Detection Strips Circa 1914 
This kind of analyzer is primarily stationary, but some portable options are also available. The lead acetate tape is prone to interference from low and high humidity and SO2. Dry conditions can cause the analyzer to underreport results, whereas high humidity can cause the glass components to fog and the tape to become moist (humid conditions). These conditions can distort the color as it perceived by the optic system, thus skewing the final test results.
Lead acetate cassette tape gas detectors are highly expensive, especially due to the necessity to replace cassettes frequently - every 1 to 4 weeks or longer depending on the frequency of sampling, amount of tape on the roll and the environment wherein the sampling is carried out. Since lead acetate cassette tape gas detectors have a set shelf life, it is necessary to store them carefully to avoid any potential damage caused by factors such as ambient humidity.
Other Methods of H2S Analysis Exist
Metal oxide semiconductors (MOS), sulfur titrators, flame photometric detectors (FPDs), sulfur chemiluminescence and field olfactometers are all useful technologies/methods for H2S detection. However, they are not described in detail in this article due to various reasons, including subjectivity of results, complicated testing procedures, special training required, lack of portability, slow response time or an inability to determine individual sulfur compounds.
Field Olfactometer 
Choosing the Correct H2S Analysis Technology is Essential
Although different types of methods are available for H2S detection, no single method can be considered to be the ideal technology for all situations, but some methods have clear advantages over other methods. The EPA recommended method is SO2 conversion, but this method is only available as a stationary analyzer. Although the initial investment for electrochemical cells is inexpensive, they have many interferences and need frequent calibrations.
Colorimetric gas detection tubes are inexpensive and user-friendly, but are completely manual, yield subjective results and have a number of interferences. H2S analysis can be quickly performed using lead acetate cassette tape based analyzers, but these analyzers are somewhat expensive and need replacement tapes, which increases the lifetime cost of the instrument.
In summary, Jerome gold film sensor H2S analyzers offer both stationary and portable solutions that can yield accurate and repeatable results for unknown, low-level concentrations of H2S. They are convenient and durable enough for frequent use and sensitive enough to make sure compliance with odor and other regulations.
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This information has been sourced, reviewed and adapted from materials provided by Arizona Instrument.
For more information on this source, please visit Arizona Instrument.