Arsenic Testing in the USA

By:

Ivars Jaunakais

In the USA the controversy over what level of arsenic in drinking water is acceptable continues. However, for the Water Quality Market one more immediate interest is in how to measure the arsenic levels in the drinking water quickly and accurately in the field. Because of the increase in interest in Field test kits for Arsenic and the anticipated reduction of the US EPA Arsenic standard from 50 ppb (μg/L) to 10 ppb (μg/L), an overview of the commercial test kits that are available is needed.   In the USA currently there are 5 Field Arsenic kits readily available that give quantitative results below 50 ppb (μg/L).  All field kits work (see Chart #1 for Comparison) for the analysis of arsenic in an aqueous sample by the reduction of arsenic, in particular inorganic arsenic compounds such as trivalent and pentavalent arsenic, in an acidic aqueous sample to arsine gas. Interference from Hydrogen Sulfide gas is eliminated by 4 out of 5 kits with a short incubation with the oxidizing agent Oxone^® (Dupont).  Hydrogen gas is also generated.  The reduction in all these kits utilizes zinc and includes the following reactions:

As₄O₆ + 12 Zn +24H⁺→ 4AsH₃(gas) + 12 Zn^{2 +} + 6H₂O    and

Zn +2H⁺ → Zn²⁺ + H₂ (gas)

All five kits use recent advances in utilizing solid weak acids such as Sulfamic and Tartaric acids, which reduce the hazards in running and transporting these new test kits.  

The kits are similar in chemical reaction design.  The analyses are in a closed reaction bottle (all plastic), with an appropriate volume of sample (50 to 250 ml), and utilize the headspace above the liquid reaction mixture where the arsine gas accumulates during the reduction phase of the chemistry.  The bottle screw caps that close the reaction bottles also are used to hold a mercuric bromide indicator test strip in the headspace.  In this way, the mercuric bromide strip is appropriately located for colorimetric reaction with the arsine gas, and yet avoids direct contact with the aqueous mixture.  After the reduction reaction time of 10, 20, or 30 minutes is allowed to go on, the mercuric bromide strip is removed and compared to the standardized color chart for matching to the level found in the sample tested.  A blank or white strip indicates no arsenic present.  A very light yellow to dark brown color change indicates that arsenic is present and the color intensity is directly proportionately related to the concentration of arsenic in the sample.

Prices per test vary from about $0.50 to $2.40. All test kits have reliable performance results, with the exception of the HACH EZ Arsenic Test Kit, as shown in chart #2.  Where minimum-testing time is required then the Arsenic QUICK Test Kit would be ideal and the important thing is that the quality of results are not compromised.   If the analysis requires detection levels below 20 μg/L the ideal kit would be the Low Range Arsenic Check where yellow color intensities allow distinguishable color blocks at 20, 15, 10, 6, 4, and 2 to be part of the color block. This almost rivals instrumental analysis sensitivity for arsenic.

Chart #1

Comparison of kits:

 *Time in minutes for Arsine gas incubation