Chemistry: analytical and immunological testing – Metal or metal containing
Reexamination Certificate
2001-11-09
2004-02-24
Warden, Jill (Department: 1743)
Chemistry: analytical and immunological testing
Metal or metal containing
C436S177000, C436S175000
Reexamination Certificate
active
06696300
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to analysis of the level of arsenic in an aqueous sample.
BACKGROUND OF THE INVENTION
Controversy currently exists over what level of arsenic in drinking water is acceptable. But there is no controversy that there is a need to be able to determine the arsenic level in natural waters, drinking water and groundwater. Also beneficial is being able to determine the arsenic level in humans, soil, pharmaceuticals, prepared biological materials and foods, and to monitor the arsenic level in certain industrial processes.
Commercially available tests for the analysis of arsenic are exemplified by tests of Merck and Hach, and work by the reduction of arsenic, in particular inorganic arsenic compounds such as trivalent and pentavalent arsenic compounds, in an acidic aqueous environment to arsine gas. According to the Hach test description, hydrogen gas is also generated. Thus, using zinc in the arsenic reduction. reaction, reduction to arsine gas may include the following reactions:
As
4
O
6
+12Zn+24H
+
→4AsH
3
↑+12Zn
2+
+6H
2
O
Zn+2H
+
→Zn
2+
+H
2
↑
In these analyses, a closed reaction bottle providing a headspace above the aqueous reaction mixture, is used for the arsenic reduction reaction, and the bottle cap holds a mercuric bromide test strip in the headspace. In this way, mercuric bromide indicator is appropriately located for reaction with arsine gas, yet spaced away from the aqueous mixture to reduce contact by the aqueous mixture. These analyses instruct that after a reaction time of 30 minutes, the indicator on the test strip may be visually evaluated for any change from white to yellow to brown.
In the Merck analysis, liquid hydrochloric acid is used for sample acidification, and acidification is carried out after zinc powder is added to the aqueous sample. An evaluation of pH after the acidification but without the zinc powder addition, indicates that the acidification adjusts pH to less than 0.8.
In the Hach analysis, a 50 ml aqueous sample is adjusted to a pH of about 9, and then Oxone® is added to the alkaline sample to oxidize hydrogen sulfide to sulfate ion to prevent interference with the analysis. Thereafter, according to test details, EDTA salts, which are typically used as a chelating agent, are added. Then, the sample is acidified using sulfamic acid powder, to a pH of about 1. Then, reduction of arsenic to arsine gas is carried out, during which the reaction mixture is swirled twice. A reactive pad impregnated with mercuric bromide, reacts with the arsine gas. To quantify both organic and inorganic arsenic, a liquid sample is subjected to a boiling water bath for 30 minutes after the pH adjustment and Oxone® addition.
Despite advances in the analysis of arsenic, there remains a significant need for an improved arsenic test.
SUMMARY OF THE IMVENTION
In accordance with the present invention, an improved method for the analysis of arsenic is provided. In the improved method, arsenic is reduced to arsine gas in an acidic aqueous reaction environment, and in the presence of an effective amount of an agent for increasing the rate of arsine gas production; and the arsine gas is reacted with an indicator for arsine gas. Beneficially, in accordance with the invention, a rate-increasing metal cation is used as the agent.
The inventive method is useful for analysis of an aqueous sample, which may be an extract. In accordance with the invention, a soil sample may advantageously be subjected to a digestion reaction in the presence of an oxidizing agent, to obtain an aqueous extract for the arsenic reduction reaction. Other extracts may, of course, be prepared, and thereafter analyzed as described herein.
In addition, the inventive method is useful for analysis of arsenic level in humans. Thus, in accordance with the inventive method, a urine sample may be analyzed for arsenic. Advantageously in this and other applications of the invention, a pad carrying the indicator, may be covered with a removable, gas permeable protective layer.
Beneficially, in accordance with one aspect of the invention, Fe(II) or Fe(III) is used as the rate-increasing agent. In a highly preferred embodiment, a mixture of iron and nickel cations is used. In this embodiment, substantially complete arsenic reduction may be effected within about 10 minutes.
Other rate-increasing metal cations may be advantageously used in the inventive method. Beneficially, these other metal cations are divalent metal cations or are metal cations of greater valency than divalent.
When a sample may include an interfering oxidizable substance such as hydrogen sulfide, the inventive method advantageously may further include prior to the arsenic reduction reaction, an oxidation reaction to produce, for instance, non-interfering sulfate ion. In accordance with the inventive method when the oxidation reaction is used, the sample may be adjusted prior to the oxidation reaction, to an acidic pH, and iron cation may beneficially be present during the oxidation reaction. Alternatively, the oxidation reaction may be carried out without prior adjustment of the sample pH, and iron cation may beneficially be present during the oxidation reaction; or, if desired, the sample pH may be adjusted to an alkaline pH.
Advantageously, in accordance with another beneficial aspect of the inventive method when the oxidation reaction is used, the arsenic reduction reaction may be carried out without the intervening step of adding EDTA salts described in, the Hach analysis. As can be understood, both time and money may be saved.
In accordance with yet another aspect of the inventive method, an acidifying agent in powder, granule or other non-liquid form is used to acidify the reaction environment. Preferably, the acidifying agent may be a non-corrosive type acidifying agent such as L-tartaric acid.
Additional advantages and beneficial features of the present invention are set forth in the detailed description, and in part will become apparent to those skilled in the art upon examination of the detailed description or may be learned by practice of the invention.
DETAILED DESCRIPTION OF THE INVENTION
In accordance with the present invention, it has been discovered that in an acidic aqueous reaction environment the rate of arsine gas production is increased by the use of certain metal cations, and that as a result, a relatively greater percentage of arsenic is reduced to arsine gas within a comparable time period. For example, investigation indicates that within the thirty minute time period allowed for the arsenic reduction reaction in the foregoing commercially available analyses, substantially less than 50%, possibly only 25% or even less, of arsine gas is detected.
Based on these and other discoveries, the present invention is directed to an improved method for the analysis of arsenic, that beneficially can provide a higher percentage of arsine gas production in a comparable or significantly shorter (for example, ten minutes) time period. Furthermore, because of relatively increased arsine gas production, the inventive method can provide increased sensitivity for low levels of arsenic. Thus, this method is useful for low levels of arsenic in the range of 2 or 3 ppb to about 5 or 7 ppb or more, up to about 10 ppb, as well as for higher levels of arsenic up to 50 or 100 ppb or more, even in excess of 500 ppb. Moreover, in a preferred aspect, the inventive method relies upon a non-corrosive type acidifying agent. Also in accordance with the inventive method., the arsenic reduction reaction may be controlled whereby, for instance, undesired splashing of the reaction mixture is reduced. In addition, the inventive method can provide greater reproducibility of results.
In accordance with the present invention, an aqueous sample is used for the analysis. The aqueous sample may be an extract. In accordance with the invention, an aqueous extract of soil may beneficially be prepared by adding arsenic-free water and an oxidizing agent, and digestin
Jaunakais Ivars
Jaunakais Lea Marianne
Lewis Corlyss Brown
Industrial Test Systems, Inc.
Kroboth Timothy R.
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