Pentavalent forms of arsenic in raw water can be easily removed to a great extent from water through co-precipitation process, unlike in case of its trivalent form. To determine the efficacy of arsenic removal filter based on co-precipitation process, it is very much necessary to quantify them. What is best laboratory process to analyze water sample in this regard?
You can read the following papers to measure As(V). For As(III), atomic absorption spectroscopy may be useful.
Dhar, R. K., Zheng, Y., Rubenstone, J., van Geen, A., 2004. A rapid colorimetric method of measuring arsenic concentrations in groundwater. Anal. Chim. Acta. 526, 203–209.
Wang, Z., Bush, R. T., Liu, J. (2013): Arsenic(III) and Iron(II) co-oxidation by oxygen and hydrogen peroxide: divergent reactions in the presence of organic ligands. Chemosphere 93, 1936–1941
If by 'raw water' you mean potable water, then use HPLC to separate the various As species, and use ICP-MS as the detector for the HPLC to measure the amount of each species after separation by the column.
If that is not available, measure As, by whatever method you have, both before and after you pass the water through an ion-exchange column that sorbs As(V). That is, measure total As and then As(III), which is not sorbed by resins that can sorb As(V). The ion-exchange can be done in the field or lab using columns you make yourself, or from pre-prepared columns available from MetalSoft (Dr. Meng), which you can find by Googling.
I think that the usefulness of the method will depend on the As level in the water. Sensitivity of colorimetric methods should be insufficient at low element levels. However, hydride generation followed by atomic absorption or atomic fluorescence will be suitable in this case. See the following article:
Anthemidis, Aristidis N.; Kalogiouri, Natasa P. : Advances in On-Line Hydride Generation Atomic Spectrometric Determination of Arsenic . ANALYTICAL LETTERS 46: 1672-1704
You can find the necessary information in the book
"Analytical Chemistry of Arsenic in Drinking Water", authors -
Andrew D. Eaton, Hsaio Chiu Wang, Jack Northington
The more recent use of hyphenated analytical techniques such as ICP/OES and ICP/MS allows the determination of arsenic without the need for hydride generation. In order to determine the individual arsenic species, such as As (III), As (V), in a water sample, the spectroscopic detector must be coupled with a chromatographic separation technique such as ion chromatography or high performance liquid chromatography. An accurate analysis is closely linked to correct sample handling.
You can find information in the book:
"Standard Methods For the examination of Water and Wastewater. 17 Edtion" American Public Health Association. ISBN 087553-161-X (or a newer edition).
There are a web page (http://www.standardmethods.org/)
where you can obtain this information too.
Voltammetry can also speciate between As(III) and total inorganic As (III+V) without any sample pretreatment or separation steps.
How to measure them by CV?I think voltammetry method may be interfered with other co-existing redox-active species.
I`ve never tried CV or the Hg drop based CSV methods, but there are a couple of approaches to doing it with a gold electrode, notably separation by pH and / or deposit potential. We use only the latter, but not all instruments are capable of measuring total inorganic As without quantitative reduction of all AsV to AsIII - a process we found unreliable in real samples. Bi, Sb and high levels of Cu or sulfide can interfere, but these aren`t typically a problem in groundwater, and there are ways around them if they arise.
Anal Chim Acta. 2009 Jul 30;646(1-2):6-16.
ICP-MS is a suitable technique to estimate Arsenic ( 5) and (3) in water sample..
The type of analytical method used and the knowledge of the different processes that affect inter-conversion of arsenic species is important to reach to a representative data. Depending on the type of the sample matrix, preservation by EDTA and acids help in As(III)/As(v) stabilization. Furthermore, filtration, optimum storage conditions and suitable sample containers play significant role in maintaining integrity of arsenic species. HPLC-ICP-MS is the most commonly used hyphenated system for speciation analysis of arsenic. HPLC is a versatile separation technique since it uses different types of stationary phases (Normal, reversed) and various mobile phases. Separation can also be enhanced by adding ion pair reagents to the mobile phase. Changing mobile phase during analysis (gradient elution) is also another merit of liquid chromatography. ICP-MS can serve as an element specific detector and offers better sensitivity.
I have tried both CSV with a variety of Au electrodes and HPLC-ICP/MS. Both techniques have advantages and drawbacks, but I rather prefer HPLC-ICP/MS (maybe less precise, but much faster and less interferences).
There is a relatively recent review by Komorowicz and Baralkiewicz in Talanta, 84:247, 2011
The simplest method, that I have applied and recommend, is use of aluminosilicate cartridges. As(V) in the water samples is removed by the cartridges and As(III) remains in the filtrates and by analysing total As in raw and filtrate solution you can determine As(total) and As(III), and by this also As(V). See also article by Meng et al. 2001, "TREATMENT OF ARSENIC IN BANGLADESH WELL WATER USING A HOUSEHOLD CO-PRECIPITATION AND FILTRATION SYSTEM"
in case you want to compare your analyses to other labs, note that our institute is providing PT testing for arsenic and chromium (incl. speciation) - see http://www.kwrwater.nl/page.aspx?id=10123 for our program. Please let us know whether you are interested, since we can better compare with more participants.
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