## 1 Introduction  
Arsenic participates in complex biological and chemical processes, and naturally occurs in the environment (soil, air, water, etc.) in different oxidation states and species that have different chemical behaviours and toxicities [1, 2]. Inorganic species are more toxic than organic arsenic compounds like monomethyl- and dimethyl arsenic, which are predominant in living organisms [3]. The reduced inorganic species, trivalent arsenic As(III), is approximately 60 times more toxic than oxidized, pentavalent arsenic As(V) [4, 5]. Therefore, arsenic speciation analysis of environmental samples is necessary. According to the World Health Organization (WHO) report, the maximum permissible concentration of arsenic in drinking water is 10 \(\mu\)g L\({}^{-1}\)[6]. In groundwater, arsenic is predominantly present as As(III) and As(V) [7]. The stability of sample plays a significant role in species content determination. Between sampling and laboratory analyses, changes in the species composition should be avoided. Otherwise, a suitable procedure for the preservation of the species should be implemented. It should be noted that there are inconsistencies in published data on the preservation procedures for stabilizing aqueous inorganic As(III/V) redox species. Oxidation of As(III) is the most commonly observed change, although reduction of As(V) has also been reported [8]. The stability of As(III) in groundwater was reported to be 2 days [8], 3 weeks [9], and even 3 months [10]. The addition of ascorbic acid, HCl, HNO\({}_{3}\) and H\({}_{2}\)SOas preservative agents has been described previously in the literature. It has been published that ascorbic acid oxidizes As(III) in natural water samples as well as HNO\({}_{3}\)[8]. HCl is not suitable when inductively coupled plasma mass spectrometry (ICP-MS) is used for detection, due to the \({}^{40}\)Ar\({}^{35}\)Cl\({}^{+}\) molecular interference on monoisotopic \({}^{75}\)As\({}^{+}\)[11; 12]. H\({}_{2}\)SO\({}_{4}\) showed good preservation activity in model water samples but it is a poor preservative in natural water samples [13]. Groundwater often has high iron concentration [14]. Oxygen alone is not able to oxidize As(III), so dissolved ferric ions, other redox active ions or surface active substances are necessary for this reaction [15]. Complexation of iron in combination with a moderate acidification for avoiding the precipitation might stabilize arsenic species in water samples until laboratory analysis [16]. Disodium ethylenediaminetetraacetic acid (EDTA) is usually used as preserving agent when determining arsenic speciation by chromatographic techniques (ion chromatography (IC)) or high-performance liquid chromatography (HPLC)) with ICP--MS detection. EDTA binds the Fe, Mn and Al cations, preventing their interaction with arsenic and the subsequent precipitation of complexes formed [16]. Hydroxy acids also coordinate metal ions well and are used in the farming and metal industries as complexing agents for micronutrients in soil fertilizers and for cleaning metal surfaces, such as those of aluminium, copper, iron and their alloys. For example, oxalate, the conjugate base of oxalic acid, is an excellent ligand for metal ions [17; 18; 19]. The efficiency of preservatives for As(III)/As(V) stabilization depends on the sample matrix and microbial activity. There is no universal preservative that is effective for all water samples, so individual sample matrices need to be tested for species stability before a particular preservative can be employed. To produce representative data on species distribution, knowledge of the various processes that influence the interconversion of arsenic species is necessary. Filtration, refrigeration at 4 \({}^{\circ}\)C and storage in dark conditions are recommended in arsenic-species stabilization, as well as preparation of standards in distilled-deionised water [20]. The preservation of arsenic species also depends on the analytical technique to be used.