## 3 Results and Discussion
### Preservation Studies of As(III/V) Stability in Natural Water Samples
The aim of the study was to examine the effect of the proposed preservatives and the combination of preservatives in natural samples with higher and lower contents of arsenic and iron. Different natural samples from different areas were used. Samples were collected from wells and are presented in Table 2. As(tot) and As(III) content was determined on the first day, a few hours after collecting the samples and adding the preservatives. The verification of results, as well as the determination of iron was done by ICP-MS. pH values were determined in unpreserved samples. Preservations were made in the same way as for the model samples and from one original sample ten subsamples were made. All the preservatives were added in such way that their final concentration amounted to \(2\times 10^{-3}\) mol L\({}^{-1}\), while the amount of acetic acid was 5%. For all subsamples voltammograms were acquired until the complete oxidation of As(III) or reduction of As(V) occurs. The resulting graphs are presented in Figures 4-7. Figure 3: Graphs show the stability of the model solutions in (**a**) double-distilled water without preservatives and (**b**) with CA/HAc as a preservative. \(\gamma\) (As(III)) = \(\gamma\) (As(V)) = 10 \(\upmu\)g L\({}^{-1}\), \(\gamma\) (As (tot)) = 20 \(\upmu\)g L\({}^{-1}\). Figure 4: As(III) and As(tot) stability over time, dependent on the preservation treatment used—natural water sample A.  
Figure 5: As(III) and As(tot) stability over time, dependent on the preservation treatment used—natural water sample B. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sample** & **Place of Origin** & **As (tot)/ug L\({}^{-1}\)** & **As (III)/ug L\({}^{-1}\)** & **As (V)/ug L\({}^{-1}\)** & **Fe/mg L\({}^{-1}\)** & **pH** \\ \hline A & Osijek & 110.0 & 48.5 & 61.5 & 130.0 & 8.42 \\ B & Osijek & 164.8 & 118.0 & 46.8 & 88.3 & 8.02 \\ C & Zagreb & 9.86 & 8.5 & 1.36 & 10.5 & 8.32 \\ D & Zagreb & 16.62 & 8.46 & 8.16 & 10.15 & 8.24 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Natural groundwater samples—the sampling site, arsenic, iron content and pH value of the samples. Figures 4 and 5 show the effect of different treatment methods on As(III/V) preservation in natural water samples from Osijek county. Sample A had a lower inorganic As(tot) concentration than the other sample (B) from the same area, but higher concentration of Fe (Table 2). Sample B had the highest concentration of As(tot) and As(III) of all tested samples. In these two samples, the As redox stability was improved by the addition of CA and acidification with HAc. These subsamples were stable for 8 days, while in untreated samples, the oxidation of As(III) was observed on the second day after sampling. Figure 6: As(III) and As(tot) stability over time, dependent on the preservation treatment used—natural water sample C.  
Figure 7: As(III) and As(tot) stability over time, dependent on the preservation treatment used—natural water sample D.  
Although a good preservation effect by acidification of natural samples was reported [21], the addition of HAc initially lowered the content of As(III) in sample B. In samples collected in Zagreb county, which is not in the Pannonian Basin, lower As concentrations were determined. In all four samples, the As redox stability was substantially improved by preservation with CA and acidification with HAc, as well as with addition of tartrate and oxalate. In samples C and D, little difference between the treated and untreated subsamples during the first five days was noticed, as can be seen in Figures 6 and 7. To stabilize arsenic species in their original state, the redox activity of the coexisting Fe(III) and Fe(II) should be minimized. The formation of metal complexes can help to decrease the free metal-ion concentration, thereby inhibiting arsenic species' interconversion. The mobilization of arsenic in groundwater involves the desorption of arsenic from ferric oxide precipitates of aquifer sediments and/or oxidation of arsenopyrites (FeAsS), so As and Fe occur together in most groundwaters [37].