AsV in marine systems

Marine Pollution Bulletin 67 (2013) 228–233

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Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul

Baseline

A new approach to determine the ratio of redox active species such as SeIV/SeVI and AsIII/AsV in marine systems A. Aksu a, N. Balkis a,⇑, I. Salihoglu b a b

Istanbul University, Marine Science and Management Institue, Chemical Oceanography, Vefa 34134, Turkey Near East University Marine Faculty, Cyprus

a r t i c l e

i n f o

Keywords: Oxidation–Reduction Potential (ORP) pH rH Nernst equation Redox active species Electrostatic activity coefficient (Fel)

a b s t r a c t The ratio of redox active species contributes to the researches about marine systems in many ways. Are marine systems reductant or oxidant? For this purpose, redox active species are analyzed by using high technology instrumental analyzers such as AAS, ICP, and HPLC. Then, all ion pair species are compared to each by calculating their ratios. These technologies are very expensive, and it takes long time to obtain the results. In this study, we suggested a basic method by using pH and Eh. Therefore, the Nernst equation expression was rearranged by using relative hydrogen (rH) and electrostatic activity coefficient (Fel). Additionally, the ratio of the redox active ion pair species SeIV/SeVI and AsIII/AsV was calculated. Ó 2012 Elsevier Ltd. All rights reserved.

Speciation is the identification and quantitative determination of different forms or phases in which a given element occurs in a given substance established in so-called speciation analysis (Lobinski, 1997). The species distribution of elements in groundwater is controlled by the geochemical conditions, in particular the redox environment. The nature of the redox reactions that control the Eh in seawater is very complicated. It is not known exactly which chemical species control the ORP, and it is not an equilibrium situation, so all simple chemical equations will only be an approximation of what is taking place. Compounds containing arsenic and selenium occurring in trace amounts in various ecosystems have recently become a subject of close monitoring. Although these elements rarely even in a polluted environment reach a level of toxic concentration, a small difference between their admissible and toxic doses absorbed by living organisms in view of their common presence that means their presence requires careful control (Dojlido, 1995). Measurements of AsIII and AsV are very important because AsIII is very toxic. In general, relatively small amounts of selenium are found dissolved in water (Furr et al., 1979; Nriagu and Wong, 1983; Lemly, 1985). The most common forms of selenium are selenic and selenious acids in alkaline waters. SeIV/SeVI and AsIII/AsV ratios have to known in marine systems. But, analysis of these elements is very difficult and expensive. In this study, these elements are established firstly by theoretical calculation and experimental measuring. Oxidation and reduction potential parameters (ORP, Eh),

⇑ Corresponding author. Tel.: +90 212 440 00 00/26053; fax: +90 212 526 84 33. E-mail address: [email protected] (N. Balkis). 0025-326X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.marpolbul.2012.11.002

relative hydrogen unit (rH), activity (a), electrostatic activity coefficient (Fel) are used within the Nernst equation to determine the theoretical ratio of element species. Thus, these results form a base for future studies. Oxidation and reduction potential parameter (ORP, Eh) of marine aquaria is a measure of the relative oxidizing power of the water. ORP, at its heart, is very very complicated. It is, perhaps, the single most complicated chemical feature of marine aquaria that aquarists will typically encounter. The chemical species that control ORP in one aquarium might not even be the same chemicals that control ORP in another aquarium or in natural seawater. Most aquarium authors have recommended a range of 300–450 mV. Because, the ocean often has ORP in this range, and the authors have successfully operated aquaria in this range (Homles-Farley, 2008). Relative hydrogen unit (rH): After determining the redox potential using Nernst relation, a correlation was made with other physicochemical parameters of given redox system. In this manner, the so-called relative hydrogen (rH) score can be determined, which gives information on the ‘‘redox capacity’’ of the system. In order to evaluate the specificity of the redox environment of a solution between the electrode potential and the pH of solution, a specific factor was declared and named as rH score that can define the state of a system that reached the equilibrium (Clark, 1923; Clark, 1960). Thus, rH characterizes the redox capacity of a redox system. There have been equations proposed that purport to ‘‘correct’’ ORP for changes in pH, giving a new parameter, sometimes called rH. This parameter was proposed in the 1920s by W.M. Clark. One form of this correction is shown below:

rH ¼ mV=29 þ ð2  pHÞ

A. Aksu et al. / Marine Pollution Bulletin 67 (2013) 228–233

The use of the relation given by Nernst – integrating Clark’s conception – to establish the relative hydrogen (rH) score is discussed by Pinto (2008). There are three effects to ions in solution. These are electrostatic interaction, ion complexing or specific interaction and activity. Electrostatic interaction: Ions were reacted with water molecules (and) other major ions in sea water. These interactions result in nonideal solutions. Ions with higher charge are more effective than ions with lower charge at this shielding effect (Murray, 2004). Ion complexing or specific interaction: There are specific interactions between ions–solutes come close enough that they make direct contact and are considered a new species. These new species are called ion pairs (when ions are separated by H2O molecules but share their first hydration shell) or complexes (Murray, 2004). Activity (a): Ions in solution interact with each other as well as with water molecules. At low concentrations (Ci) and low background salt concentrations, these interactions can possibly be ignored but at higher concentrations ions behave chemically like they are less concentrated than they really are. Equilibrium constants calculated from the Standard Free Energy of reaction (e.g., DGr°) are expressed in terms of this effective concentration, which is formally called the activity that means the concentration available for the reaction. Thus, we define the activity as (Murray, 2004):

Activity ðai Þ ¼ Effective concentration ðC i Þ To understand the chemical behavior of ionic solutes in natural waters, it is necessary to know the thermodynamic activity rather than the total concentration (Millero, 1971). The activity and total concentration are related by

A ¼ ½iT Y T ðiÞ where Yt(i) is the total or stoichiometric activity coefficient of i. The value of it is related to the nonideal behavior of i due to ionic interactions. These ionic interactions are controlled by the composition of the natural waters. The effect of composition on the activity of electrolytes can be estimated by using various ionic interaction models (Pitzer, 1973; Whitfield, 1975; Pytkowizt, 1979; Dickson and Whitfield, 1981). A number of workers (Millero, 1975, 1977, 1979; Pitzer, 1973; Whitfield, 1975; Pytkowizt, 1979; Dickson and Whitfield, 1981) have applied these models to seawater, rivers, lakes, and brines. These models can be divided into two major types; 1. Specific interaction model and 2. Ion-pairing model. The specific interaction models yield reliable estimates for the major ionic components, while the ion-pairing models yield reliable estimates for the minor ionic components. Most of the early models used to estimate the activity coefficients of ions were based on extensions of the Debye–Huckel equation;

lnY i ¼ AI1=2 =ð1 þ CaI1=2 Þ where A and C are Debye–Huckel parameters, and a is the ion size parameter which is an adjustable term (Robinson and Stokes, 1959). The theory of Debye and Huckel gives specific consideration of only the long-range electrical interactions between ions. Even here, physical properties such as the dielectric constant are given values appropriate to the pure solvent. At higher concentrations, ion– solvent interactions and short-range interactions between ions become important. Solvation and association should not be ignored. These effects give contributions to the logarithm of the activity coefficient, which are proportional to the solute concentrations even in solutions of nonelectrolytes. Consequently, at concentrations where such terms are comparable to the square-root term, the Debye– Hückel theory can no longer adequately describe the thermodynamic properties. Refinement of the electrical contributions is not very useful unless these noncoulombic interactions are also accounted for Newman adds though. Disappointedly enough, it must be admitted

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that the rigorous justification of the D–H theory is the most important progress that has been made by the recent developments in the field of electrolyte theory (Newman, 1967, 1973). It is well known that besides nonspecific electrostatic long-range interactions accounted for by the Debye–Hückel term. The main contribution to activity coefficients of ions in concentrated solutions is due to the cation– cation or anion–anion interactions are small because the ions with the same sign of charge usually stay away from one another. This fact was recognized by Bronsted as early as 1922 (Whitfield, 1973). Adopting the Bronsted postulate on specific interaction between ions and the Debye–Hückel equation for the nonspecific electrostatic term. The specific interaction models give an estimate of cT. Bronsted–Guggenheim Model (Whitfield, 1973)

log c þ MX ¼ log cEL þ mBMX ½MX where c + MX is the mean activity coefficient for the salt MX. cel represents the long-range electrostatic interactions.

cel ¼ AðZ 2 ÞðI1=2 =1 þ BaI1=2 Þ BMX is the short-range interaction coefficient between M and X. If MX molecule has lower concentration, short-range interaction should be ignore in salty water. Nernst equation:

E ¼ E0 þ ðRT=nFÞ 2:303 log aox =ared or

E ¼ E0 þ ðRT=nFÞ2:303 log½ox=½red: E is the Half-cell redox potential (mV), E0 is the Standard electrode potential (mV), R is the universal gas constant (8.314 joule/ (mol Kelvin), T is the Absolute temperature (Kelvin), F is the Faraday constant (1F = 96,496 C mol1), and n is the number of moles of electrons transferred. The redox potential (E) of the redox system is calculated starting from the value of the standard redox potential (E0) of the system. Additionally, the value of some physicochemical constants and the ratio of chemical activities aox/ared or chemical concentrations (ox)/(red) are used in the equation. The temperature is assumed to be 25 °C, and the electromotive force voltage (E, E0) is used as millivoltage. Then, the expression of the Nernst equation is rearranged in the following form;

E ðMvÞ ¼ E0 ðmVÞ þ ð59; 16=nÞ log aox =ared or,

E ðmVÞ ¼ E0 ðmVÞ þ ð59; 16=n Þ log½ox=½red The Istanbul Strait (Bosphorus) is located between the Marmara Sea and the Black Sea forming part of the Turkish Strait System which consists of the Dardanelles Strait, the Marmara Sea and the Istanbul Strait. This system which is approximately 300 km in length connects the Mediterranean Sea via Agean Sea to the Black Sea. The system has an important influence on oceanographic condition in the Black and the Marmara Sea. The Bosphorus current system has two layers stratified flow with different salinities. While the upper layer flows from the Black Sea with low salinity, the lower layer flows from the Marmara Sea, which is originated from the Mediterranean Sea with high salinity in the opposite direction of the upper layer. The Golden Horn is an estuary, which flows in west–southeast direction and joins the Bosphorus at Istanbul. It is approximately 7.5–8 km in length and also has a current system with two layers with different salinities. For this purpose, sea water samples are collected from the Bosphorus and Golden Horn in January, February, March, April, May, June, July, October, November, and December 2008 (Fig. 1). The sampling depths are 0.5 m, 10 m, and 20 m, the halocline layer and the deep.

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As(III) can be found directly by using HG-AAS method. Arsenic is converted into H3As by the treatment of the sample with 0.4% borohydride solution (containing 2 g of NaBH4 + 2.5 g of NaOH in 500 ml aqueous solution) (Andrea, 1977).

3BH4 þ 3Hþ þ 4H3 AsO3 ! 3H3 BO3 þ 4AsH3 " þ3H2 O To a 5 ml volume of sample of inorganic As (containing both AsIII and AsV). 1 ml of 10% KI + Ascorbic acid is added and let to stand for 30 min. This enables all inorganic arsenic to convert into AsIII, which can subsequently be analyzed with HG-AAS. Then, the procedure A is applied. As(V) can be calculated from the difference of B–A (Driehaus and Jekel, 1992). Se(IV) can be found directly by using HG-AAS method. Selenium is converted into H2Se by the treatment of the sample with 0.4% borohydride solution (containing 2 g of NaBH4 + 2.5 g of NaOH in 500 ml aqueous solution) (Cutter, 1978, 1982).

3BH4 þ 3Hþ þ 4H2 SeO3 ! 3H3 BO3 þ 4H2 Se " þ3H2 O To a 5 ml volume of sample of inorganic Se (containing both SeIV and Se ), 1 ml of 6 N HCl is added and let to stand for 30 min. This enables all inorganic selenium to convert into SeIV, which can subsequently be analyzed with HG-AAS. Then, the procedure A is applied. Se(VI) can be calculated from the difference of B–A (Cutter, 1978, 1982). Eh is named as Oxidation–Reduction Potential (ORP). The ORP determined with platinum electrode probe. And, the hydrogen ion concentration determined with glass electrode probe. Then, the pH values are calculated by getting the negative logarithm of the hydrogen ion concentrations. The oxidation–redox potential results for As and Se ions and the rH values are given in Tables 1–4. The ORP values vary between 335 mV and 150 mV. ORP values must be between 300 mV and 450 mV in healthy marine system (Clarck Mansfield, 1960; Gorzyslaw and Jozef, 2003). Only six measurement values are above the 300 mV, so the examined sea water has reduction condition. The relative hydrogen (rH) values are calculated by using Clarck’s equation. Relative hydrogen (rH) values vary between 26.42 and 21.35 in redox system. In the case of the redox system – taking the criterion of ‘‘neutrality’’ into consideration – it can be considered that systems have; VI

(a) rH < 28.3 – are reducing systems that can release electrons to other systems with lower rH and (b) rH > 28.3 – are oxidant systems that can accept electrons from systems with higher rH (Garban, 2008). ORP and rH parameters help to determine the chemical composition of the sea water. Due to that reduction occurs in the sample of sea water that is examined.

41.5

The Nernst equation is used to determine SeIV/SeVI and AsIII/AsV ratio. Here, rH values are used instead of the Eh in the Nernst equation. Because for a particular environment (an aqueous solution), pH characterizes the reaction of environment (i.e., acidity/alkalinity), and rH characterizes the redox (oxidation/reduction ratio) capacity of the environment (Garban, 2008; Clarck Mansfield, 1920). Activity is used instead of the concentration in Nernst equation. Activity coefficient is the most important part of the activity. The Debye–Hückel term contributes to the activity coefficient particularly. Activity coefficient is applicable for I (Ionic strength) < 0.01 in Debye Hückel equations. In extended Debye Hückel equation, I (Ionic strength) must be below 0.1, but the ionic strength of the sea varies between 0.31 (salinity 14‰) and 0.72 (salinity 33‰). Since the seas surrounded our country have high salinity values. A different activity coefficient is needed. In high salinity seas, there are some interactions between the ions, so the longrange electrostatic interactions contribute to the activity coefficients. According to Bronsted–Guggenheim model electrostatic activity coefficient:

fel ¼ AðZ 2 ÞðI1=2 =1 þ B a I1=2 Þ ! B a  1: A  0:5 The rearranged Nernst equation is used for arsenic and selenium as the following manner;

fel ¼ AðZ 2 ÞðI1=2 =1 þ B a I1=2 Þ ! B a  1: A  0:5 The rearranged Nernst equation is used for arsenic and selenium as the following manner;

H3 AsO4 2H þ 2e ! H3 AsO3 þ 2H2 O     rHðEðmVÞÞ ¼ rH0 þ ð59:16=2Þ log fAs ðVÞ CAsðVÞ =fAsðIIIÞ CAsðIIIÞ 1=2 fAsðVÞ ¼ ðð0:5 52 I1=2 ds Þ=ð1 þ I ds ÞÞ 1=2

H2 SeO4 þ 2H þ 2e ! H2 SeO3 þ H2 O rHðEðmVÞÞ ¼ rHo þ ð59:16=2Þ  log f SeðVIÞ CSeðVIÞ=f SeðIVÞ CSeðIVÞÞ fSeðVIÞ ¼ ðð0:5 52 Ids 1=2Þ=ð1 þ Ids 1=2ÞÞ fSeðIVÞ ¼ ðð0:5 32Ids 1=2Þ=ð1 þ Ids 1=2ÞÞ

BLACK SEA

Bosphorus

41 MARMARA SEA

40.5

40

TURKEY 26.5

27

27.5

28

28.5

29

29.5

1=2

fAsðIIIÞ ¼ ðð0:5 32 Ids Þ=ð1 þ Ids ÞÞ

30

Fig. 1. Study area.

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A. Aksu et al. / Marine Pollution Bulletin 67 (2013) 228–233 Table 1 Theoretically calculated and experimentally measured redox active species ratio of AsV/AsIII and SeVI/SeIV in March 2008. March

Stations

Depths

Eh (mv)

pH

rH

rH0AS

Theoretically calculated

Experimentally measured

AsV/AsIII

AsV/AsIII

K0

0.5 10 20 50 68

226 226 231 243 249

8.17 8.18 8.23 8.13 8.01

24.13 24.15 24.43 24.64 24.61

35.65 35.67 35.77 35.57 35.33

1.13 1.13 1.15 1.18 1.20

2.42 0.94 1.96 0.69 1.02

M8

0.5 10 27 40

217 223 235 241

8.29 8.32 8.07 7.98

24.06 24.33 24.24 24.27

35.89 35.95 35.45 35.27

1.10 1.12 1.16 1.18

0.45 1.79 1.89 1.00

P.P

62 0.5 10 20 32

241 224 223 225 245

7.85 8.18 8.28 8.27 7.95

24.01 24.08 24.25 24.30 24.35

35.01 35.67 35.87 35.85 35.21

1.18 1.12 1.12 1.13 1.19

2.63 2.01 2.79 2.10 0.42

SeVI/SeIV

SeVI/SeIV

K0

0.5 10 20 50 68

226 226 231 243 249

8.17 8.18 8.23 8.13 8.01

4.13 24.15 24.43 24.64 24.61

18.06 18.08 18.18 17.98 17.74

4.46 4.46 4.52 4.67 4.75

2.92 2.24 2.38 6.98 2.50

M8

0.5 10 27 40 62

217 223 235 241 241

8.29 8.32 8.07 7.98 7.85

24.06 24.33 24.24 24.27 24.01

18.30 18.36 17.86 17.68 17.42

4.35 4.43 4.57 4.64 4.64

1.54 0.64 6.51 2.86 2.80

P.P

0.5 10 20 32

224 223 225 230

8.18 8.28 8.27 7.96

24.08 24.25 24.30 23.95

18.08 18.28 18.26 17.64

4.44 4.43 4.45 4.55

1.80 1.80 1.80 1.80

Table 2 Theoretically calculated and experimentally measured redox active species ratio of AsV/AsIII and SeVI/SeIV in June 2008. June

Stations

Depths

Eh (mv)

pH

rH

rHAs0

Theoretically calculated AsV/AsIII

Experimentally measured AsV/AsIII

K0

0.5 10 20 49 68

155 161 167 184 185

8.1 8.1 8.06 7.82 7.73

21.54 21.75 21.88 21.98 21.84

35.51 35.51 3.43 34.95 34.77

0.93 0.95 0.96 1.01 1.01

2.62 3.15 1.14 2.21 1.69

M8

0.5 10 22 40 62

164 169 179 185 186

8.1 8.01 7.8 7.75 7.73

21.86 21.85 21.77 21.88 21.87

35.51 35.33 34.91 34.81 34.77

0.96 0.97 1.00 1.01 1.02

2.25 1.83 3.64 3.30 1.13

P.P

0.5 10 20 32

150 164 260 255

8.09 8.09 8.05 7.75

21.35 21.84 25.07 24.29

35.49 35.49 35.41 34.81

0.92 0.96 1.24 1.22

3.07 1.93 1.56 1.19

SeVI/SeIV

SeVI/SeIV

K0

0.5 10 20 49 68

155 161 167 184 185

8.1 8.1 8.06 7.82 7.73

21.54 21.75 21.88 21.98 21.84

17.92 17.92 1.84 17.36 17.18

3.68 3.74 3.81 3.98 3.99

0.23 2.01 1.72 1.47 1.16

M8

0.5 10 22 40 62

164 169 179 185 186

8.1 8.01 7.8 7.75 7.73

21.86 21.85 21.77 21.88 21.87

17.92 17.74 17.32 17.22 17.18

3.78 3.83 3.93 3.99 4.01

1.21 2.92 1.87 1.58 2.90

P.P

0.5 10 20 32

150 164 260 255

8.09 8.09 8.05 7.75

21.35 21.84 25.07 24.29

17.90 17.90 17.82 17.22

3.64 3.78 4.89 4.82

1.11 1.16 1.91 2.90

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Table 3 Theoretically calculated and experimentally measured redox active species ratio of AsV/AsIII and SeVI/SeIV in September 2008. September

Stations

Depths

Eh (mv)

pH

rH

rHAs0

Theoretically calculated AsV/AsIII

Experimentally measured AsV/AsIII

K0

0.5 10 20 46 68

191 188 187 196 199

8.13 8.13 8.12 7.8 7.65

22.85 22.74 22.69 22.36 22.16

35.57 35.57 35.55 34.91 34.61

1.03 1.02 1.02 1.04 1.05

1.47 0.13 0.66 0.64 0.19

M8

0.5 10 15 40 62

168 172 181 191 192

8.1 8.08 7.92 7.66 7.61

21.99 22.09 22.08 21.91 21.84

35.51 35.47 35.15 34.63 34.53

0.97 0.98 1.00 1.03 1.03

0.62 1.27 1.21 1.34 1.43

P.P

0.5 10 20 32

224 223 225 245

8.18 8.28 8.27 7.95

24.08 24.25 24.30 24.35

35.67 35.87 35.85 35.21

1.12 1.12 1.13 1.19

2.01 2.79 2.10 0.42

SeVI/SeIV

SeVI/SeIV

K0

0.5 10 20 46 68

191 188 187 196 199

8.13 8.13 8.12 7.8 7.65

22.85 22.74 22.69 22.36 22.16

17.98 17.98 17.96 17.32 17.02

4.06 4.03 4.02 4.11 4.15

0.93 16.64 4.40 4.07 0.98

M8

0.5 10 15 40 62

168 172 181 191 192

8.1 8.08 7.92 7.66 7.61

21.99 22.09 22.08 21.91 2.84

17.92 17.88 17.56 17.04 16.94

3.82 3.86 3.95 4.06 4.07

4.27 7.24 3.08 3.83 2.70

P.P

0.5 10 20 32

157 156 158 193

8.1 8.08 8.1 7.64

21.61 21.54 21.65 21.9

17.92 17.88 17.92 17.00

3.70 3.69 3.71 4.08

6.34 0.89 0.90 1.92

Table 4 Theoretically calculated and experimentally measured redox active species ratio of AsV/AsIII and SeVI/SeIV in December 2008. December

Stations

Depths

Eh (mv)

pH

rH

rHAs0

Theoretically calculated AsV/AsIII

Experimentally measured AsV/AsIII

K0

0.5 10 20 49 68

302 291 283 280 265

8.18 8.17 8.18 7.93 7.84

26.79 26.37 26.12 25.52 24.82

35.69 35.65 35.67 35.17 34.9

1.39 1.35 1.32 1.31 1.26

1.20 1.20 0.22 0.30 0.24

M8

0.5 10 14 40 62

335 305 274 267 265

7.91 7.99 7.93 7.8 7.77

27.37 26.50 25.31 24.81 24.68

35.13 35.29 35.17 34.91 34.85

1.52 1.40 1.29 1.26 1.26

1.20 1.35 1.20 0.86 0.65

P.P

0.5 10 20 32

300 289 283 283

8.59 8.66 8.65 8.29

27.52 27.29 27.06 26.34

36.49 36.63 36.61 35.89

1.38 1.34 1.32 1.32

0.56 0.53 1.05 0.61

SeVI/SeIV

SeVI/SeIV

K0

0.5 10 20 49 68

302 291 283 280 265

8.188 8.17 8.18 7.93 7.84

26.79 26.37 26.12 25.52 24.82

18.10 18.06 18.08 17.58 17.40

5.47 5.31 5.20 5.16 4.95

9.51 6.41 2.50 1.28 1.19

M8

0.5 10 14 40 62

335 305 274 267 265

7.91 7.99 7.93 7.8 7.77

27.37 26.50 25.31 24.81 24.68

17.54 17.70 17.58 17.32 17.26

5.98 5.52 5.08 4.98 4.95

3.55 3.56 3.86 4.12 2.60

The results are determined by using hydride generation atomic absorption spectrometry (HG-AAS). The ratios of SeIV/SeVI and AsIII/AsV are given in Tables 1–4. When theoretically calculated and experimentally measured, results are compared. It is clearly seen that they are approximately equal to one another.

In conclusion, the ratio of redox active species such as Fe2+/Fe3+, Se /SeVI, AsIII/AsV provide valuable information about the marine systems. Generally, HG-AAS equipment is used to measure SeIV/ SeVI and AsIII/AsV ratio. But, this is a high-cost and time-consuming method. We are suggesting a new Nernst equation. Here, rH is used IV

A. Aksu et al. / Marine Pollution Bulletin 67 (2013) 228–233

instead of Eh, and electrostatic activity coefficient is used instead of activity coefficient. rH and Fel are put into the new Nernst equation. And, the ratios of SeIV/SeVI and AsIII/AsV are calculated accurately. This calculation is advantageous for the researches as pH, and Eh meters are used. These advantages are beneficial because of being helpful at gain of time and money. Acknowledgements We thank the captain, crew, scientists, and technicians on board of R/V ARAR; the Institute of Marine Sciences; and the Management of Istanbul University for their support during the collection of water samples. We also thank Dr. Günay Kural for her English edition. This work was supported by the General Directory of Istanbul Water and Sewerage Administration (ISKI). References Andrea, M.O., 1977. Determination of arsenic species in natural waters. Anal. Chem. 49, 820–823. Clarck Mansfield, W., 1920. The Determination of Claridation – Reduction Hol Pint Sko Hydrogen Ions. Williams and Wilkins Co., Baltimore. Clarck Mansfield, W., 1960. Oxidation–Reduction Potentials of Organic Systems. Williams and Wilkins Co, Baltimore. Cutter, G.A., 1978. Species determination of selenium in natural waters. Anal. Chim. Acta. 98, 59–66. Cutter, G.A., 1982. Selenium in reducing waters. Science 217, 819–831. Dickson, A.G., Whitfield, M., 1981. Mar. Chem. 10, 315–333. Dojlido, J.R., 1995. Chemistry of Surface Waters. Wyd, Ekonomia i Srodowisko (in Polish). Driehaus, W., Jekel, M., 1992. Determination of As(III) and total inorganic arsenic by on-line pretreatment in hydride generation atomic absorption spectrometry. Fresenius J. Anal. Chem., 323–343. Furr, A.K., Parkinson, T.F., Young, W.D., Berg, C.O., Gutenmann, W.H., Pakkala, I.S., Lisk, D.J., 1979. Elemental content of aquatic organisms inhabiting a pond contaminated with coal fly ash. NY Fish Game J. 26, 154–161.

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