Journal of Dispersion Science and Technology Potentiometric Titration

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Journal of Dispersion Science and Technology

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Potentiometric Titration of Na2HAsO4, NaPO3, Na4P2O7, and their Binary Mixtures in NaNO3 Melt at 350°C Using a Novel Solid State Glass/TiO2 as pH Indicator Electrode Issa M. El Nahhala; Nasser Abu Ghalwaa a Chemistry Department, Al Azhar University of Gaza, Gaza, PNA

To cite this Article Nahhal, Issa M. El and Ghalwa, Nasser Abu(2007) 'Potentiometric Titration of Na2HAsO4, NaPO3,

Na4P2O7, and their Binary Mixtures in NaNO3 Melt at 350°C Using a Novel Solid State Glass/TiO2 as pH Indicator Electrode', Journal of Dispersion Science and Technology, 28: 6, 876 — 882 To link to this Article: DOI: 10.1080/01932690701459504 URL: http://dx.doi.org/10.1080/01932690701459504

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Journal of Dispersion Science and Technology, 28:876–882, 2007 Copyright # Taylor & Francis Group, LLC ISSN: 0193-2691 print/1532-2351 online DOI: 10.1080/01932690701459504

Potentiometric Titration of Na2HAsO4, NaPO3, Na4P2O7, and their Binary Mixtures in NaNO3 Melt at 35088C Using a Novel Solid State Glass/TiO2 as pH Indicator Electrode Issa M. El Nahhal and Nasser Abu Ghalwa Chemistry Department, Al Azhar University of Gaza, Gaza, PNA

A solid-state glass/TiO2 electrode was fabricated using a transparent conductive titanium oxide film on a glass substrate. The coating of the glass substrate was achieved by a novel simple chemical vapor deposition (CVD) procedure. This electrode can be used as an indicator electrode in potentiometric acid-base titration. This electrode behaves reversibly and responds to the oxide ion concentration in molten NaNO3 . Na2HAsO4, NaPO3, Na4P2O7, and their binary mixtures were potentiometrically titrated with Na2O2 as titrants in molten NaNO3 at 35088 C, using the above mentioned indicator electrode.

Downloaded At: 10:09 11 January 2011

Keywords

Titanium oxide, conductive thin film titanium oxide, indicator electrode, potentiometric titration

1

INTRODUCTION In the last few decades, there was a considerable research in studying the chemistry of fused salts,[1] because of their application as coolants in nuclear reactors, heat transfer liquids and reaction media for chemical[2] and electrochemical processes[3] and also in fused cells.[4 – 8] Ta/Ta2O5, Zr/ZrO2, Nb/Nb2O5 electrodes were prepared by the electrochemical oxidation of Nb in KNO3 melt. These electrodes can be used as indicator electrodes in several potentiometric acid-base titrations in molten KNO3 at 3508C. The Nb/Nb2O3 electrode was also used as an oxide indicator in potentiometric titration of NaPO3, Na4P2O7, Na2HAsO4 against K2CO3 and Na2O2 in molten KNO3 at 625 K.[9 – 13] Ta/Ta2O5 electrode has been used as a pH indicator electrode in several potentiometric titrations in aqueous media,[14] and as an indicator electrode for the potentiometric titrations of NH4VO3 and Na2HAsO4 with Na2O2 in molten KNO3 at 3508C.[12] Vanadium (V) oxide and vanadium-titanium oxides supported on titanium were examinened by electrochemical measurements for complexometric titrations.[15] Many of metal oxide electrodes were used for the electro-oxidation of organic compounds (such as phenol, methanol, etc.).[16 – 18] Received 15 July 2006; Accepted 9 August 2006. This work was accomplished through a grant from the Deanery of Scientific Research in the Al-Azhar University of Gaza. Address correspondence to Issa M. El Nahhal, Chemistry Department, Al Azhar University of Gaza, P. O. Box 1277, Gaza, PNA. E-mail: [email protected]

Palladium film electrode was prepared and used as indicator electrode in acid-base, redox, complexation titration, and titration in nonaqueous solution.[19] Apotentiometric method was used to study the adsorption properties of a film tin-antimony oxide electrode for antimony (III) determination.[20] In our previous article, a solid-state pH sensor was fabricated using a transparent conductive titanium oxide film on a glass substrate. The coating of the glass substrate was achieved by a novel simple chemical vapor deposition (CVD) procedure. The electrode Glass/TiO2 is used as pH sensor and is very sensitive indicator for oxide ions in potentiometric titration of K2Cr2O7 (acid) against K2CO3 and Na2O2 (bases) in molten salts.[21] In this study, we aim to use and investigate the glass-TiO2 electrode as an oxide ion indicator for titration of acids and bases in molten NaNO3 at 3508C was investigated. The work was extended to show the ability of such electrode can be used as an indicator electrode for potentiometric titration of Na2HAsO4, NaPO3, Na4P2O7, and their binary mixtures in NaNO3 melt at 3508C. 2

EXPERIMENTAL

2.1 Preparation of Glass/TiO2 Electrode Undoped conductive titanium oxide (TiO2) films were prepared on cleaned soda-lime glass plates with the dimensions 100 mm 50 mm 2 mm as previously reported.[20] Glass plates were cleaned physically by wiping the solid surface with Ethanolamine wetted cotton cloth. Moreover, the plates were chemically cleaned by dipping in concentrated chromic

876

POTENTIOMETRIC TITRATION OF NA2HAsO4, NAPO3, NA4P2O7


acid for 15 minutes followed by rinsing with distilled water. The glass substrates were placed on a preheated custom-made hot plate with a heating element rated at 1750 W, 220 volts. The plate’s temperature was monitored with a Chromel Alumel thermocouple and controlled electronically from 3508C to 5508C. Fumes of concentrated titanium chloride (99%, Riedel de Hae¨n, Seelze, Hannover), carried by 99.9% oxygen gas, were forced to flow at a rate of 5 L/hour through two rows of fine pipettes (melting point capillary tubes). Each row consisted of 50 tubes 10 cm long and 0.1 mm internal diameter. The prepared titanium oxide coated glass plates were cut into small pieces (100 mm 3 mm). The resulting sensor was washed with distilled water and stored in 0.4 M aqueous KCl. 2.2 Chemicals and Instrumentals NaNO3 melt was used as ground (supporting) electrolyte. The melt, viz. NaNO3 (A.R. Merck, Germany) was worked out at 3508C. The analytically pure salt was freed from the last traces of water by melting and bubbling through it pure dry oxygen for a period of one hour. The solidified mass was then crushed in a mortar and kept in desiccators until required. The acids were potentiometrically titrated in NaNO3 3508C: NaPO3, Na4P2O7 and Na2HAsO4 (A.R. Merck, Germany). The following materials were used as titrants: Na2O2 was mainly used as oxide donors. The potential of the indicator electrode relative to that of the reference electrode was measured on a valve voltmeter (VVM), model 1M-180, Gloucester Ltd., England. Potentials were measured to +5 mV. The reference electrode was Ag/Agþ, NaNO3. The reversibility of the Ag/Agþ, system and its suitability to function as a reference electrode was established by a number of authors.[12 – 18] A silver wire 99.9 % pure was dipped in NaNO3 containing 2.04 wt% AgNO3 (A.R. quality, Merck, Germany). The reference half-cell was separated from the main melt by a solid Pyrex-glass tubing. This possessed sufficient electrical conductance at high temperature to permit definite potentials to be measured, provided that a valve voltmeter was used to measure the potentials. 2.3 Potentiometric Titrations in Fused NaNO3 Titration was carried out in tall-unlipped Pyrex glass beakers (4.9 cm in diameter, 12 cm long). The attack on glass by molten NaNO3 was negligible and containers were usable for a large number of experiments. The titration vessel was maintained in a small electrically heated crucible-type furnace. Regulation of the temperature of the furnace was affected through a variable transformer. The temperature of the melt was measured with the help of a Pt –Pt –10% Rh or Ni-NiCr thermocouple and a temperature device (+38C). The thermocouple was separated from the main melt by means of a tight fitting Pyrex-glass tube. NaNO3 (50.00 g), together with the required quantity of oxide ion acceptor (the acid), was melted over a period of 2 hours. The electrodes (indicator and reference) were then

877

dipped slowly into the melt. After the steady-state potential was attained, the titrations of the acid were carried out by the addition of definite amounts of the base to the melt, waiting until the steady potential is established and then measured.

3

RESULTS AND DISCUSSION

3.1 The Potentiometric Titration of NaPO3, Na4P2O7 and their Binary Mixture in Molten NaNO3 In this part the potentiometric titration of acids NaPO3 and Na4P2O7 in NaNO3 melt at 3508C was carried out using Na2O2 as a titrant. The Glass/TiO2 electrode was used as indicator. The experimental results of the investigations are represented in Figures 1–3. Figure 1 represents the variation of the potential of the Glass/TiO2 electrode during the titration of NaPO3 as an acid with Na2O2 as the oxide donor. The titration curve shows two different potential arrests representing a neutralization reaction with two different steps. The effect of the initial concentration of the oxide ion acceptor on the width of the potential arrest is clearly represented in Figure 1, in which four different initial concentrations of the acid were used.

FIG. 1. 3508C.

Potentiometric titration of NaPO3 with Na2O2 in NaNO3 melt at


878

I. M. EL NAHHAL AND N. A. GHALWA

FIG. 3. Potentiometric titration of a mixture of NaPO3 (a) þNa4P2O7 (b) with Na2O2 in NaNO3 melt at 3508C. FIG. 2.

Potentiometric titration of Na4P2O7 with Na2O2 in NaNO3 melt at

3508C.

Figure 2 represents titration curves of Na4P2O7 as an acid using Na2O2 as abase. Different initial concentrations of the acid were used. The titration curves of Figure 2 show that there is only one potential shift, which corresponds, to a single neutralization step. The potential arrest is depending also on the initial concentration of the acid. The results of Figures 1 and 2 may lead to the assumption that during the neutralization of NaPO3 a transformation step of NaPO3 to Na4P2O7 is taking place. If this is the case, a mixture of both materials NaPO3 and Na4P2O7 can be titrated against Na2O2 in the molten state using the Glass/TiO2 as an oxide ion indicator electrode. Figure 3 represents the data obtained during the titration of mixtures of both acids against Na2O2 as a base. Each titration curve shows two potential shifts corresponding to the two different neutralization steps taking place in the melt. The potential arrests before the potential shift depend essentially on the initial concentration of the mixture. In Figure 3, different mixtures of NaPO3 and Na4P2O7 were used. In each curve the first step potential arrest corresponds to the transformation of NaPO3 to Na4P2O7 and the second step is due to the neutralization of the whole material Na4P2O7 produced in the first step and that used in the mixture.

Figure 4 represents the relation between the amounts of acid and base at each potential shift in three different cases. The data were taken from the results of Figures 1 and 2. Three straight lines were obtained. The first line is from the results of the first step in the titration of NaPO3 against Na2O2 (cf. Figure 1). The second is coming from the data of the second neutralization step of Figure 1. The third line is from the result of the titration of Na4P2O7 against Na2O2 (cf. Figure 2). The straight lines of Figure 4 can be used as a calibration curve for each titration. From these lines the exact amount of the base required to neutralize the acid to the required neutralization step can be calculated. Figure 1 shows clearly that the titration of the oxide ion acceptor, NaPO3, against Na2O2 as an oxide ion donor takes place in two equivalent steps. Each step can be identified by the potential shift occurring in the measured potential of the Glass/TiO2 electrode. The second step of the titration of NaPO3 is identical with the potential shift of the titration of the oxide in acceptor, Na4P2O7 against Na2O2. This means that the indicator electrode is very sensitive to these systems. Calculation of the molar ratio of titrant base to the acid in NaPO3 in (Figures 1 and 2) showed that the first potential shift occurs at 1:2 abase: acid ratio, whereas the second potential shift occurs at a ratio of 1:1. This indicates that the neutralization of NaPO3 occurs in two equivalent steps, the first

879


is a confirmation that the two steps are exactly equivalent and that the total neutralization of NaPO3 occurs in two steps according to: 2NaPO3 þ Na2 O2 ¼ Na4 P2 O7 þ 1=2O2

½3

Na4 P2 O7 þ Na2 O2 ¼ 2Na3 PO4 þ 1=2O2

½4

NaPO3 þ Na2 O2 ¼ Na3 PO4 þ 1=2O2

½5

During the course of titration, the oxide ion is involved in the reaction and is determined by the equilibrium taking place is each case according to the reaction controlling the titration. Consider the first neutralization step: 2 2PO ¼ P2 O4 3 þO 7

½6

This equilibrium is governed by the equilibrium constant K1 K1 ¼

½P2 O4 7 2 2 ½O ½PO 3

½7

From which the oxide ion concentration is given by


½O2 ¼

½P2 O4 7 2 K1 ½PO 3

½8

Substituting in equation FIG. 4. Relation between the quantities of oxide ion donor (Na2O2) and the concentration of oxide ion acceptor (NaPO3 or Na4P2O7). corresponds to the transformation of NaPO3 to Na4P2O7 and the second corresponds to the complete neutralization of the acid into Na3PO4. This neutralization reactions can be represented by the following equations:

EGlass=TiO2 ¼ E0Glass=TiO2

½1

2. Neutralization step: 2 3 P2 O4 7 þ O2 ¼ 2PO4 þ ð1=2ÞO2

½2

It is clear that the amount of base consumed in each step is exactly the same (1 mol in each step). The dependence of the width of the potential arrest occurring before each potential shift on the initial concentration of the acid is a good measure for the equivalent amounts of the titrant in each step. The experimental results of such titrants are used as calibration curves for potentiometric titrations in molten salts (Figure 4). The transformation of NaPO3 to Na4P2O7 during the course of titration at a specified potential step enables the simultaneous determination of both materials in a molten mixture with a practically acceptable degree of accuracy under such experimental conditions, where the temperatures are relatively high. Figure 4 shows that the straight line obtained from the different initial concentrations of the acid in the first step has a slope of one-half that obtained from the data of the second step that obtained from the direct titration of Na4P2O7. This

½9

the electrode potential will be governed by the system P2O42 7 / 42 PO2 3 and is a function of the concentration ratio of [P2O7 ] to [PO2 3 ] according to: EGlass=TiO2 ¼ E0Glass=TiO2

1. Transformation step: 2 4 2PO 3 þ O2 ¼ P2 O7 þ ð1=2ÞO2

RT ln½O2 2F

EGlass=TiO2 ¼ E01

RT RT ½P2 O4 7 ln½1=K1 ln 2F 2F ½PO3 2

RT ½P2 O4 7 ln 2F ½PO3 2

½10 ½11

42 When all PO2 3 transforms to P2O7 , the system in the reaction medium changes and, hence, the potential shifts to a value that corresponds to the new system, which is representing the second step:

2 P2 O4 ¼ 2PO3 7 þO 4

K2 ¼

2 ½PO3 4 2 ½O2 ½P2 O4 7

½12 ½13

In the same way, the concentration of oxide ion in this case is given by: ½O2 ¼

2 ½PO3 4 4 K2 ½P2 O7

½14

where K2 is the equilibrium constant of reaction (13). Substitute for the oxide ion in Equation (9) gives: EGlass=TiO2 ¼ E0Glass=TiO2

2 RT RT ½PO3 4 ln½1=K2 ln ½15 4 2F 2F ½P2 O7

880


or EGlass=TiO2 ¼ E02

2 RT ½PO3 4 ln 2F ½P2 O4 7

½16


and the potential of the electrode will be governed by the new system and is a function of the concentration ratio of [PO32 4 ] to [P2O42 ]. 7 These results are similar to the previously reported results on metal/metal oxide electrodes.[9 – 13] 3.2 The Potentiometric Titration of Na2HAsO4 in Molten NaNO3 This part deals with the potentiometric titration of disodium monohydrogen arsenate in NaNO3 melt with Na2O2 similar to the previously studies on the tantalum/tantalum oxide electrode.[12] The Glass/TiO2 electrode was used as indicator electrode. Figure 5 represents the variation of the potential of the Glass/TiO2 electrode during the titration of Na2HAsO4 as an acid with Na2O2 as the oxide donor. The titration curve shows that there is only one potential shift, which representing a single neutralization step. The effect of the initial concentration of the oxide ion acceptor on the width of the potential

arrest is clearly represented in Figure 5, in which four different initial concentrations of the acid were used. Figure 6 represents the relation between the amounts of acid and base at each potential shift in one case. The data were taken from the results of titration of Na2HAsO4 (cf. Figure 5). One straight line was obtained which can be used as a calibration curve for each titration. From the line the exact amount of the base required to neutralize the acid to the required neutralization step can be calculated. The results of experiments in which the acid Na2HAsO4 is potentiometrically titrated with Na2O2 in NaNO3 melt at 3508C are seen from Figure 5, contradict those previously reported by Baraka et al.[12] Distinct drops in the potential of the Glass/TiO2 electrode characterize the equivalence points. The neutralization of this acid with Na2O2 takes place according to: 2 3 2HAsO2 4 þ O2 ¼ 2AsO4 þ H2 O þ 1=2O2

½17

2 2HAsO2 ¼ 2AsO3 4 þO 4

½18

This equilibrium is governed by the equilibrium constant K3 K3 ¼

2 ½AsO3 4 2 ½O ½HAsO2 4

FIG. 5.

Potentiometric titration of Na2HAsO4 with Na2O2 in NaNO3 melt

½19

From which the oxide ion concentration is given by ½O2 ¼

at 3508C.

2

2 ½AsO3 4 2 K3 ½HAsO2 4

½20

FIG. 6. Relation between the quantity of oxide ion donor (Na2O2) and the concentration of oxide ion acceptor (Na2HAsO4).

881


TABLE 1 The molal amounts of acid, experimental and theoretical amounts of a base, Be, Bt, and recovery percentage (R%) for acid-base titrations using Glass/TiO2 indicator electrode First step

Aa (mol) 0.0490 0.0980 0.1470 0.1960 Ab (mol) 0.0490a þ 0.0335b 0.0980a þ 0.0671b 0.1470a þ 0.0895b 0.1960a þ 0.1119b

Second step

Be (mol)

Bt (mol)

R%

Be (mol)

Bt (mol)

R%

0.0223 0.0482 0.0705 0.0964

0.0245 0.0490 0.0735 0.0980

91.02 98.36 95.91 98.36

0.0475 0.0930 0.1430 0.1935

0.0490 0.0980 0.1470 0.1960

96.93 94.89 97.27 98.72

0.0205 0.0482 0.0695 0.0923

0.0218 0.0512 0.0733 0.0969

94.03 94.14 94.81 95.25

0.0764 0.1641 0.2256 0.2846

0.0825 0.1651 0.2356 0.3079

92.60 99.39 95.75 92.43


Aa Titration of NaPO3 with Na2O2. Ab Titration of a mixture of NaPO3 (a) and Na4P2O7 (b) with Na2O2.

Substituting in Equation (9), the electrode potential 22 will be governed by the system AsO32 and is a 4 /HAsO4 32 function of the concentration ratio of [AsO4 ] to [HAsO22 4 ] according to: EGlass=TiO2 ¼ E0Glass=TiO2

2 RT RT ½AsO3 4 ½H2 O ln½1=K3 ln 2 2F 2F ½HAsO2 4

½21 EGlass=TiO2 ¼ E01

2 RT ½AsO3 4 ½H2 O ln 2 2F ½HAsO2 4

½22

The values of recovery percentage (R%) for the all above titrations are calculated as: R% ¼

Be 100 Bt

½23

Where Be ¼ experimental amount of a base required for the completion of the neutralization process (mol). Bt ¼ theoretical amount of a base calculated from the stoichiomtric equations of neutralization reactions (mol). The calculated values of the recovery percentage are listed in Tables 1 and 2. It is clear from these data that the Glass/TiO2 electrode can be used as an indicator electrode for the determination of oxide ion concentration in molten nitrates with the satisfactory recovery percentage not less than 91.02%. These differences in the recovery percentage may be attributed to the impurities in the salts from one side and to the experimental difficulties and errors in carrying out the titrations at high temperatures from other side.

This electrode was tested as indicator electrode since, the glass was chosen as a substrate for the investigated electrode. The electrode behaves reversibly in the nitrate melts. Its response to the acidity-basicity in the nitrate melt is governed by the sensitivity of the electrode towards changes in oxide-ion concentration in the nitrate melt. This means that the investigated electrode can be used as oxide ion indicator electrode in the nitrate melts. The stability and reproducibility of the electrode were excellent. Upon immersion in the nitrate melt, the potential always stabilized within a few minutes. The electrode has a long-term stability, where one electrode was used for potentiometric titrations (4 months).

TABLE 2 The molal amounts of acid, experimental and theoretical amounts of a base, Be, Bt, and recovery percentage (R%) for acid-base titrations using Glass/TiO2 indicator electrode

Aa (mol) 0.0224 0.0336 0.0448 0.0670 Ab (mol) 0.0270 0.0540 0.0810 0.1080

Be (mol)

Bt (mol)

R%

0.0215 0.0325 0.0430 0.0661

0.0224 0.0336 0.0448 0.0670

95.98 96.72 95.98 98.65

0.0128 0.0256 0.0389 0.0525

0.0135 0.0270 0.0405 0.0540

94.81 94.81 96.05 97.22

Aa Titration of Na4P2O7 with Na2O2. Ab Titration of Na2HAsO4 with Na2O2.

882


4

CONCLUSIONS A solid-state Glass/TiO2 electrode was fabricated using a transparent conductive titanium oxide film on a glass substrate. The coating of the glass substrate was achieved by a novel simple chemical vapor deposition (CVD) procedure. The electrode was successfully tested and used as an indicator electrode for potentiometric titration of Na2HAsO4, NaPO3, Na4P2O7, and their binary mixtures in NaNO3 melt at 3508C. The electrode was found very sensitive toward the changes in the oxide ion concentration in the nitrate melt. It has long-term stability and can be used to determine the oxide ion concentration in melt nitrate with recover percentage of over than 91%.


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[8] Lee, C.G., Nakano, H., Nishina, T., Uchida, I., and Kuroe, S. (1998) J. Electrochem. Soc., 145: 2747. [9] Baraka, A., Abdel-Rohman, A.I., and El-Taher, E.A. (1983) Mater. Chem. Phys., 9: 583. [10] Baraka, A., Abdel-Rohman, A.I., and El-Taher, E.A. (1983) Mater. Chem. Phys., 9: 447. [11] Baraka, A., Abdel-Razik, A., and Abdel-Rohman, A.I. (1985) Surface Technol., 25: 31. [12] Baraka, A., Abdel-Rohman, A.I., and El-Taher, E.A. (1985) Indian J. Chem. Sect. A, 24A: 349. [13] Attia, A.A. (2002) Electrochim. Acta, 47: 1241. [14] Comniellis, Ch. and Pulgarin, C. (1992) J. Appl. Electrochem., 23: 108. [15] Behrens, R. and Umland, F. (1988) J. Less-Common Met., 137: 353. [16] Boyarchuk, T.P., Khailova, E.G., and Cherginets, V.L. (1993) Electrochim. Acta, 38: 1481. [17] Aramata, A., Toyoshima, I., and Enyo, M. (1992) Electrochim. Acta, 37: 1317. [18] Shahine, S. and El Basiouny, M.S. (1980) J. Electroanal. Chem., 108: 271. [19] Xu, Z., Jianyan, l., and Fence, H. (2000) Jixie Gongyebu Shanghai Cailiao Yanjiuso, 36(1): 8. [20] Marinina, G.I., Sonkina, N.A., and Reznik, M.F. (1991) Zh. Anal. Khim., 46 (2): 334. [21] Ghalwa, N. and El Nahhal, I.M. (2007) J. Disp. Sci Technol., 28 (5): 757.