Coulometric generation of H + ions by the anodic oxidation of esters of gallic acid

Transcription

1 J. Serb. Chem. Soc. 70 (2) (2005) UDC : JSCS 3267 Original scientific paper Coulometric generation of H + ions by the anodic oxidation of esters of gallic acid LJILJANA N. JAK[I] 1,*, RADMILA M. D@UDOVI] 2 and RANDJEL P. MIHAJLOVI] 2,# 1 Faculty of Mining and Geology, University of Belgrade, Dju{ina 7, Belgrade and 2 Faculty of Science, University of Kragujevac, Radoja Domanovi}a 12, Kragujevac, Serbia and Montenegro ( randjel@knez.uis.kg.ac.yu) (Received 2 February, revised 16 June 2004) Abstract: The application of esters of gallic acid (propyl, butyl and dodecyl gallate) for the coulometric generation of H + ions in aqueous media is described. The current potential curves recorded for these depolarizers, as well as for tris(hydrosymethyl)aminomethane ( THAM ) and the employed solvent showed that the investigated depolarizes are oxidized at potentials lower than the oxidation potentials of the other components in the solution. The H + ions formed by the oxidation of these depolarizers (in 1 mol/l sodium perchlorate in water) were used for the titration of THAM with potentiometric end-point detection using a hydrogen/palladium indicator electrode. The current efficiency was 100 %. Keywords: coulometric potentiometric titration, esters of gallic acid, tris(hydroximethyl)aminomethane, (H 2 /Pd) ind electrode. INTRODUCTION The first application of coulometric titrations of acids and bases was carried out by Szebelledy and Somogyi. 1 They titrated sodium hydroxide with H + ions obtained by the oxidation of water on a platinum anode. Schreiber and Cooke 2 determined micro quantities of HCl and NaOH by generating OH and H + from water with KCl as the supporting electrolyte. De Ford et al. 3 electrolytically generated hydrogen and hydroxyl ions outside the cell in an electrolyte which flowed over separate electrodes. Marinenko 4 titrated tris(hydroxymethyl)aminomethane by adding sulpohuric acid and coulometrically back-titrating and standardizing the latter. He reported that the direct coulometric titration of bases with H + ions generated by the oxidation of water at a platinum anode was not feasible, since the bases were also oxidized, which resulted in a negative error of several procent. * Author for correspondende. # Serbian Chemical Society active member. 243

2 244 JAK[I], and MIHAJLOVI] To avoid the oxidation of bases at the platinum anode, Hoyle et al. 5 generated H + ions separately from the test solution ( externally generated titrant ) but the results obtained in the titration of 4-aminopyridine were still too low by 2 6 %. These authors considered that this might be due to the formation of peroxodisulphate anions (S 2 O 2 8 when sodium sulphate was used as the background electrolyte) or peroxodiperchlorate (Cl 2 O 8, when sodium perchlorate was used) at the anode in the course of the electrolysis. To avoid both the oxidation of the to be titrated base and the formation of peroxodiperchlorate or peroxodisulphate anions, these authors used sodium hydrazine sulphate as the depolarizer in the generation of H + ions from water. Hoyle et al. 6 showed that a bright platinum anode in sodium hydrazine sulphate Na(N 2 H 5 )SO 4 is 100 % efficient in the generation of five hydrogen ions per four electrons: N 2 H + 5 =N 2 +5H + +4e 2H + +2e =H 2 Moreover, hydrazine is more readily oxidized than is either THAM or 4-aminopyridine, so that the titration of these bases may be carried out directly. They offered as experimental proof of the efficiency of this hydrazine platinum anode in the generation of hydrogen ion the direct, high-precision coulometric titration of THAM and the supplementary analysis of this same material by titration with perchloric acid standardized coulometrically. In our previous papers 7 it was shown that the titration of bases in aqueous media could be carried out with H + and D + ions generated by the oxidation of hydrogen and deuterium dissolved in palladium. This paper is aimed at investigating the possibility of generating H + ions by the anodic oxidation some of esters of gallic acid (propyl, butyl and dodecyl gallate) in aqueous media. EXPERIMENTAL Reagents All depolarizers, THAM (HOCH 2 ) 3 CNH 2 and chemicals for the buffer solutions were of analytical-reagent grade from Merck or Fluka. Sodium perchlorate was prepared by neutralizing perchloric acid with sodium hydroxide to ph 7. The solution was then evaporated on a water bath and the crystals obtained recrystallized and dried at 150 C. The supporting electrolyte was an aqueous 1 mol/l sodium perchlorate solution. The solutionofthamwaspreparedbydissolvinganaccurately weighed amount of THAM. Portions of 2 3 ml of these solutions were delivered from a microburette. Electrodes The cathode and anode were platinum spirals 0.5 mm in diameter, each having an area of 27 mm 2 immersed in the titrated solution. The indicator electrode (H 2 /Pd) ind was a spiral of 0.5-mm diameter palladium wire (99.9 % pure; Johnson Matthey Metals, London) with a surface area of 30 mm 2 saturated with hydrogen ob-

3 HYDROGEN IONS GENERATION 245 tained by the electrolysis of dilute sulphuric acid at a current of 2 ma. The potential of the (H 2 /Pd) ind electrode prepared in this manner was measured (vs. SCE) as a function of time in 1 mol/l sodium perchlorate solution (Fig. 1). A stable potential was established within a few minutes provided the electrode was used one hour after being saturated. A saturated calomel electrode type K 401 Radiometer, was used as the reference electrode. The compartments of the electrolytic vessel were separated by a G-4 sintered-glass disk. The volume of the anolyte was 20 ml and that of the catholyte 5 ml. Procedure The supporting electrolyte was poured into the cathode and anode compartments of the electrolyte vessel up the same level. The platinum cathode was dipped into the catholyte and, after the addition of about 100 mg of the depolarizer (ester of gallic acid), the platinum anode, the (H 2 /Pd) ind indicator electrode and the SCE were immersed in the anolyte. The current was then switched on, and H + ions were generated and the potentials were read. The titration end-point was determined from the second derivative. After the titration of the supporting electrolyte, a measured volume of base (THAM) was added to the anolyte and the titration was carried out in the same manner. RESULTS AND DISCUSSION Fig. 1. Dependence of the H 2 /Pd electrode potential (vs. SCE) on time in a sodium perchlorate solution (1 mol/l). Anodic oxidation of esters of gallic acid The oxidation of esters of gallic acid (propyl-, butyl- and dodecyl-gallate) in aqueous media is represented by the Scheme 1. In order to establish whether the reactions can be applied for the quantitative generation of H+ ions, first the current potential curves for the esters, titrated base (THAM) and solvent were recorded, and then the current efficiency was determined. From the current potential curves (Fig. 2) it can be seen that in aqueous media propyl, butyl and dodecyl-gallate are oxidized at potentials much more negative than the oxidation potentials of the other components present in the solution.

4 246 JAK[I], and MIHAJLOVI] Scheme 1. The oxidation potential (vs. SCE) of the esters of gallic acid depends on the nature of the ester and decreases in the order: dodecyl gallate > butyl gallate > propyl gallate. Fig. 2. Change of the anode potential (vs. SCE) with current density in aqueous 1 mol/l sodium perchlorate solutions; 1. solvent; 2 THAM; 3 dodecyl gallate; 4. butyl gallate; 5. propyl gallate. In order to establish whether the oxidation of the esters of gallic acid in water is rapid and quantitative, the generated hydrogen ions were used for the titration of THAM. Tris(hydroxymethyl)aminomethane is a weak base (pk b = 5.98) which is used as a primary standard in volumetric methods. 9 Coulometric titration of THAM was carried out with potentiometric end-point detection, using glass electrode SCE and (H 2 /Pd) ind SCE as electrode couples.

5 HYDROGEN IONS GENERATION 247 Potentiometric detection of the end-point In order to investigate the ph-response of the (H 2 /Pd) ind -electrode, the potential of this electrode (vs. SCE) in open circuit was measured in freshly prepared buffer solutions: acetate (ph ), phosphate ( ), glycine NaOH ( ) in an inert atmosphere obtained by nitrogen bubbling. It was found (Fig. 3) that the potential of the (H 2 /Pd) ind -electrode showed a sub-nernstian dependence on ph with a slope of 30 mv/ph. Fig. 3. Plots of the potential of the (H 2 /Pd) ind -electrode (vs. SCE) versus the ph value of buffer solutions. In the coulometric potentiometric titration of THAM, with the (H 2 /Pd) ind indicator electrode, the potential was usually established in less than 30 s, however, in the vicinity of the TEP, it was established in about 1 min. Fig. 4. Coulometric potentiometric titration curve of THAM in aqueous sodium perchlorate solution (1 mol/l).

6 248 JAK[I], and MIHAJLOVI] The titration curve of THAM (Fig. 4) shows that the potential jump at the equivalence end-point for a base concentration of mol/l was about 90 mv. The results obtained in the coulometric determination of THAM with potentiometric end-point detection are given in Table I. TABLE I. Results of the coulometric titrations of THM with H + ions obtained by the oxidation of esters of gallic acid in aqueous sodium perchlorate solution (1 mol/l); I = A; current density 4 ma cm -2 Depolarizer Taken / mg Found / % 1 Electrode couples Propoyl gallate Glass SCE Propyl gallate (H 2 /Pd) ind SCE Propyl gallate Glass SCE Propyl gallate (H 2 /Pd) ind SCE Butyl gallate (H 2 /Pd) ind SCE Dodecyl gallate (H 2 /Pt) ind SCE 1 Number of determination 6 On the basis of the obtained results, it might be concluded that oxidations of propyl, butul and dodecyl gallate in aqueous solution proceed with 100 % current efficiency and that the generated hydrogen ions can be applied for the determination of THAM. In addition, the investigations showed that the self-made hydrogen/palladium indicator electrode could be used for end-point detection in aqueous media. Acknowledgement: This work was funded by the Ministry of Science, Technology and Development of the Republic of Serbia (Project No. 1571). IZVOD KULOMETRIJSKO GENERISAWE H + JONOVA ANODNOM OKSIDACIJOM ESTARA GALNE KISELINE QIQANA N. JAK[I] 1, RADMILA M. YUDOVI] 2 i RAN\EL P. MIHAJLOVI] 2 1 Rudarsko-geolo{ki fakultet, \u{ina 7, Beograd i 2 Prirodno-matemati~ki fakultet, Radoja Domanovi}a 12, Kragujevac Prikazana je primena estara galne kiseline (propil-, butil- i dodecil-galata) za kulometrijsko generisawe H + jonova u vodenoj sredini. Snimqene krive struja potencijal za ove depolarizatore kao i za tris(hidroksimetil)aminometan (THAM) i upotrebqeni rastvara~ pokazuju da se ispitivani depolarizatori oksiduju na potencijalima ni`im od potencijala oksidacije ostalih komponenata u rastvoru. H + jonovi nastali oksidacijom depolarizatora (u rastvoru 1 mol/l natrijum perhlorata) upotrebqeni su za titraciju THAM sa potenciometrijskom detekcijom zavr{ne ta~ke, uz vodonik/paladijumovu indikatorsku elektrodu. Iskori{}ewe struje je 100 %. (Primqeno 2. februara, revidirano 16. juna 2004)

7 HYDROGEN IONS GENERATION 249 REFERENCES 1. L. Szebelledy, Z. Somogyi, Z. Anal. Chem. 315 (1938) R. Schreiber, D. W. Cooke, Anal. Chem. 27 (1955) D. De Ford, N. J. Pitts, J. C. Johns, Anal. Chem. 23 (1951) G. Marinenko, Nat. Bur. Stds. Tech. Note 543 (1970) W. C. Hoyle, W. F. Koch, H. Diehl, Talanta 22 (1975) W. C. Hoyle, W. F. Koch, H. Diehl, Talanta 22 (1975) R. P. Mihajlovi}, V. J. Vajgand, Lj. N. Jak{i}, Talanta 38 (1991) R. Mihajlovi}, Z. Stani}, M. Antonijevi}, Anal. Chim. Acta 497 (2003) W.F.Koch,D.I.Biggs,H.Diehl,Talanta 22 (1975) 637.