LABORATORIUM Z CHEMII FIZYCZNEJ POTENTIOMETRIC MEASUREMENTS OF PH POLITECHNIKA ŚLĄSKA WYDZIAŁ CHEMICZNY. Agata Blacha-Grzechnik.

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1 POLITCHNIKA ŚLĄSKA WYDZIAŁ CHMICZNY KATDRA IZYKOCHMII I TCHNOLOGII POLIMRÓW POTNTIOMTRIC MASURMNTS O PH Prowadzący: Miejsce ćwiczenia: Agata BlachaGrzechnik Katedra izykochemii i Technologii Polimerów, Sala 210 LABORATORIUM Z CHMII IZYCZNJ 1

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3 I. TH AIM O TH XRCIS The aim of the exercise is to determine the ph value of the given sample and to familiarize with the potentiometric methods of ph value determination. II. INTRODUCTION AND THORTICAL BACKGROUND The acidity of the solution is usually given by its ph value defined as: ph = log a + H3O The given equation is strictly formal, since it is impossible to determine the activity of just one type of ions. Practically, the ph value of the solution is found by the comparison with the ph scale, which is based on the few standard buffer solutions with ph values determined by measuring M of appropriately constructed electrochemical cell: Pt, H 2 (p = Pa) H + (a = 1) buffer solution H 2 (p = Pa), Pt (1) Hydrogencalomel cell is typically applied in the potentiometric measurements of ph: () Pt, H 2 (p = Pa) solution KCl (sat) Hg 2 Cl 2(s), Hg (+) Assuming that the activity of water is constant, the M of this cell is given by: where cell o RT = cell a a ) H Cl + π ln( + d (2) o cell is the difference between standard potentials of halfcells, Σπ d is the sum of all of the diffusion potentials, which is constant when the same calomel electrode is used, and can be introduced into o ' value, similarly as the part (RT 1 ) ln(a Cl ): In this case the M is given by: o' o RT cell = cell + Σπ d ln( a ) Cl (3) o' o 2.303RT cell = cell + ph (4) Depending on the concentration of potassium chloride, the value of o' will be different, so as the formula for the ph. 1

4 Instead of troublesome hydrogen electrode, one can use the quinhydrone electrode in the described cell. Pt C 6 H 4 O 2, C 6 H 4 (OH) 2, H + The quinhydrone electrode is reversible towards hydronium ions. This electrode is of a redox type, in which the electrode reaction is a simple exchange of protons between quinone and hydroquinone, given by: C 6 H 4 O 2 + 2H + + 2e C 6 H 4 (OH) 2 The calomel electrode can also be replaced with another reference electrode with constant potential, like silver chloride electrode: Ag, AgCl KCl (saturated) The M value of the cell constructed of quinhydrone electrode and silver chloride electrode (or any other reference electrode with constant potential) is given by: ' 2, 303RT ph = (5) The value can be determined by additional measurement, taking one of the standard buffer solutions as the solution of the quinhydrone electrode. When the silver chloride electrode is used as the reference electrode, the can be calculated. The main advantages of the quinhydrone electrode are its simplicity, good reproducibility, quick potential setting and insensitivity to the presence of small quantities of reducing or oxidizing agents. The glass electrode is widely applied in the potentiometric measurements of ph. This electrode is a membrane electrode selective towards hydrogen ions in the broad ph region. Depending on the glass composition, the glass electrode can be also selective towards Na +, K + or NH + 4 ions. Typically, the glass electrode is built from the glass tube ended with the bubble and filled with the solution of known ph containing chloride anions. The silver chloride electrode is immersed in this solution. The glass bubble is made of the glass with high specific conductivity. In the range from 2 to 11 ph, the glass electrode is selective only towards H + ions. 2

5 Rys. 1. The scheme of a glass electrode. The glass electrode is used with phmeters for the ph determination. In this case the cell is constructed of the glass electrode (as an indicator electrode) and the reference electrode, f.e. calomel electrode. Both electrodes can also be combined in one. In both cases the scheme of the cell is given by: Ag, AgCl (s) KCl (aq) sol. 1 of known ph glass bubble sol. 2 unknown KCl (sat) Hg 2 Cl 2(s), Hg The M of this cell is given by: RT ' a ln a + (1) + H =, stąd: ph ph ( ') H (2) 2,303RT = (6) ( 2) (1) where is the sum of all constant values. It has to be noted that before the real ph measurements, the phmeter should be calibrated using the solution of known ph, so that value can be determined. The calibration should be done each time, when new series of sample is to be measured, because the potential of glass electrode is not constant in the solution and may vary in time. The glass electrode cannot be used for the determination of ph of the strongly alkaline solutions, because of the too low hydrogen ions concentration and the chemical influence of the solution on glass. III. PROCDUR Apparatus Compensator for the M measurements, digital phmeter N517 with the combined electrode, saturated calomel electrode, platinum electrode, Plexiglas plate as the electrode holder, flasks, pipettes, weighing glasses, beakers, wires. 3

6 Reagents 0.1 M HCl solution, 0.1 M NaOH solution, acidic potassium phtalate (C 6 H 5 O 4 K), borax (Na 2 B 4 O 7 10H 2 0), succinic acid ((CH 2 COOH) 2 ), potassium dihydrogen phosphate (KH 2 PO 4 ), disodium hydrogen phosphate 12 hydrate (Na 2 HPO 4 12H 2 O) (for the buffer solution preparation); quinhydrone (C 6 H 4 (OH) 2 C 6 H 4 O 2 ). Measurements Prepare the buffer solution of ph given in the instruction using the appropriate tables. Then determine the real ph value of the prepared solution. The ph value can be calculated based on the M of the cell built from the indicator electrode (quinhydrone electrode) and reference electrode (calomel electrode) immersed in the prepared solution. The M value should be measured by the compensator according to given instructions. When the real ph value is determined, this solution should be used for the calibration of phmeter. The calibration should be done before the ph value of the sample is measured. a) Cell construction and M measurements Clean the 50 ml beaker firstly with distilled water and then with small amount of prepared buffer solution. Then, clean the calomel and platinum electrodes with distilled water and with tissue papers, and put them into the Plexiglas plate. Pour the prepared buffer solution into the beaker, so that the ending of the platinum electrode is fully immersed. Then add about 10 mg of quinhydrone into the solution. Connect the electrodes with the compensator, so that the calomel electrode is connected with the () pole and the quinhydrone electrode with (+) pole. Gently mix the solution and then measure the M of the constructed cell. After the measurement, clean the beaker and the electrodes with distilled water. The calomel electrode should be put into the saturated solution of KCl. b) Calculation of the real ph value of the prepared solution Before measuring the ph value of the given sample, the real ph value of the prepared solution has to be calculated. irstly, derive the formula for the M of the constructed cell as a function of ph and temperature. The general scheme of the cell is: () Hg, Hg 2 Cl 2 KCl (saturated) (buffer solution) C 6 H 4 O 2, C 6 H 4 (OH) 2, H + Pt (+) where: C 6 H 4 (OH) 2 hydroquinon C 6 H 4 O 2 quinon 4

7 Potential of the saturated calomel electrode: = 0, 223V Hg, Hg2Cl2 Cl Normal potential of the quinhydrone electrode: 0 + = 0, 699V Pt Ph ( OH ) 2, PhO2, H Based on the given data and the measured value of M, calculate the real ph value of the prepared standard buffer solution. c) phmeter calibration ollow the instructions for the phmeter. In the first step the phmeter calibration should be done. Prepare 50 ml beaker and combined electrode for the measurements, i.e. clean them with the small amount of prepared buffer solution. Then, dry the electrode, put it on the holder and immerse it in the prepared buffer solution so that the electrode ending (glass bubble) is fully immersed. The calibration is done by adjusting the phmeter (using the knob) to the real value of the prepared buffer solution. Note: after the calibration, the phmeter settings should not be changed! d) Sample ph measurement After cleaning the electrode and the beaker with water, the measurement of the sample ph value should be done. The preparation procedure is similar to the one given for the phmeter calibration. Though, in this case instead of the buffer solution the sample should be poured into the beaker. After the electrode is immersed in the sample, few minutes should be waited until the value recorded by the phmeter is stabilized. Then, the ph value of the sample should be read. 5

8 IV. SATY AND WASTS UTILIZATION RULS NOT: In the case of the undesirable contact with substance, please inform the lecturer. Reagent Classification Hazards irst aid Waste disposal Hydrochloric acid HCl Sodium hydroxide NaOH Acetic potassium phtalate C 6 H 5 O 4 K Disodium Tetraborate 10 hydrate (borax) Na 2 B 4 O 7 10H 2 0 Is corrosive to skin and eyes. Is corrosive to skin and eyes. Irritating to skin and eyes. Irritating to eyes and skin. In case of swallowing: drink plenty of water, avoid vomiting. Call the doctor if filling unwell. Contact the eyedoctor. In case of swallowing: drink plenty of water, avoid vomiting. Call the doctor. In case of inhalation: fresh air. In case of inhalation: fresh air. Disodium hydrogen phosphate 12 hydrate Na 2 HPO 4 12H 2 0 Succinic acid C 2 H 4 (COOH) 2 Potassium dihydrogen phosphate KH 2 PO 4 Is corrosive to eyes. Irritating to eyes In case of inhalation: fresh air. Can be introduced into sewage system Quinhydrone C 6 H 4 (OH) 2 C 6 H 4 O 2 Sample Harmful if swallowed. Toxic to water systems. Irritating for digestive system. In case of inhalation: fresh air. In case of swallowing: drink plenty of water. Call the doctor In case of inhalation: fresh air. Call the doctor. denoted with O 6

9 V. CALCULATIONS In the results analysis, the derivation of the formula for the M should be put, together with the detailed calculations of the real value of the standard buffer solution. Moreover, the uncertainty of the measured and calculated values should be included. The final results should be presented in the form of a table: M of the cell [V] Theoretical value of the standard buffer solution Real value of the standard buffer solution Measured ph value of the sample VI. QUSTIONS 1. xplain the idea behind the halfcell classification (1st and 2nd type; with examples). 2. xplain the term halfcell potential. Give the Nernst equation. 3. xplain what the reference electrode is. Give the examples with names, constructions and the halfcell reactions. 4. Name the halfcells that are applied as the ph indicators. 5. Give the scheme of a glass electrode. 6. Give the scheme of the hydrogen halfcell, the electrode reactions, the formula for its potential. What is the standard hydrogen electrode? VII. LITRATUR 1. Praca zbiorowa, Chemia izyczna (1980), p , p.25.13h, 2. R.Brdicka, Podstawy Chemii izycznej, p G.M.Barrow, Chemia izyczna, p , p B.A. Averill, P. ldredge, Principles of General Chemistry (available for free: 7