Electrochemistry Lecture 3 - PharmD Clinical - ShO PDF

Electrochemistry: Potentiometry Dr. Sherif Okeil, Fall semester 2024/2025 Overview of today’s lecture Introduction to potentiometry Instrument: Potentiometer Salt bridge and junction potential Reference electro...

Electrochemistry: Potentiometry Dr. Sherif Okeil, Fall semester 2024/2025 Overview of today’s lecture Introduction to potentiometry Instrument: Potentiometer Salt bridge and junction potential Reference electrodes Fall semester 2024/2025 Dr. Sherif Okeil | Electrochemistry 2 Potentiometry What is potentiometry? Potentiometry = method of analysis in which we determine the concentration of an ion or substance by measuring the potential developed when a sensitive electrode is immersed in the solution of the species to be determined. measuring the potential of electrochemical cells without drawing appreciable current Fall semester 2024/2025 Dr. Sherif Okeil | Electrochemistry 3 Basics of Potentiometry Basic idea: Analyte is an electroactive species that is part of a galvanic cell. → Depending on its concentration the potential will change (Nernst equation). 1. Case: for metals (metal/ion half-cell) metal [M0] is immersed in solution of its ions [Mn+] Solution pressure (oxidation) Solution pressure: Tendency of M0 to 𝐸 0.059 [𝑜𝑥𝑖𝑑𝑖𝑧𝑒𝑑 𝑓𝑜𝑟𝑚] 25°𝐶 = 𝐸0 + log go to solution 𝑛 [𝑟𝑒𝑑𝑢𝑐𝑒𝑑 𝑓𝑜𝑟𝑚] 𝑀0 ⇌ 𝑀𝑛+ + 𝑛𝑒 − vs Ionic pressure: Tendency of Mn+ ion to 𝟎. 𝟎𝟓𝟗 [𝑴𝒏+ ] Ionic pressure (reduction) 𝑬𝟐𝟓°𝑪 = 𝑬𝟎 + 𝐥𝐨𝐠 deposit on M0 rod 𝒏 𝟏 n = number of electrons gained or lost Fall semester 2024/2025 Dr. Sherif Okeil | Electrochemistry 4 Basics of Potentiometry 2. Case: for non-metals (N) (non-metal/ion half-cell) e.g. Cl2/2Cl- Inert electrode has to be used for electron transfer (such as platinum electrode) 𝑁 𝑛− ⇌ 𝑁 + 𝑛 𝑒 − Nernst equation for non-metals: 0.059 1 𝐸25°𝐶 = 𝐸0 + log 𝑛− 𝑛 [𝑁 ] Or 0.059 𝐸25°𝐶 = 𝐸0 − log [𝑁 𝑛− ] 𝑛 Fall semester 2024/2025 Dr. Sherif Okeil | Electrochemistry 5 Basics of Potentiometry 3. Case: for redox systems (ion/ion half-cell) e.g. Fe3+/Fe2+ → inert platinum electrode is used 0.059 [𝑜𝑥𝑖𝑑𝑖𝑧𝑒𝑑 𝑓𝑜𝑟𝑚] 𝐸25°𝐶 = 𝐸0 + log 𝑛 [𝑟𝑒𝑑𝑢𝑐𝑒𝑑 𝑓𝑜𝑟𝑚] Examples: 1 𝑭𝒆𝟐+ ⇌ 𝑭𝒆𝟑+ + 𝒆− 0.059 [𝐹𝑒 3+ ] 𝐸25°𝐶 = 𝐸0(𝐹𝑒 3+ /𝐹𝑒 2+ ) + log 𝑛 [𝐹𝑒 2+ ] 2 𝑴𝒏𝟐+ + 𝟒𝑯𝟐 𝑶 ⇌ 𝑴𝒏𝑶− + 𝟒 + 𝟖𝑯 + 𝟓𝒆 − 0.059 𝑀𝑛𝑂4− [𝐻+ ]8 𝐸25°𝐶 = 𝐸0(𝑀𝑛𝑂4− /𝑀𝑛2+ ) + log 𝑛 [𝑀𝑛2+ ] Fall semester 2024/2025 Dr. Sherif Okeil | Electrochemistry 6 Basics of Potentiometry Some notes on the electrode potential determined by Nernst equation: If the ionic concentration is 1 molar, E25°C = Eo which is called standard electrode potential. The sign of the potential is similar to the charge on the metal electrode. The potential of single electrode can't be measured directly, but measured against Half-cell, whose reference electrode (standard electrode which Reference potential has to be has known and fixed potential) through electrode determined electrochemical cell. Fall semester 2024/2025 Dr. Sherif Okeil | Electrochemistry 7 Instrument: Components for potentiometric measurement For any potentiometric measurement we must have: 1- Indicator electrode: The electrode which is used for the determination of the ion concentration 2- Reference electrode: The potential of the indicator electrode cannot be measured alone. It has to be connected to a reference electrode of known constant potential. → The two electrodes form the two half-cells of the electrochemical cell. → EMF (=electromotive force): E = Ecathode – Eanode (Note: As the reference electrode potential is constant, the change in e.m.f. of the cell is due to the change of the indicator electrode potential, which is related to the ion concentration to be measured.) 3- Potentiometer: Device measuring the e.m.f. formed. 4- Salt bridge: to connect the two electrode solutions and complete the circuit. Fall semester 2024/2025 Dr. Sherif Okeil | Electrochemistry 8 Potentiometer: Measurement of EMF Potentiometer is used to measure the e.m.f. of the galvanic cell under zero current (i.e. without withdrawing any current from the cell or imposing current from the outer source which will cause polarization or chemical change in the cell). Potentiometer consists of: External source of D.C 1- Galvanic Cell: consisting of reference and indicator electrode C: Sliding contact to connected with salt bridge give variable potential Voltage 2- Voltage divider: which supplies the galvanic cell with an external divider potential opposing that of the galvanic cell. Idea: The voltage divider supplies the galvanic cell with variable potential till we obtain zero current Galvanometer G Galvanic cell (detected by the galvanometer), i.e potential difference obtained by the voltage divider equals that of the galvanic cell. According to Ohm’s law: E = RAC I Fall semester 2024/2025 Dr. Sherif Okeil | Electrochemistry 9 Salt bridge Salt bridge = a liquid junction between the two half cells which allows transfer of charge without mixing the two electrode solutions. It may be in the form of a bent tube or inverted U- shaped tube, filled with agar gel prepared in saturated KCl or KNO3 solution. Ions in the salt bridge must not pass to the two half cells. This can be achieved by blocking the two ends of salt bridge with cotton wool or gelatin or agar. Fall semester 2024/2025 Dr. Sherif Okeil | Electrochemistry 10 Salt bridge: Junction potential Junction potential: a potential developed at both boundaries of the junction (interface with anode and cathode solution) Reason for formation of a junction potential: Difference in the migration rates of anions and cations of the bridge salt ⟹ this difference results in unequal charge distribution at the boundaries ⟹ development of a potential In short: An electric potential is generated Occurs Whenever Dissimilar Electrolyte Solutions are in Contact by a separation of charge Develops at solution interface (salt bridge) Small potential (few millivolts) Junction potential puts a fundamental limitation on the accuracy of direct potentiometric measurements i.e. you don’t know the contribution to the measured voltage Fall semester 2024/2025 Dr. Sherif Okeil | Electrochemistry 11 Salt bridge: Junction potential To reduce liquid junction potential: 1- Choose the electrolyte of salt bridge, that its cations and anions have nearly the same mobility, so that, they move by the same rate, leading to equal distribution of charges. e.g KCl or KNO3 (K+ = 73.5 , Cl- = 76.3 , NO3- =71.5). 2- Use high concentration of the salt for preparation of the bridge, to reduce the effect of difference in rates of migration of other ions in the electrode solutions (to keep migration happen from salt bridge to solution not the reverse and so can control junction potential by using only salts with cations and anions of near mobilities). Fall semester 2024/2025 Dr. Sherif Okeil | Electrochemistry 12 Reference electrodes Requirements of a reference electrode: 1. Have a constant potential 2. Its potential must be known and definite Types of reference electrodes Primary reference electrode Secondary reference electrodes Normal Hydrogen Electrode 1. Calomel Electrode (NHE) 2. Silver, Silver chloride saturated potassium chloride electrode (Ag0/AgCl, saturated KCl electrode) Fall semester 2024/2025 Dr. Sherif Okeil | Electrochemistry 13 Normal Hydrogen Electrode (NHE) 1. Electrode reaction: 2𝐻 + + 2𝑒 − ⇌ 𝐻2 2. Half-cell presentation: Pt(s)|H2(g,1atm)|H+(aq),1M || 3. Design: 4. Nernst equation and potential: 𝟎. 𝟎𝟓𝟗 𝑬𝟐𝟓°𝑪 = 𝑬𝟎 + 𝐥𝐨𝐠 [𝑯+ ]𝟐 𝟐 In NHE: [H+] = 1M ⇒ log 1 = 0 i.e. E25°C = E0 = 0V Fall semester 2024/2025 Dr. Sherif Okeil | Electrochemistry 14 Normal Hydrogen Electrode (NHE) Advantage: It is a primary reference electrode, as its potential is considered to be zero. Disadvantages: 1- Difficult to be used 2- Catalytic poison e.g S2- will interfere with the catalytic activity of platinum black. 3- We can't keep H2 gas at one atmosphere during all determinations 4- It needs replating of platinum black. → Thus instead we use secondary reference electrodes. Fall semester 2024/2025 Dr. Sherif Okeil | Electrochemistry 15 Calomel Electrode 1. Electrode reaction: 𝐻𝑔2 𝐶𝑙2 + 2𝑒 − ⇌ 2𝐶𝑙 − + 2𝐻𝑔0 2. Half-cell presentation: Hg° | Hg2Cl2 , KCl (sat. or 1 N or 0.1 N) || 3. Design: 4. Nernst equation and potential: 𝐻𝑔2 𝐶𝑙2 + 2𝑒 − ⇌ 2𝐶𝑙 − + 2𝐻𝑔0 The sparingly soluble Hg2Cl2 dissociates in very small amounts giving the mercurous ion: 𝐻𝑔2 𝐶𝑙2 ⇌ 2𝐻𝑔22+ + 2𝐶𝑙 − The mercurous ion is the species undergoing the reaction: 2𝐻𝑔22+ + 2𝑒 − ⇌ 2𝐻𝑔0 Fall semester 2024/2025 Dr. Sherif Okeil | Electrochemistry 16 Calomel Electrode 𝟎. 𝟎𝟓𝟗 𝑬𝟐𝟓°𝑪 (𝑯𝒈/𝑯𝒈𝟐+ ) = 𝑬𝟎(𝑯𝒈/𝑯𝒈𝟐+ ) + 𝐥𝐨𝐠 [𝑯𝒈𝟐+ 𝟐 ] 𝟐 𝟐 𝟐 𝟐 As the concentration of the mercurous ion is controlled by the solubility product of the salt, we can say: 𝐾𝑠𝑝 (𝐻𝑔2𝐶𝑙2) = [𝐻𝑔22+ ]2 [𝐶𝑙 − ]2 𝐾𝑠𝑝 (𝐻𝑔2𝐶𝑙2) ⟹ [𝐻𝑔22+ ]2 = [𝐶𝑙− ]2 And insert this term into the Nernst equation, then you get: Corresponds to the 𝟎. 𝟎𝟓𝟗 𝐾𝑠𝑝 (𝐻𝑔2𝐶𝑙2) concentration of the 𝑬𝟐𝟓°𝑪 (𝑯𝒈/𝑯𝒈𝟐+ ) = 𝑬𝟎(𝑯𝒈/𝑯𝒈𝟐+ ) + 𝐥𝐨𝐠 𝟐 𝟐 𝟐 [𝐶𝑙 − ]2 KCl solution Electrode potential depends on the chloride ion concentration (E=0.246 V for saturated KCl, E=0.285 V for 1N KCl and E=0.338 V for 0.1N KCl) Fall semester 2024/2025 Dr. Sherif Okeil | Electrochemistry 17 Silver-silver chloride electrode 1. Electrode reaction: 𝐴𝑔𝐶𝑙 + 𝑒 − ⇌ 𝐴𝑔0 + 𝐶𝑙 − 2. Half-cell presentation: Ag° | AgCl, KCl (sat. or 1 N or 0.1 N) || 3. Design: 4. Nernst equation and potential: 𝐴𝑔𝐶𝑙 + 𝑒 − ⇌ 𝐴𝑔0 + 𝐶𝑙 − The sparingly soluble AgCl dissociates in very small amounts giving the silver ion: 𝐴𝑔𝐶𝑙 ⇌ 𝐴𝑔+ + 𝐶𝑙 − The silver ion is the species undergoing the reaction: 𝐴𝑔+ + 𝑒 − ⇌ 𝐴𝑔0 Fall semester 2024/2025 Dr. Sherif Okeil | Electrochemistry 18 Silver-silver chloride electrode 𝟎. 𝟎𝟓𝟗 𝑬𝟐𝟓°𝑪 (𝑨𝒈/𝑨𝒈+ ) = 𝑬𝟎(𝑨𝒈/𝑨𝒈+ ) + 𝐥𝐨𝐠 [𝑨𝒈+ ] 𝟏 As the concentration of the silver ion is controlled by the solubility product of the salt, we can say: 𝐾𝑠𝑝 (𝐴𝑔𝐶𝑙) = [𝐴𝑔+ ][𝐶𝑙 − ] 𝐾𝑠𝑝 (𝐴𝑔𝐶𝑙) ⟹ [𝐴𝑔+ ] = [𝐶𝑙− ] And insert this term into the Nernst equation, then you get: Corresponds to the 𝟎. 𝟎𝟓𝟗 𝐾𝑠𝑝 (𝐴𝑔𝐶𝑙) concentration of the 𝑬 𝟐𝟓°𝑪 (𝑨𝒈/𝑨𝒈+ ) = 𝑬 𝟎(𝑨𝒈/𝑨𝒈+ ) + 𝐥𝐨𝐠 𝟏 [𝐶𝑙− ] KCl solution Electrode potential depends on the chloride ion concentration (E=0.197 V for saturated KCl, E=0.205 V for 3.5M KCl) Fall semester 2024/2025 Dr. Sherif Okeil | Electrochemistry 19 Thank you Fall semester 2024/2025 Dr. Sherif Okeil | Electrochemistry 20