Electroanalytical – Coulometry and Conductivity Ch 24, 7th e, WMDS) Homework Assignment Chapter 22 – next Wed Textbook page 694,5 Problems 1,2,4,9,11 Notes: on 4, assume an acceptable error is less than 0.1% on 9, remember that ISEs measure activity. also, this a standard depletion reaction where the added Ag+ removes S-2 limited by the amount of Ag+. Assume the depletion reaction goes to completion limited by the silver because for Ag2S, Ksp = 1 x 10-50

Electroanalytical – Coulometry and Conductivity Ch 24, 7th e, WMDS) Coulometry is the general name for methods that measure the amount of electricity required to react exactly with an analyte. It is generally is a redox reaction, though there are variations which we will discuss later. Coulometry may be done at either constant potential or constant current. The number of coulombs of electric charge Q = it where i is the current in amperes (Coulombs/sec) and t time in seconds.

Electroanalytical – Coulometry and Conductivity Ch 24, 7th e, WMDS) If i varies during the electrolysis, Q = ∫ i tdt. A charge of 96,485 Coulombs (1 Faraday) is equal to one mole of electrons. Thus a measurement of Q allows us to calculate the number of moles of electrons involved in that specific half cell reaction. The most common way to use coulometry is by electrically generating a chemically reactive species within an electrolysis cell.

Electroanalytical – Coulometry and Conductivity Ch 24, 7th e, WMDS) For example, at an anode, Ce+3 = Ce+4 + e. The Ce+4 generated may then be used as an oxidant titrant. In order for the electrically generated titrant to be useful analytically it should have a rapid reaction between the titrant and the analyte and a known quantitative reaction with the analyte

Electroanalytical – Coulometry and Conductivity Ch 24, 7th e, WMDS) Typical problem: A 25.0 mL sample of a solution containing phenol was treated with 5 mL of 1 M KBr and 10 mL of 1.0 M HCl. A platinum electrode was made anodic so that the bromide ion was oxidized to Br2. A constant current of 8.00 mA was passed through the electrolysis cell for 2 min and 38 s in order to reach the endpoint for the titration. The reaction is Phenol + 3Br2 = Phenol(Br)3 + 3HBr How many mg of phenol were present in the original sample?

Electroanalytical – Coulometry and Conductivity Ch 24, 7th e, WMDS) Solution to Problem, previous slide t of 2:38 s = (2X60) + 38 or 158 s Q = (8 x 103 A)(158s) = 1.264 As = 1.264 Coulombs The anode reaction is 2Br = Br2 + 2e The number Faradays = 1.264C/96,485 C/F = 1.31 x 105 F or 1.31 x 105 moles of electrons The number of moles of Br2 generated = 1.31 x 105 /2 = 6.55 x 106 mol

Electroanalytical – Coulometry and Conductivity Ch 24, 7th e, WMDS) Solution, continued From the balanced chemical equation, the number of moles of phenol = 6.55 x 10 6 /3 = 2.18 x 10 6 mol The MM of phenol is 94.11 g/mol, so the mass of phenol in the sample = (94.11 g/mol )(2.18 x 10 6 mol) = 2.05 x 10 4 g = 0.205 mg.  

Electroanalytical – Coulometry and Conductivity Ch 24, 7th e, WMDS) The use of coulometric generated species allow the use of several unusual reagents that would not be possible with conventional titrants such as Cr+2, Ag+2, Cu+, Cl2, Ti+3, U+5, Br2, I2 Additional reactants and the electrochemical half-cell for their generation are shown on the next slide.

Electroanalytical – Coulometry and Conductivity Ch 24, 7th e, WMDS) Selected Coulometric Reagents – Hargis, p 365 Reagent Generator Reaction Subs. Determined H+ 2H2O  4H+ + O2 + 4e bases OH- 2H2O + 2e  2OH- + H2 acids Ag+ Ag  Ag+ + e X-, CN-, SCN-, S-2 Hedta-3 HgNH3edta + NH4+ + 2e  Hg + 2 NH3 + Hedta-3 Ca+2, Cu+2, Pb+2, Zn+2, etc Br2 2Br-  Br2 + 2e Phenols, anilines, alkene, mercaptans, As+3, Sb+3, U+4 Fe+2 Fe+3 + e  Fe+2 Ce+4, MnO4-, Cr2O7-2, VO3-

Electroanalytical – Coulometry and Conductivity Ch 24, 7th e, WMDS) Apparatus for Electrically generating OH

Electroanalytical – Coulometry and Conductivity Ch 24, 7th e, WMDS) The coulometric generation of OH- has the following advantages over conventional titrations with hydroxide: 1) by being produced in situ very small amounts may be generated. 2) Eliminates problems with carbonate formation

Electroanalytical – Coulometry and Conductivity Ch 24, 7th e, WMDS) An especially important use of coulometry is the generation of the Karl Fischer reagent for the determination of water. The active species is iodine generated from the oxidation of iodide in absolute methanol in the presence of sulfur dioxide and amines. The general equation is amine-I2 + amine-SO2 + H2O + excess amine  2 amineH+I- + amine-SO3 Commercial development of this method is shown at Karl Fisher method .

Electroanalytical – Coulometry and Conductivity Ch 24, 7th e, WMDS) Advantages of Coulometic Methods 1) No need to standardize the titrant solution 2) Very sensitive; the analysis of small samples 3) Can use titrants that would other wise be unstable 4) four (or five) significant digits are possible, limited only by the accuracy of the time and current measurements.

Electroanalytical – Coulometry and Conductivity Ch 24, 7th e, WMDS) Conductiometric Methods Electrical conduction is a measure of the ability of a solution to carry (or conduct) an electric current. Conductance is the reciprocal of resistance (ohm-1 or mho, now called siemens) , i.e., C = 1/R. Measurements are made with two identical electrodes (generally platinum).

Electroanalytical – Coulometry and Conductivity Ch 24, 7th e, WMDS) Conductiometric Methods Because the solution has both resistive and capacitative elements, and dc currents will not flow through capacitors, the equipment for the measurement of conductance consists of a Wheatstone bridge circuit modified to operate with an ac current. (see Figure 24.10, page 751 of textbook)

Electroanalytical – Coulometry and Conductivity Ch 24, 7th e, WMDS) Uses of conductance measurements detection of presence of ions in solvents, as in the purity control of deionized water, etc detector system for ion chromatography monitoring the course of a titration; the only requirement is that the reaction be accompanied with changes in conductance. Produces straight-line titration curves.  

Electroanalytical – Electrodeposition Ch 24, 7th e, WMDS) Electrodeposition – (or electroseparation) are those methods of electroanalytical chemistry that involve the quantitative electrochemical oxidation or reduction of all or an appreciable amount of the analyte at the surface of an electrode. In order to assure completeness of the electrolysis, the process is done at a voltage > Eo for the electrode. For example, in order to assure 99.9% completion, the potential of electrode exceeds the Eo value by E = Eo + 0.059/n log [Ci(1 – 0.999] or 178/n mV more negative than Eo  

Electroanalytical – Electrodeposition Ch 24, 7th e, WMDS) Electrodeposition – the most common use of electrodeposition is known as electrogravimetry The electrodes are weighed prior to the deposition, and then afterward. The increase in mass (usually on the cathode) is the amount of that material present. Sample Problem: A 1.2563 g sample containing copper was analyzed by dissolving it in sulfuric acid and carrying electrolysis. The cathode weighed 22.3165g before electroylsis and 22.5433 g afterward. What is the % copper in the original sample? mass of Cu = 22.5433 – 22.23165 = 0.2268 g % Cu = (0.2268/1.2563 x 100 = 18.05%

Electroanalytical – Electrodeposition Ch 24, 7th e, WMDS) In addition to the voltage calculated on the previous slide, the electrode is likely to have an overpotential which would be added to the voltage calculated. Overpotential (η) is the voltage that must be applied in an electrolytic cell in addition to the theoretical potential required to liberate a given substance at the working electrode. The value depends on the electrode material and on the current density.  

Electroanalytical – Electrodeposition Ch 24, 7th e, WMDS) Often the fact that a given electrode has an overpotential value of important; for example, the η for H2 on a Pt (~-0.46 v) extends the cathodic range so that some metals whose Eored are more negative than H2 (such as Pb+2, Eored = -0.13v or Ni+2, Eored = -0.26 v) may be deposited at the cathode. Otherwise, the reduction of H+ to H2 would interfere.  

Electroanalytical – Electrodeposition Ch 24, 7th e, WMDS) The apparatus to carry out electrodepositon is shown in Fig 24.1 of your textbook or the next slide. Generally the device includes a potentiostat and a reference electrode so the voltage applied to the working electrode is known and can be maintained. The apparatus also needs to have means to vary the potential difference between the two working electrodes and monitor the current. Stirring is important, and several commerical units also have built in heaters for doing the electrolysis at elevated temperatures.  

Electroanalytical – Electrodeposition Ch 24, 7th e, WMDS) Apparatus for Electrodepositiona) circuit b) Pt cathode c) Pt anode  

Electroanalytical – Electrodeposition Ch 24, 7th e, WMDS) Metal ions may be deposited separately from a mixture for their analysis if their E values differ by more than 0.178/n mV. If the difference is less than 0.178/n mV, the chemist may rely on the use of masking agents to shift the potential of the components to this difference. Sometimes, the inclusion of potential buffers are used to prevent the oxidation/reduction of some species, as in the addition of hydrazine (NH2NH2) to prevent the oxidation of [CuCl2]- .    

Electroanalytical – Electrodeposition Ch 24, 7th e, WMDS) Electrodeposition is also used to remove metal ions that interfere with other methods of analysis. This is generally done using mercury as the cathode as shown in Fig 24.2; the ions are reduced to the metallic form and are absorbed in the mercury, forming an amalgam (Hg + M) mixture which effectively removes them from the solution.