AIT

      a redox reaction transfers electrons between the reactant species and the electrode produces a measurable current the greater concentration of reactive species, the greater the current measurement of currents can be used to determine concentrations voltammetry - an electrical current is measured as a function of applied potential used to identify and quantify

 M + + e  M (s)  reaction will only occur if both the following conditions apply: ◦ the ion is close enough to the electrode ◦ the voltage applied at the electrode is enough to allow the reaction to occur (the reduction potential)   some ions will always be close to the electrode by sheer chance voltage as the controlling factor for whether reaction will occur

Current Reduction potential Applied potential Initial potential Too low, so no reaction can occur As potential approaches redn V, some ions react Current is zero Current is low and increasing As potential pass redn V, all ions near electrode react Current is high After redn V, ions newly arrived near electrode react Current is high and constant

 a measurable change in current as a consequence of a voltage change  this is known as a wave  whole scan is a voltammogram

 an analogy between spectroscopy and voltammetry peak wavelength/frequency intensity wave voltage current

 for both quantitative and qualitative analysis: ◦ the wave position (voltage) is characteristic of a particular species ◦ the wave height (current) is proportional to concentration

   diffusion (simple random motion), electrostatic attraction, and convection   current-concentration only linear, if diffusion is the only mechanism minimise the other two processes as much as possible

 not stirring the solution controls convection     not possible to prevent electrostatic attraction between the positive ions and the negative electrode reduced by addition of a high concentration of non-reactive ions, known as the supporting electrolyte KCl or KNO3 at concentrations around 0.1 M the very high level of other ions masks attraction to the electrode

electrostatic attraction diffusion

   supporting electrolyte has two other functions: ◦ masks matrix interference due to different levels of background ions in different samples ◦ ensures that the solution will have enough electrical conductivity voltammetry only ever uses up a tiny fraction of the reducible species in the sample multiple scans can be run on the one sample without changing its overall concentration

 the most commonly used form of voltammetry  one of the electrodes is made from a capillary of mercury, forming a drop at the end  known as a dropping mercury electrode (DME)  scan is called a polarogram

limiting current residual current diffusion current

Applied Pote ntial

half-wave potential

 

Measure the half-wave potential and diffusion current Applied potential: each scale division is equal to 0.5 V, becoming more negative from 0 V Current: each scale division is equal to 1 uA starting from 0.

  (a) -1.1 V (b) 7.5 uA

auxiliary electrode DME Hg reservoir reference electrode N 2 bubbler

    Dropping mercury electrode – the electrode at which the analyte reaction occurs Reference electrode –an electrode which maintains a constant voltage regardless of the solution and reactions occurring Auxiliary electrode – provides a path through which current can flow and be measured; usually a platinum wire Nitrogen bubbler – dissolved oxygen produces two visible polarographic waves, at around –0.1 and –0.9 V bubbling nitrogen through the solution for 5 minutes removes the oxygen

  one pair (DME & ref.) to control voltage one pair (DME & aux.) for current path and measurement Auxiliary Current DME Voltage Reference

 does not seem like the most obvious choice  one significant advantage: it presents a fresh surface to the solution

every second or so

 allows a much more reproducible control of potential than a fixed electrode, where the reduced metal (for example) becomes coated to it  Hg oxidised >+0.4 V, so a Pt or graphite working electrode must be used

 presence of complexing agents (ligands) shifts E ½ 

working voltage range of +0.4 to –1.8V

◦ >+0.4: the mercury drop will be oxidised ◦ < –1.8V (varies with pH) water is reduced to hydrogen gas

  limited sensitivity – DC polarography is limited to about 5 mg/L for most species difficulty in measurement – due to the waveform shape and the oscillations  improve the former and get rid of the latter by changing the way that: ◦ the voltage changes ◦ the current is measured

    most obvious problem is the oscillations “digitise” the current measurement, so that a single measure per drop was taken measurement is timed at just before the drop falls off (knocker) slightly improved sensitivity polarogram output measurement point

   sensitivity is limited by the relatively high level of background current it “hides” analyte response three causes: ◦ other species – apart from oxygen, not solvable ◦ voltage changes – the drop charges like a capacitor as the V changes ◦ drop growth – high bkgd current at start of drop growth

   V changes – apply V increases in steps (pulses), since capacitor behaviour fades if V is constant drop growth – measure at end of drop life: bkgd current has faded away 10 x improvement in sensitivity V pulsed change continuous voltage change time

     pulse still gives difficult to measure wave DP measures at two points (start and end of drop) current plotted is difference 20 x increase in sensitivity change of shape to peak (like 1 st derivative titration curve)

    sensitivity – realistic detection limits for differential pulse polarography are around 50 ug/L, multi-component analysis – provided the half-wave potentials are at least 100 mV apart, equipment that is relatively simple and not particularly expensive – typically $40,000 for a computer-controlled device capable of polarography and voltammetry, a wide range of analytes - metallic ions, non-metallic ions and organic species.

   contaminated mercury – which can be purified by distillation with special apparatus, relatively slow – due to purging time matrix interference – due to complex formation, which can make a species not analysable because the half-wave potential is outside the measurable range

       an electrode which measures dissolved oxygen (DO) not an ion-selective electrode relies on current, not potential, measurement the oxygen is not an interference but the analyte non- scanning: V held at -0.8V

current is proportional to the oxygen concentration calibrated using a saturated solution (9 mg/L at 25  C)

     the most sensitive form analyses much more of the sample than normal polarography requires stirring & longer reaction period cannot do a very slow scan Hg drop electrode still used  Step 1 (slow) - fixed voltage, with stirring for 90s to 10 minutes ◦ M + + e => M (Hg amalgam)  Step 2 (normal speed) – scan ◦ M (Hg amalgam) => M + + e

 not all analyte is reduced – time dependent  can measure at ng/L (not ug/L) level  limited to those which form an amalgam with Hg ◦ copper, lead, cadmium, zinc, indium and bismuth

 

What is the problem with using a DME for this analysis?

reduced analyte in step 1 falls to the bottom of the cell and is lost   

What could be done to get around this problem, still using a mercury drop as the electrode?

both steps are done with a single drop called Hanging Drop Mercury Electrode (HDME)