Polarography PDF

# Introduction - Polarographic technique is applied for the qualitative or quantitative analysis of electroreducible or oxidisable elements or groups. - It is an electromechanical technique of analyzing solutions that measures the current flowing between two electrodes in the solution as well as th...

# Introduction - Polarographic technique is applied for the qualitative or quantitative analysis of electroreducible or oxidisable elements or groups. - It is an electromechanical technique of analyzing solutions that measures the current flowing between two electrodes in the solution as well as the gradually increasing applied voltage to determine respectively the concentration of a solute and its nature. - Created by: Jaroslav Heyrovsky # Principle - The principle in polarography is that a gradually increasing negative potential (voltage) is applied between a polarisable and non-polarisable electrode and the corresponding current is recorded. - Polarisable electrode: Dropping Mercury electrode - Non-polarisable electrode: Saturated Calomel electrode - From the current-voltage curve (Sigmoid shape), qualitative and quantitative analysis can be performed. - This technique is called as polarography, the instrument used is called as polarograph and the current-voltage curve recorded is called as polarogram. # A Typical Polarogram - The point of inflection in the current-voltage curve is known as Half-wave potential and is characteristic or typical for every element or functional group - **Qualitative aspect**. - The diffusion current measured is proportional to the concentration of that particular compound - **Quantitative aspect**. - Thus, polarography can be used for both qualitative and quantitative analysis of compounds. # Polarographic Measurements - Polarographic measurements are governed by Ilkovic equation. - **Ilkovic Equation** - The linear relationship between the diffusion current (id) and the concentration of electroactive species is shown by the Ilkovic equation: $id = 607 nCD1/2m2/3t1/6,$ - id = diffusion current in microamperes; - n = number of electrons required per molecule of electroactive substance, - D = diffusion coefficient, in square cm per second, - C = concentration, in millimoles per L, - m = mass of mercury flow from the DME, in mg per second, - t = drop time, in seconds. # Advantages of Polarography - Organic and inorganic samples can be analyzed. - Even low concentrations of the samples can be detected (10-3 M to 10-4 M). - Rapidity of the technique. - Qualitative and quantitative analysis can be performed easily. - Mixture of compounds or elements could be analyzed without separation. # Mercury Electrodes - In polarography, mercury is used as a working electrode, because mercury is a liquid. - The working electrode is often a drop suspended from the end of a capillary tube. - There are 3 examples of electrodes. - HMDE (Hanging mercury drop electrode). - DME (dropping mercury electrode) - Most commonly used electrode. Mercury drops form at the end of the capillary tube as a result of gravity. The optimum interval between drops for most analysis is between 2 to 5 seconds. - DSME (static mercury drop electrode) # Why Dropping Mercury Electrode? - Hg yields reproducible current-potential data. - This reproducibility can be attributed to the continuous exposure of fresh surface on the growing mercury drop. - With any other electrode (such as Pt in various forms), the potential depends on its surface condition and therefore on its previous treatment. - The vast majority of reactions studied with the mercury electrode are reductions. - At a Pt surface, reduction of solvent is expected to compete with reduction of many analyte species, especially in acidic solutions. - The high overpotential for H+ reduction at the mercury surface. Therefore, H+ reduction does not interfere with many reductions. # Problems with Mercury Electrode - A mercury electrode is not very useful for performing oxidations, because Hg is too easily oxidized. - The electrode cannot be used over +0.4 V (vs S.C.E.), because Hg dissolves and anodic wave is recorded. Lesser than -1.8 V, hydrogen is liberated. - The capillary is difficult to maintain since dust or other articulate matter can block the capillary. # Method of Analysis - The polarographic apparatus consists of a polarisable electrode (DME) and a reference electrode (SCE). - Between these electrodes, the required potential range (0 to - 3 V) can be applied. - It consists of a sample cell made up of glass, in which the sample solution to be analyzed is kept. - The capillary is dipped into the solution to be analyzed and the height of the mercury reservoir is adjusted so that droptime of about 2-7 sec is set. # Supporting Electrolytes - Supporting electrolytes like potassium chloride is added to the sample solution to eliminate migration current. - The oxygen present in the sample solution is removed by passing nitrogen or using alkaline pyrogallol solution. - Maximum suppressors are added in the required concentration range. - When all these things are done, the initial and final potential is set in the instrument and the current-voltage curve is recorded. - From the polarogram, half wave potential and diffusion current is determined and thus qualitative and quantitative analysis is performed. # Polarographic Maxima - An increase of current above the limiting value in the form of a maximum is often noticed on polarographic waves known as polarographic maxima. - The origin of the maximum is sometimes connected with the mechanism of the electrode process as in the case of catalytic discharge of ions, while in many cases, it is caused by the increased transport of the depolarizer towards the electrode by a streaming motion of the solution. - The presence of this reproducible polarographic maxima leads to error in determining half wave potential and diffusion current. # Elimination of Polarographic Maxima - Maximum suppressors which increases the viscosity eliminate the polarographic maxima such as: - Gelatin (0.002% to 0.01%). - Dyes (methyl red) - Surfactants (triton) # Different Types of Current in Polarography - **Residual current (i)**: It is the sum of the relatively larger condenser current and a very small Faradic current. Condenser current is due to the formation of Helmholtz double layer at the mercury surface and Faradic current is due to the traces of impurities. - **Migration current (im)**: It is due to migration of cations from the bulk of the solution towards cathode due to diffusive force, irrespective of concentration gradient. - **Diffusion current (id)**: Diffusion current is due to the actual diffusion of electroreducible ion from the bulk of the sample to the surface of the mercury droplet due to concentration gradient. - **Limiting current (id)**: Beyond a certain potential, the current reaches a steady value called as the limiting current. At this point, the rate of diffusion of ions is equal to the rate of reduction and the state of electrode is said to be concentration polarised. # Factors Affecting Diffusion Current - **Concentration**: Diffusion current is directly proportional to the concentration of the electroreducible ions. - **Temperature**: Diffusion current also varies with respect to temperature (directly proportional). - **Viscosity of the medium**: Diffusion coefficient depends on the viscosity of the medium. Hence, diffusion current also varies. - **Capillary characteristics**: The bore size of the capillary, drop time in seconds and the pressure of the mercury will all affect the diffusion current as they alter the flow characteristics of mercury droplet. - **Presence of maximum suppressors**: Maximum suppressors like gelatin, dye stuffs and surfactants will affect the diffusion current. # Pharmaceutical Applications - Dissolved oxygen and peroxides. - Trace metals and metal-containing drugs. - Antiseptics and insecticides. - Vitamins. - Hormones. - Antibiotics. - Alkaloids. - Blood serum and cancer diagnosis. # Advantages of Polarography - Only small volume of sample is required. - Turbid and coloured solutions can be analyzed. - It can be used for the determination of substances, which are not electrochemical active (indirect). - Prior separation of excepients is not required. - Its sensitivity is sufficient for the determination trace elements and toxic impurities. - High speed analysis which is important for QC.