Polarographic titration

Polarographic titration seems of limited application owing to the discontinuity of the dme moreover, the addition of the titrant increments requires stirring whereas the polarographic measurements require an unstirred solution. 

In a wider sense the subject of voltammetric titration would include the polarographic mode as a type of amperometric titration however, we have already treated this in Section 3.3.1.3, because we prefer to use the term voltammetric titration in the strict sense, i.e., for faradaic non-polarographic titration techniques, a survey of which is given in Table 3.3. 

Zhan and Mao [60] used a simple, fast, and selective alternating current oscilloscop-ic polarographic titration method for the determination of primaquine and other alkaloid phosphate in pharmaceutical preparation. The titration was carried out with a standard lead solution in hexamethylene tetramine buffer containing 1 M sodium chlorate (pH 5.5). The results obtained by this method are comparable to those obtained by pharmacopoeial method. 

Figure 4.20 A polarographic titration curve. A polarogram has oscillations about a mean current-voltage curve owing to the regularly changing area of the mercury drop.

Rosner (124) oxidized the liberated iodide with hydrogen peroxide and then estimated the iodine formed colorimetrically (see also (101)). Smythe (129) allowed the reaction to proceed in a bicarbonate-COi buffer system and followed the evdution of COi manometrically. Iodoacetate has been used to block the effect of —SH groups in polarographic titrations (32,125). For results obtiuned with iodoacetate and iodoacetamide see (10,17,106, 119,120). 

The titration of ascorbic acid using coulometrically generated I2 and Br2 is described in this experiment. Details are also given for the polarographic analysis of ascorbic acid. 

Aqueous titration with IN sodium hydroxide is the usual malic acid assay. Maleic and fumaric acid are deterrnined by a polarographic method. Analytical methods have been described (40). 

Tests for purity include the Karl Fischer titration for water this can be applied directly. Acetic acid and methylamine can be detected polarographically. 

The methods dependent upon measurement of an electrical property, and those based upon determination of the extent to which radiation is absorbed or upon assessment of the intensity of emitted radiation, all require the use of a suitable instrument, e.g. polarograph, spectrophotometer, etc., and in consequence such methods are referred to as instrumental methods . Instrumental methods are usually much faster than purely chemical procedures, they are normally applicable at concentrations far too small to be amenable to determination by classical methods, and they find wide application in industry. In most cases a microcomputer can be interfaced to the instrument so that absorption curves, polarograms, titration curves, etc., can be plotted automatically, and in fact, by the incorporation of appropriate servo-mechanisms, the whole analytical process may, in suitable cases, be completely automated. 

Electrochemical analytical techniques are a class of titration methods which in turn can be subdivided into potentiometric titrations using ion-selective electrodes and polarographic methods. Polarographic methods are based on the suppression of the overpotential associated with oxygen or other species in the polarographic cell caused by surfactants or on the effect of surfactants on the capacitance of the electrode. One example of this latter case is the method based on the interference of anionic surfactants with cationic surfactants, or vice versa, on the capacitance of a mercury drop electrode. This interference can be used in the one-phase titration of sulfates without indicator to determine the endpoint... 

Majer65 in 1936 proposed measuring, instead of the entire polarographic curve, only the limiting current at a potential sufficiently high for that purpose if under these conditions one titrates metal ions such as Zn2+, Cd2+, Pb2+, Ni2+, Fe3+ and Bi3+ with EDTA66, one obtains a titration as depicted in Fig. 3.55 i, decreases to a very low value, in agreement with the stability constant of the EDTA-metal complex and the titration end-point is established by the intersection of the ij curves before and after that point correction of the i values for alteration of the solution volume by the titrant increments as in conductometric titration is recommended. 

Fig. 3.83. Potentiometric dead-stop (1) and reversed dead-stop (2) end-point titrations, (a) Metrohm Polarecord 626 PARC Model 174A Polarographic Analyzer, (b) Radiometer ISS 820 Ion Scanning System, (c) Tacussel PRG4 Polarograph. 

Oxidative stress Lipid oxidation Oxygen absorption Manometric, polarographic Diene conjugation HPLC, spectrophotometry (234 nm) Lipid hydroperoxides HPLC, GC-MS, chemiluminescence, spectrophotometry Iodine liberation Titration Thiocyanate Spectrophotometry (500 nm) Hydrocarbons GC Cytotoxic aldehydes LPO-586, HPLC, GC, GC-MS Hexanal and related end products Sensory, physicochemical, Cu(II) induction method, GC TBARS Spectrophotometry (532-535 nm), HPLC Rancimat Conductivity F2-iP GC/MS, HPLC/MS, immunoassays... 

Amperometric titrations are inherently more precise than polarography and are more generally applicable because the analyte need not itself be electroactive. Titrations involving the DME are not affected by changes in capillary characteristics as are conventional polarographic determinations, whilst working at a predetermined temperature is unnecessary provided that it remains reasonably constant throughout the titration. 

Implementation Confirmation of the in situ monitors results is obtained when river water samples are brought to the lab and tested for dissolved oxygen using a lab dissolved oxygen probe (a polarographic electrode-based measurement) and the classic Winkler oxygen titration method. 

Amperometric titration is a better method than polarographic methodfor quantitative analysis. 

A 20-50 mg sample of chlorpromazine was poiarographically titrated with O.OIM tungstosilicic acid in 3.5% HCl [146], Alternatively, a solution or extract of the analyte in 0.5N HCl (25 mL, containing 25-200 pg of chlorpromazine) can be shaken with 2 drops of saturated aqueous bromine solution, set aside for 1 minute, flushed with nitrogen, and examined with a cathode ray polarograph [147]. 

Procaine has been determined by polarographic [58-60] and oscillopolarographic titration [61,62] in dmg forms, eye drops, injections, suppositories, and in the commercial product "Menovasin". 

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