Oxidation and reduction titration

Lactose is a reducing sugar, i.e. it is capable of reducing appropriate oxidizing agents, two of which are usually used, i.e. alkaline copper sulphate (CUSO4 in sodium potassium tartrate Fehling s solution) or chloroamine-T (2.1). 

CujO precipitates and may be recovered by filtration and weighed the concentration of lactose can then be calculated since the oxidation of one mole of lactose (360 g) yields one mole of CujO (143 g). However, it is more convenient to add an excess of a standard solution of CUSO4 to the lactose-containing solution. The solution is cooled and the excess CUSO4 determined by reaction with KI and titrating the liberated with standard sodium thiosulphate (Na2S203) using starch as an indicator. 

The end point in the Fehling s is not sharp and the redox determination of lactose is now usually performed using chloramine-T rather than CUSO4 as oxidizing agent. 

The I2 is titrated with standard Na2S40g (sodium thiosulphate)  

One millilitre of 0.04 N thiosulphate is equivalent to 0.0072 g lactose monohydrate or 0.0064 g anhydrous lactose. 

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Oxidation and reduction titrations may be used to measure many species, especially metals in high or low valency states, iodine and iodides, and easily oxidized organic compounds. 

In an oversimplified way, it may be stated that acids of the volcanoes have reacted with the bases of the rocks the compositions of the ocean (which is at the fkst end pokit (pH = 8) of the titration of a strong acid with a carbonate) and the atmosphere (which with its 2 = 10 atm atm is nearly ki equdibrium with the ocean) reflect the proton balance of reaction 1. Oxidation and reduction are accompanied by proton release and proton consumption, respectively. In order to maintain charge balance, the production of electrons, e, must eventually be balanced by the production of. The redox potential of the steady-state system is given by the partial pressure of oxygen (0.2 atm). Furthermore, the dissolution of rocks and the precipitation of minerals are accompanied by consumption and release, respectively. 

OXIDATION AND REDUCTION PROCESSES INVOLVING IODINE I0D0METRIC TITRATIONS... 

Molarity Solubility rules Acids and bases Oxidation and reduction Net ionic equations Titrations... 

This chapter introduces the theory of redox titrations and discusses some common reagents. A few of the oxidants and reductants in Table 16-1 can be used as titrants.2 Most reductants react with 02 and require protection from air to be used as titrants. 

At unit activities of the oxidant and reductant, the potential depends only on pH the slope of the line for a plot of potential versus pH is governed by the ratio m/n. Potential-pH diagrams are a concise means to display the redox properties of a system. We will take uranium as an example. The +6, +5, +4, and + 3 oxidation states are known in aqueous solution. The determination of +6 uranium by coulometric titration has been investigated by many workers and the lower oxidation states have all been used as coulometric titrants. Hydrolyzed uranium species exist in a noncomplexing solution, but the chemistry is simplified considerably if the discussion is limited to solutions more acidic than about pH 4. Some of the half-reactions to be considered are listed next with E° vs. NHE ... 

In Fe(II)-dichromate titrations, Winter and Moyer observed a time dependence of the potential after the end point. When potential readings were taken soon after each addition, an asymmetrical titration curve was observed, but when a time interval of 10 to 15 min was allowed after each addition, the curve approached the theoretical shape. We have noted that automatically recorded titration curves for the Fe(II)-dichromate titration show a considerably smaller potential jump than manually observed curves, the difference being due to lower potentials after the end point. But curves plotted with 15 s of waiting for each point differed only slightly from curves plotted with 150 s of waiting. Ross and Shain also studied the drift in potential of platinum electrodes with time and noted hysteresis effects in recorded potentiometric titration curves. These effects, due to oxidation and reduction of the platinum surface, are discussed below. 

Surface oxide formation undoubtedly is involved in the Fe(II)-dichromate titration curves, which Smith and Brandt found to be different when the direction of titration was reversed (Figure 15-2, right). Kolthoff and Tanaka found that the rate of oxidation with dichromate was slow, whereas the rate of reduction with Fe(II) was fast. Ross and Shain found the same sort of behavior and noted also that the rates of oxidation and reduction decreased in more dilute solutions. The oxidized surface in a dichromate solution may be largely covered with adsorbed dichromate, as chromium surfaces have been shown to be in some experiments with radio-chromium, so that it is relatively ineffective as an electron-transfer surface for the Fe(III)-Fe(II) system. 

REDOX titrations are titrations that involve the processes of oxidation and reduction. These two processes always occur together and are of huge importance in chemistry. Everything from simple ionic reactions to the generation of energy within human mitochondria depends on these two processes. 

In contrast, when the CPI complex was titrated both oxidatively and reductively and monitored at both 820 and 703 nm, the data points also fell on a theoretical Nemst curve for a one-electron transition, but the results yielded a lower value of-i-427 mV, 65 mV less positive than that ofthe chloroplast lamellae or TSF-I particles. These results indicate that chloroplast particles obtained by harsher detergent treatment or samples that have been altered through aging, for example, would result in a lower redox potential. This finding probably can explain many (although perhaps not all) ofthe discrepancies in the redox-potential values reported by various groups over a period of forty years. 

Because of the close analogy between acid-base and redox behavior, it will come as no surprise that one can use redox titrations, and also simulate them on a spreadsheet. In fact, the expressions for redox progress curves are often even simpler than those for acid-base titrations, because they do not take the solvent into account. (Oxidation and reduction of the solvent are almost always kinetically controlled, and therefore do not fit the equilibrium description given here. In the examples given below, they need not be taken... 

The shape of the amperometric titration curve in this case, where both the titrant and the substance titrated undergo reversible redox reactions, is illustrated in Figure 3.21A. In the case where the substance titrated does not have a reversible voltammetric wave, the titration curve will have the shape illustrated in Figure 3.2IB. Prior to the equivalence point, the applied voltage is too small to cause both oxidation and reduction of the redox couple of the substance titrated. If the titrant has an irreversible wave, the titration curve will look like that in Figure 3.21C. This type of titration is commonly called a dead-stop titration, because the indicator current falls to zero at the equivalence point. 

Another important amperometric titrant is bromine solution, which undergoes stoichiometric oxidation-reduction reactions with many substances such as As(III), Sb(III), ammonium salts, and others. Often the titration involves adding an excess of KBr to an acidified solution of the substance to be oxidized and then titrating it with potassium bromate solution. Bromine is thereby generated in situ. 

The effect of host-guest complexation on the observable CV response, and vice versa the ability for oxidation and reduction to change the stability of the complex, is best understood with a square scheme (Scheme 1). The equilibria running vertically are chemical (C) steps associated with complexation. Those equilibria running horizontally are the electrochemical (E) steps. The square scheme is a thermodynamic cycle such that when the two redox potentials ( i and 2) are determined in a CV experiment and a is measured independently (e.g., using an NMR titration, see Binding Constants and Their Measurement, Techniques), it is trivial to determine the stability of the oxidized or reduced complex, This... 

Photometric redox titrations can be carried out in a so-called self-indicator system i.e. the color of a solution changes during the titrations. The analytical wavelength should be matched to the pair of the oxidant and the reductant, which is the main disadvantage of this method. Another method is to use an indicator, the oxidized form of which has a different color than the reduced one. This can be utilized in the fiber optic sensor. One indicator can be used for the titrations with various oxidants and reductants. 

Fig. 1. Effect of CCCP on the pH-dependence of the low midpoint potential of cytochrome b-559 from spinach chloroplasts. Midpoint redox potentials of several chloroplast suspensions (50 pg chloro-phyll/ml) were determined at different pH values by titration with potassium ferricyanide and dithionite as oxidant and reductant, respectively. The following redox mediators were used 20 >iM 2,3,5,6-tetramethyl-p-phenylenediamine, 20 pM 1,2-naphthoquinone, and 20 pM duroquinone. Buffers were potassium phosphate (pH 6.5, and 7) and 50 mM tricine-KOH (pH 7.5, 8.0, and 8.5). Chloroplast suspensions were supplemented, when indicated, with 33 pM CCCP.

Masking by oxidation or reduction of a metal ion to a state which does not react with EDTA is occasionally of value. For example, Fe(III) (log K- y 24.23) in acidic media may be reduced to Fe(II) (log K-yyy = 14.33) by ascorbic acid in this state iron does not interfere in the titration of some trivalent and tetravalent ions in strong acidic medium (pH 0 to 2). Similarly, Hg(II) can be reduced to the metal. In favorable conditions, Cr(III) may be oxidized by alkaline peroxide to chromate which does not complex with EDTA. 

In Sections 10.11-10.16 it is shown how the change in pH during acid-base titrations may be calculated, and how the titration curves thus obtained can be used (a) to ascertain the most suitable indicator to be used in a given titration, and (b) to determine the titration error. Similar procedures may be carried out for oxidation-reduction titrations. Consider first a simple case which involves only change in ionic charge, and is theoretically independent of the hydrogen-ion concentration. A suitable example, for purposes of illustration, is the titration of 100 mL of 0.1M iron(II) with 0.1M cerium(IV) in the presence of dilute sulphuric acid ... 

It is possible to titrate two substances by the same titrant provided that the standard potentials of the substances being titrated, and their oxidation or reduction products, differ by about 0.2 V. Stepwise titration curves are obtained in the titration of mixtures or of substances having several oxidation states. Thus the titration of a solution containing Cr(VI), Fe(III) and V(V) by an acid titanium(III) chloride solution is an example of such a mixture in the first step Cr(VI) is reduced to Cr(III) and V(V) to V(IV) in the second step Fe(III) is reduced to Fe(II) in the third step V(IV) is reduced to V(III) chromium is evaluated by difference of the volumes of titrant used in the first and third steps. Another example is the titration of a mixture of Fe(II) and V(IV) sulphates with Ce(IV) sulphate in dilute sulphuric acid in the first step Fe(II) is oxidised to Fe(III) and in the second jump V(IV) is oxidised to V(V) the latter change is accelerated by heating the solution after oxidation of the Fe(II) ion is complete. The titration of a substance having several oxidation states is exemplified by the stepwise reduction by acid chromium(II) chloride of Cu(II) ion to the Cu(I) state and then to the metal. 

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