Titration oxidants

D 4739 D 4742 Base Number Determination by Potentiometric Titration. Oxidation Stability of Gasoline Automotive Engine Oils by Thin Film Oxygen Uptake (TFOUT)... 

Fig. 9 shows the titration results for the following samples chloroplast lamellae and TSF-1 particles, both measured at 820 nm, and the CPI complex measured at 820 as well as 703 nm. Each sample was titrated oxidatively (starting with 100 pM ferrocyanide and adding ferricyanide to a maximum concentra tion of 10 mM) and reductively (starting with 1-5 mM ferricyanide and adding ferrocyanide to a maximum concentration of 10 mM). The titration is a plot of the light-induced AA V5. the actual redox-potential of the medium or the ferri-/ferrocyanide ratio as shown in Fig. 9. The plot of the data points clearly show that the titration was completely reversible and that P700 was in redox equilibrium with the ferri-/ferro-cya-nide couple. The solid line is the theoretical Nernst curve for a one-electron transition and the data points agree well with the theoretical course. The titration curve for both the chloroplast lamellae and the TSF-1, as well as D144 (data not shown here), yielded an value of+492 mV.

After removal of peracids by tetradecene addition, substantial amounts of iodometrically titratable oxidation products remained in photooxidized isooctane. Their relative rate of formation did not depend markedly on the rate of radical initiation and was found to be in the order of 40 to 45% of I. This is shown in Figure 3. Although the chemical composition of these products has not been identified, it is, however, assumed that these products were mainly hydroperoxides and probably dialklyperoxides other than those formed by self-recombination of tertiary peroxy radicals. 

The first titration oxidizes Fe to Fe. This titration gives the amonnt of Fe in solution. Zn metal is... 

Titrations based on oxidation-reduction reactions enjoy wide use. Permanganate, dichromate, and iodine and iron(II), tin(II), thiosulfate, and oxalate are commonly used oxidizing and reducing titrants, have been employed to determine components in both inorganic and organic analysis. As we saw in Chapter 7, solvent water does not play as central a role as in acid-base titrations. Oxidants or reductants strong enough to decompose water are not practical as titrants. 

For many years, analytical chemistry relied on chemical reactions to identify and determine the components present in a sample. These types of classical methods, often called wet chanical methods, usually required that a part of the sample be taken and dissolved in a suitable solvent if necessary and the desired reaction carried out. The most important analytical fields based on this approach were volumetric and gravimetric analyses. Acid-base titrations, oxidation-reduction titrations, and gravimetric determinations, such as the determination of silver by precipitation as silver chloride, are all examples of wet chemical analyses. These types of analyses require a high degree of skill and attention to detail on the part of the analyst if accurate and precise results are to be obtained. They are also time consuming, and the demands of today s high-throughput pharmaceutical development labs, forensic labs, commercial environmental labs, and industrial quality control... 

Calculations for Acid-Base Titrations Oxidation-Reduction Reactions... 

The Composition of Solutions Dilution Acid-Base Titrations Oxidation States Method of Balancing Oxidation-Reduction Reactions... 

Several different kinds of reactions other than acid-base reactions can be used for titrations. Oxidation reactions can be used, for example, in detennining dissolved oxygen levels in water the precipitation of CF ion with a standard solntion of Ag is one of the oldest titration procedures, and chelation with the anion of the strong chelating agent ethylenediantinetetraacetic acid (EDTA) can be used for determining Ca ion concentration in water (water hardness) (Chapter 3, Section 3.10). 

The study of nonaqueous titrations covers a very broad field. It can include acid-base titrations, oxidation-reduction titrations, precipitation-titrations, and titrations involving complex formation. In order to limit things somewhat, I would like to discuss only nonaqueous acid-base titrations. However, I do not mean to imply that other types of titrations carried out in nonaqueous media are of lesser importance. For example, some excellent oxidation-reduction titrations have been carried out by Dr. Stone of Michigan State and as an example of a precipitation titration, we developed a method several years ago for titrating sulfate with barium that is carried out at least partly in nonaqueous solution. Other examples could be given but to mention all of them would lead to an excessively prolonged discussion. 

Andrews deration An important titration for the estimation of reducing agents. The reducing agent is dissolved In concentrated hydrochloric acid and titrated with potassium iodale(V) solution. A drop of carbon tetrachloride is added to the solution and the end point is indicated by the disappearance of the iodine colour from this layer. The reducing agent is oxidized and the iodate reduced to ICl, i.e. a 4-eiectron change. 

Excess standard acid is added, and the excess (after disappearance of the solid oxide) is estimated by titration with standard potassium manganate(VII). 

When the oxidation is complete, the excess of hypo-iodite is estimated by cidifying the solution, and then titrating the iodine thus formed against... 

Several variations of the chemical method are in use. In the one described below, a freshly prepared Fehling s solution is standardised by titrating it directly against a standard solution of pure anhydrous glucose when the end-point is reached, I. e., when the cupric salt in the Fehling s solution is completely reduced to cuprous oxide, the supernatant solution becomes completely decolorised. Some difficulty is often experienced at first in determining the end-point of the reaction, but with practice accurate results can be obtained. The titrations should be performed in daylight whenever possible, unless a Special indicator is used (see under Methylene-blue, p. 463). 

A selected list of redox indicators will be found in Table 8.26. A redox indicator should be selected so that its if" is approximately equal to the electrode potential at the equivalent point, or so that the color change will occur at an appropriate part of the titration curve. If n is the number of electrons involved in the transition from the reduced to the oxidized form of the indicator, the range in which the color change occurs is approximately given by if" 0.06/n volt (V) for a two-color indicator whose forms are equally intensely colored. Since hydrogen ions are involved in the redox equilibria of many indicators, it must be recognized that the color change interval of such an indicator will vary with pH. 

In a back titration, a slight excess of the metal salt solution must sometimes be added to yield the color of the metal-indicator complex. Where metal ions are easily hydrolyzed, the complexing agent is best added at a suitable, low pH and only when the metal is fully complexed is the pH adjusted upward to the value required for the back titration. In back titrations, solutions of the following metal ions are commonly employed Cu(II), Mg, Mn(II), Pb(II), Th(IV), and Zn. These solutions are usually prepared in the approximate strength desired from their nitrate salts (or the solution of the metal or its oxide or carbonate in nitric acid), and a minimum amount of acid is added to repress hydrolysis of the metal ion. The solutions are then standardized against an EDTA solution (or other chelon solution) of known strength. 

Manganese(II) can be titrated directly to Mn(III) using hexacyanoferrate(III) as the oxidant. Alternatively, Mn(III), prepared by oxidation of the Mn(II)-EDTA complex with lead dioxide, can be determined by titration with standard iron(II) sulfate. 

Probably the most extensively applied masking agent is cyanide ion. In alkaline solution, cyanide forms strong cyano complexes with the following ions and masks their action toward EDTA Ag, Cd, Co(ll), Cu(ll), Fe(ll), Hg(ll), Ni, Pd(ll), Pt(ll), Tl(lll), and Zn. The alkaline earths, Mn(ll), Pb, and the rare earths are virtually unaffected hence, these latter ions may be titrated with EDTA with the former ions masked by cyanide. Iron(lll) is also masked by cyanide. However, as the hexacy-anoferrate(lll) ion oxidizes many indicators, ascorbic acid is added to form hexacyanoferrate(ll) ion. Moreover, since the addition of cyanide to an acidic solution results in the formation of deadly... 

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. 

---