Choice of indicators

Choice of indicators. The indicator chosen should be one for which the formation of the metal-indicator complex is sufficiently rapid to permit establishment of the end point without undue waiting, and should preferably be reversible. 

Diphenylcarbazide as adsorption indicator, 358 as colorimetric reagent, 687 Diphenylthiocarbazone see Dithizone Direct reading emission spectrometer 775 Dispensers (liquid) 84 Displacement titrations 278 borate ion with a strong acid, 278 carbonate ion with a strong acid, 278 choice of indicators for, 279, 280 Dissociation (ionisation) constant 23, 31 calculations involving, 34 D. of for a complex ion, (v) 602 for an indicator, (s) 718 of polyprotic acids, 33 values for acids and bases in water, (T) 832 true or thermodynamic, 23 Distribution coefficient 162, 195 and per cent extraction, 165 Distribution ratio 162 Dithiol 693, 695, 697 Dithizone 171, 178... 

Within one of these intervals there is free choice of indices, but once this is decided the other assignments are no longer random. The arbitrary assignment of indices to the N-atoms determines the indices of the adjacent C-atoms and these in turn the order of the H-atoms. 

By assuming that n = 1, the limits for this particular indicator are E 0.059ln (100) V, i.e. a working range of c. 120 mV. The ideal choice of indicator is seen to be one which is characterized by a standard redox potential as close as possible to the end point of the titration system. 

In the case of aspirin, the choice of indicator is restricted by where the inflection in its titration curve lies PP is suitable as an indicator whereas MO is not. 

To measure hardness, the sample is treated with ascorbic acid (or hydroxylamine) to reduce Fe3+ to Fe2+ and with cyanide to mask Fe2+, Cu+, and several other minor metal ions. Titration with EDTA at pH 10 in NH3 buffer then gives the total concentrations of Ca2+ and Mg2+. Ca2+ can be determined separately if the titration is carried out at pH 13 without ammonia. At this pH, Mg(OH)2 precipitates and is inaccessible to EDTA. Interference by many metal ions can be reduced by the right choice of indicators.21... 

In order that a particular indicator may be of use for a given acid-base titration, it is necessary that its exponent should correspond to a pH on the almost vertical portion of the pH-neutralization curve. When the end-point of the titration is approached the pH changes rapidly, and the correct indicator will undergo a sharp color change. The choice of indicator may be readily facilitated by means of Fig. 106 in which the... 

With use of these rules we can answer questions as to the hydrolysis of salts or the choice of indicators for titration without referring to tables of acid constants. 

In this titration of a weak acid (HCOOH) by a strong base (NaOH), the equivalence point is not at pH 7. Check Figure 19-18. Do you agree that phenolphthalein is the best choice of indicator for this titration Why ... 

FIGURE 15.9 Indicators change their colors at very different pH values, so the best choice of indicator depends on the particular experimental conditions. 

Note The choice of indicator is not critical in strong acid-strong base titrations since the slope of the transition at the equivalence point is very steep.)... 

When using an acid—base indicator, the change in color refers to the end point of the titration. With proper choice of indicator, the end point should be very close to the equivalence or stoichiometric) point. 

The Effect of Reaction Completeness on Titration Curves Figure 13-5 illustrates the effect of solubility product on the sharpness of the end point in titrations with 0.1 M silver nitrate. Clearly, the change in pAg at the equivalence point becomes greater as the solubility products become smaller, that is, as the reaction between the analyte and silver nitrate becomes more complete. By careful choice of indicator—one that changes color in the region of pAg from 4 to 6—titration of chloride ion should be possible with a minimal titration error. Note that ions forming precipitates with solubility products much larger than about 10 do not yield satisfactory end points. 

Figures 14-5 and 14-6 show that the choice of indicator is more limited for the titration of a weak acid than for the titration of a strong acid. For example. Figure 14-5 illustrates that bromocresol green is totally unsuited for titration of 0.1000 M acetic acid. Bromothymol blue does not work either because its full color change occurs over a range of titrant volume from about 47 mL to 50 mL of 0.1000 M base. An indicator exhibiting a color change in the basic region, such as phenolphthalein, however, should provide a sharp end point with a minimal titration error.

KHIIO,), in contrast to all other primary standards for bases, has the advantage of being a strong acid, thus making the choice of indicator less critical. 

It will be thus necessary to introduce a hierarchical approach. Lapkin et al. [65] have proposed four vertical hierarchy levels (i) product and process, (ii) company, (iii) infrastructure and (iv) society. Each level should reflect different boundary limits and use an appropriate choice of indicators. It is proposed also that the choice of appropriate indicators depends on the speciflcs of the industry sector and even on the types of products. The indicators should reflect speciflc by-products, wastes and emissions that are characteristic of the process or the product. Of course, alimit of the approach is how to make uniform the comparison between diflferent industrial sectors. On the other hand, we have already remarked that industrial chemical production is different from other manufacture industrial sectors, because (i) it includes very different types of productions, from several thousand tons per day in refinery to kg amounts per day in fine chemical production and (ii) it is characterized by a highly integrated structure in which a large part of the products are the input for other chemical processes. 

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