Titration of a Mixture

If a mixture of two ions is titrated, the less soluble precipitate forms first. If the solubilities A liquid containing suspended particles is said 

Consider the addition of AgN03 to a solution containing KI and KC1. Because Kip(AgI) A spCAgCl), Agf precipitates first. When precipitation of I is almost complete, the concentration of Ag+ abruptly increases and AgCl begins to precipitate. When Cl is consumed, another abrupt increase in [Ag+] occurs. We expect to see two breaks in the titration curve. The first corresponds to the Agl equivalence point, and the second to the AgCl equivalence point. 

Before Cl precipitates, the calculations for Agl precipitation are just like those in Section 7-4. 

All solutions, including AgN03, were maintained at pH 2.0 by using 0.010 M sulfate buffer prepared from H2S04 and KOH. 

Let s consider the titration of the two species Redn and Red by the compound 0x2. The two titration reactions are 

By adopting the same symbolism as that used in the preceding chapter (Sect. 16.8) but by replacing Cn, C12, and C2 with Xi, X2, and Y in order to take the dilution into account, we find for the general equation of the titration curve. 

Equation (17.27) enables the calculation of the solution equilibrium potential as a function ofY through the exponentials e, en, and 62-It is difficult to handle, but it can be simplified in some conditions. 

The principal goal of this smdy is to find the conditions that must prevail for the sequential titrations of the species Redn and Redn to be satisfactory. As we shall see, it will be possible if, in some parts of the titration curve, some exponentials become negligible. 

For the first species Redn to be solely titrated, the following inequality  

Silver iodide has a smaller solubility product than silver chloride, so Agl is less soluble than AgCl. When Ag is added to a solution of Cl and I , Agl(s) precipitates first  

Because the two solubility products are sufficiently different, the first precipitation is nearly complete before the second commences. 

The elements F. Cl, Br. I. and At are called halogens. Their anions are called halides. 

The Latin word for silver is argentum, from which the symbol Ag is derived. 

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Spectrophotometric titration curve for the complexation titration of a mixture. 

The scale of operations, accuracy, precision, sensitivity, time, and cost of methods involving redox titrations are similar to those described earlier in the chapter for acid-base and complexometric titrimetric methods. As with acid-base titrations, redox titrations can be extended to the analysis of mixtures if there is a significant difference in the ease with which the analytes can be oxidized or reduced. Figure 9.40 shows an example of the titration curve for a mixture of Fe + and Sn +, using Ce + as the titrant. The titration of a mixture of analytes whose standard-state potentials or formal potentials differ by at least 200 mV will result in a separate equivalence point for each analyte. 

Eulton, R. Ross, M. Schroeder, K. Spectrophotometric Titration of a Mixture of Galcium and Magnesium, /. Chem. Educ. 1986, 63, 721-723. 

The titration of a mixture ofp-nitrophenol (pfQ = 7.0) and m-nitrophenol pK = 8.3) can be followed spectrophotometrically. Neither acid absorbs at a wavelength of 545 nm, but their respective conjugate bases do absorb at this wavelength. The m-nitrophenolate ion has a greater absorbance than an equimolar solution of the p-nitrophenolate ion. Sketch the spectrophotometric titration curve for a 50.00-mL mixture consisting of 0.0500 M p-nitrophenol and 0.0500 M m-nitrophenol with 0.100 M NaOH, and compare the curve with the expected potentiometric titration curves. 

Sketch the spectrophotometric titration curve for the titration of a mixture of 5.00 X 10 M Bi + and 5.00 X 10 M Cu + with 0.0100 M EDTA. Assume that only the Cu +-EDTA complex absorbs at the selected wavelength. 

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. 

An interesting extension of the above experiment is the titration of a mixture of halides (chloride/iodide) with silver nitrate solution. Prepare a solution (100 mL) containing both potassium chloride and potassium iodide weigh each substance accurately and arrange for the solution to be about 0.025 M with respect to each salt. A silver nitrate solution of known concentration (about 0.05 M) will also be required. 

A supplementary section at www.whtreeman.com/qca derives a spreadsheet equation for the titration of a mixture, such as that in Figure 7-8. 

It is desirable to avoid errors in pH measurement, which limit the accuracy of the above NMR pH titration. If an NMR titration is applied to a mixture of two acids, HA and HA, each of whose chemical shifts, 6 and S, follows Equation (13), it is possible to eliminate pH from the two equations and replace it with n, the number of equivalents of titrant added. Thus Ellison and Robinson obtained Equation (14), where A = 6-6, A = 6 -6, A° = <5° -S °, and R = KJK1 20 Moreover, it is not necessary to prepare solutions of exact molarity, because n can be evaluated more readily as (6-6 )/(6° -6 ), from the variation of the chemical shift of HA during the titration. When R is near 1, this is approximately a parabolic dependence of A on n. The titration of a mixture of formic acid and 1802-formic acid permitted the evaluation of the lsO IE on acidity from the 13C NMR chemical shifts 6 and S of the carboxyl carbons. In practice, this involved fitting to the three parameters A-, A°, and R. This same equation was used with 31P chemical shifts to evaluate the lsO IE on the acidity of phosphoric acid and alkyl phosphates.21... 

This NMR titration method was subsequently applied to equilibrium IEs on acidity.30 33 Like the previous methods, it too benefits from the high sensitivity of 13C and 19F chemical shifts, and even 111 chemical shifts, to both isotopic substitution and state of protonation. Figure 1 shows the NMR titration of a mixture of tri(methyl-d)amine and tri(methyl-t/2)amine in D20, plotted according to Equation (19). The slope is 1.1618 0.0004. The intercept is -0.0061 0.0046, properly zero. The correlation coefficient is an impressive 0.999999, which is an indication of the accuracy achievable. Another remarkable result was the measurement of the relative basicity of the two exceedingly similar isotopomers of 1 -benzyl-4-methylpiperidine-2,2,6-t/3 (6). These are truly isotopomers (here stereoisomers), which bear the same number of isotopic substitutions and differ only in the position of the isotope, which is either axial or equatorial. 

Figure 6.8. Titration of halides with silver nitrate, (a) Titration of iodide with AgNOs. (b) Titration of a mixture of chloride, bromide and iodide M jjh AgNOj...

Fig. 6 Titration of a mixture of a strong and a weak acid with alkali.

If the titrant is a weak electrolyte (such as ammonia), the curve is essentially horizontal past the equivalence point, which causes less uncertainty to the extrapolation of a curve. In titration of a weak base, such as acetate ion, with a strong acid, a salt and undissociated acetic acid are formed. After the endpoint is passed, a sharp rise in conductance attends the addition of excess hydronium ions. Salts whose acidic or basic character is too weak to give satisfactory endpoints with indicator are conveniently titrated with the conductometric method. The conductometric titration of a mixture of two acids that differ in degree of dissociation is frequently more accurate than a potentiometric titration. 

Briefly explain why curve B cannot describe the titration of a mixture consisting of H3PO4 and NaH2P04. 

However, NMR titration of a mixture of the anomerie glueopyranosylimi-dazoles in a variety of solvents established eonelusively that the a-anomer was more basie than the p-anomer by around 0.3 pA units in D2O, but less in less polar solvents. This is exactly the opposite of what is predicted by any reverse anomerie effect, but is what would be predicted on the frontier orbital picture of the normal anomerie effect of Figure 2.12, since protonation would increase the electron demand of the anomerie substituent. The additional hydroxymethyl group of glucopyranosylimidazoles, ensures that they, unlike xylopyranosyl-imidazoles, remain in the conformation on protonation. On the electrostatic model, therefore, the protonated a-anomer is expected to be destabilised and the protonated p-anomer stabilised, which should make the p-anomer more basic. 

Fig. 7. The Titration of a Mixture of 0.01 Normal Hydrochloric Acid and 0.01 Normal Acetic Acid with Sodium Hydroxide.

For the titration of a mixture of acids with a single strong base, (4.3-6) can be generalized to... 

With these values, calculate in the column for VbiCA c the appropriate expression for the titration of a mixture of two monoprotic acids. 

Fig. 5.6-3 The four Gran plots for the titration of a mixture of iodide, bromide, and chloride with a soluble silver salt in the presence of a coagulating agent, as in Fig. 5.6-2.

EXPERIMENT 20 POTENTIOMETRIC TITRATION OF A MIXTURE OF CHLORIDE AND IODIDE... 

It might appear that peak width merely serves as range extension. Yet, since peak width is related to a time span, which in turn is related through linear flow velocity to volumetric rate, peak width measurements allow flow titrations to be performed in a novel way. Therefore, similarly to classical batch titrations, FIA titrations encompass a domain of determinations, which cannot be performed in any other way, because they are based on consumption of an equivalent amount of reagent and, therefore, titrations yield different information than a direct measurement (pH measurement versus titration of a mixture of a weak and strong acid). 

Analysis of mixtures is possible when the two species have different equilibrium constants and heats of reaction with the titrant. This occurs, for example, in the titration of a mixture of calcium and magnesium with EDTA. Calcium (ATf = 10 ) reacts first and exothermically AH = — 5.7 kcal/mole) magnesium (AT, = 10 - ) reacts second and endothermically AH — +5.5 kcal/mole). This titration is illustrated in Figure 17.16. 

Figure 17.16. Thermometric titration of a mixture of Ca and Mg with EDTA.

Figure 15.24 Solution conductivity during titration of a mixture of HCl and CH3COOH with NaOH. Here C and C" designate the respective endpoints. HCl is titrated first, then the weaker acetic acid is neutralized.

Thus, mix 366 mL H3PO4 with 634 mL NaOH 15-28. For the titration of a mixture of H3PO4 and H2PO4, the volume to the first end point... 

Titration of a Mixture of Strong Acids with a Strong Base and Inversely... 

Titration of a Mixture of a Strong Acid and a Weak Acid. 

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