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Chapter 19: Q6P (page 485)

Spectroscopic data for the indicators thymol blue (TB),semithymol blue (STB), and methylthymol blue (MTB) are shown in the table. A solution ofTB,STB,MTB in a1.000-cm cuvet had absorbances of 0.412at455nm,0.350 at485nm, and 0.632 at545nm. Modify the spreadsheet in Figure 19-4 to handle three simultaneous equations and find[TB],[STB]and[MTB]in the mixture.

Short Answer

Expert verified

TheTB,STB,MTBinthemixtureare,TB=1.22路10-5MSTB=9.30路10-6MMTB=1.32路10-5M

Step by step solution

01

Modify a spreadsheet and use a formula to find the values:

Make a spreadsheet,

Hence, highlight the cells G4,G5 and G6.

Use the formula in the cells,

=MMULT(MINVERSE(B4:D6);E4:E6)

Thus, the values of TB,STB,MTBare,

TB=1.22路10-5MSTB=9.30路10-6MMTB=1.32路10-5M

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Most popular questions from this chapter

This problem can be worked by calculator or with the spreadsheet in Figure 19-4. Consider compounds X and Y in the example labeled 鈥淎nalysis of a Mixture, Using Equations 19-6鈥� on page 464. Find [X] and [Y] in a solution whose absorbance is 0.233 at 272 nm and 0.200 at 327 nm in a 0.100-cm cell.

This problem can be worked with Equations 19-6 on a calculator or with the spreadsheet in Figure 19-4. Transferrin is the iron-transport protein found in blood. It has a molecular mass of 81 000 and carries twoFe3+ions. Desferrioxamine B is a chelator used to treat patients with iron overload (see the opening of Chapter 12). It has a molecular mass of about 650 and can bind oneFe3+Fe31. Desferrioxamine can take iron from many sites within the body and is excreted (with its iron) through the kidneys. Molar absorptivities of these compounds (saturated with iron) at two wavelengths are given in the table. Both compounds are colorless (no visible absorption) in the absence of iron.


(a) A solution of transferrin exhibits an absorbance of 0.463 at 470 nm in a 1.000-cm cell. Calculate the concentration of transferrin in milligrams per milliliter and the concentration of bound iron in micrograms per milliliter.

(b) After adding desferrioxamine (which dilutes the sample), the absorbance at 470 nm was 0.424, and the absorbance at 428 nm was 0.401. Calculate the fraction of iron in transferrin and the fraction in desferrioxamine. Remember that transferrin binds two iron atoms and desferrioxamine binds only one.

The figure shows spectra of1.00脳10-4MMnO4-,1.00脳10-4and an unknown mixture of both, all in1.000鈥�cm鈥�path length cells. Absorbances are given in the table. Use the least squares procedure in Figure 19-3 to find the concentration of each species in the mixture.

Visible spectrum ofMnO4-,Cr2O72-, and an unknown mixture containing both ions.

The graph shows the effect of pH on quenching of luminescence of tris(2,2'-bipyridine) Ru(II) by 2,6-dimethylphenol. The ordinate, KSV, is the collection of constants, kq /(ke + kd), in the Stern-Volmer equation. The greater KSV, the greater the quenching. Suggest an explanation for the shape of the graph and

estimate pKa for 2,6-dimethylphenol.

Two ways to analyze a mixture. Figure 19-5 shows the spectrum of the indicator bromothymol blue adjusted to several pH values. The spectrum at pHis that of the pure blue form and the spectrum at pH 1.8is that of the pure yellow form. At other pHvalues, there is a mixture of the two forms. The total concentration isand the path length isin all spectra. For the purpose of calculation, assume that there are more than two significant digits in concentration and path length. Absorbance at the dots on three of the curves in Figure 19-5 is given in the table.

(a) Prepare a spreadsheet like Figure 19-3 to use absorption at all six wavelengths to find[In-]and[HIn]in the mixture. Comment on the sum[In-]+[HIn].

(b) From[In-]in the mixture, and frompKa=7.10for HIn,calculate theof the mixture. (This calculation is the source of pH labels in the figure.)

(c) Use Equations 19-6 at the peak wavelengths ofto findin the mixture. Compare your answers to those in (a). Which answers, (a) or (c), are probably more accurate? Why?
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