CHEM 2113 Test 4 Spring 1997

All questions are worth 25 points. Omit one question by leaving completely blank or by clearly writing OMIT on the page. Show your work and circle your answers where appropriate for full credit. Good Luck!

The following data were obtained for the calibration of a calcium ion electrode:

[Ca2+], Potential (mV)

1.0 x 10-5, -53.8

1.0 x 10-4 , -27.7

1.0 x 10-3, +2.7

1.0 x 10-2, +31.9



Calculate the slope, the y-intercept, and the correlation coefficient for the linearly-related data.

An unknown serum sample containing calcium was measured, and the potential of the electrode was 20.3 mV. Calculate the concentration of the calcium ion in the solution.

2. Consider the following questions pertaining to potentiometric methods of analysis, and answer each briefly.

a) What are the three types of metallic indicator electrodes? List each type, explain what differentiates the types, and qualitatively describe the relationship between cell potential and concentration of analyte for these electrodes.

b) What is the purpose of a reference electrode? What property is most important for a reference electrode?

c) It was discussed in class that the function of a membrane electrode is 'fundamentally different' than the function of a metallic indicator electrode. Briefly describe this 'fundamental' difference, and describe how this type of electrode responds to a change in analyte concentration.

3. a) (10) The quinhydrone electrode was used prior to the introduction of the glass electrode for the determination of the pH of solutions. Quinhydrone is an equimolar mixture of quinone (Q) and hydroxyquinone (H2Q) that undergoes the following ½ reaction:

Q +2H+ + 2e- H2Q E = 0.699V

calculate the pH of a solution saturated in quinhydrone if the potential of the cell (vs SCE, E=0.242V) is 0.182V. Assume the junction potential = 0 for this cell.

b) (15) One mole of photons is called an einstein of radiation. Calculate the energy in kJ of one einstein of radiation at 525 nm.

4. Consider the spectrophotometric determination of manganese. A 50.0 ppm solution of standard manganese solution was used to prepare the solutions for analysis as follows:

mL standard; [Mn], ppm; g KIO4; Final Volume (mL); %T at 525 nm

0.00; _________; 0.8; 100.0; 100.0

5.00; _________; 0.8; 100.0; 83.9

10.00; _________; 0.8; 100.0; 68.9

25.00; _________; 0.8; 100.0; 41.6

50.00; _________; 0.8; 100.0; 17.9

Calculate the slope and the y-intercept for the linearly-related data.

A 1.427 g sample of cast iron was dissolved in nitric acid and diluted to 1.000 L. 25.00 mL of this solution was treated as above, diluted to 100.0 mL and measured spectrophotometrically. The %T was determined to be 32.3. calculate the %Mn in the unknown.

5. Manganese and chromium can be determined simultaneously in steel. A sample of steel weighing 1.000 g was dissolved in acid and treated to convert the Mn to MnO4-, and the Cr to Cr2O72-. This solution was then diluted to 100.0 mL in a volumetric flask, and the absorbance was measured at two separate wavelengths. At 440 nm A=0.108; and at 545 nm A=1.296. Calculate the %Mn and Cr in the steel sample, given the following molar absorptivity values:

Molar Absorptivities () for the following species at the given wavelengths:

Wavelength 440, 545

Cr2O72- 369, 11

MnO4- 95, 2350

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