CHEMISTRY 425

Instrumental Methods of Analysis

SPECTROPHOTOMETRIC TITRATIONS OF CHROMATE AND IRON

THEORY

In the photometric method of equivalence point detection in titrations, use is made of the difference in the molar absorptivities (at the analytical wavelength selected) of the various species present. The appearance of an absorbing species will give a linear or concentration dependent change in absorbance which will yield two straight lines that intersect at the equivalence point.

The selection of the analytical wavelength requires much care, for there are at least three components present which may absorb light: the original substance, the titrant and the resulting product or products. The usual procedure is to select some wavelength at which only one component absorbs. However, the mere fact that only one component absorbs at a particular wavelength does not necessarily mean that this particular absorbance should be selected for analysis. The absorbance may be so intense, that the %T reading may be limited to the undesirable 0-20% T region, where comparatively large errors in measuring absorbance would overshadow the inherent accuracy of the photometric titration.

For a successful photometric titration it is necessary that the measured species adhere roughly to Lambert-Beer's law, and the necessary chemical instrumental precautions must be observed to maintain the relation A = bc. To avoid effects caused by dilution, especially its effect on absorbance, it is customary to use a titrant that is at least 10 times more concentrated than the titrated solution.

In this experiment two different titrations will be performed to provide experience in photometric end point detection. The chromate concentration of an unknown solution will be determined by titration with standard hydrochloric acid. The reaction to produce dichromate is as follows:

2 CrO₂ ^2- + 2 H⁺ --> Cr₂O₇^2- + H₂O

The extent of the reaction is followed by measuring the absorbance at a wavelength selected by experiment to provide an optimally large change in absorbance (a wavelength at which the difference in values of chromate and dichromate is maximal). In another titration, iron will be determined by titration with 1,10-phenanthroline using a wavelength setting of 510 nm at which the product of the reaction, the Fe(phen)⁺ chelate, absorbs strongly ( = 11 100).

TITRATION PROCEDURE

Ideally the titrated solution is continuously circulated through an absorption cell (flow through cell) and back to the titration vessel to facilitate rapid and convenient collection of spectrophotometric titration data. Lacking this special equipment, one can carefully transfer without loss (using a transfer pipet) a portion of the analyte to the absorption cell for each measurement, taking care to return it quantitatively to the titration vessel before adding more titrant and repeating the procedure. Also, it is possible to use a custom made flask with a protruding finger cuvette. In this case care has to be taken to insure a reproducible placement of the cell. Fortunately spectrophotometric titration curves are linear so only a few points are needed before and a few after the equivalence point to locate the intersection that corresponds to the end point of the titration.

APPARATUS

Spectrophotometer and 1 cm absorption cell

Magnetic stirrer and stirring bar

Buret, 10 ml

Ring stand and buret clamp

Pipet, 10 ml

Beaker, 250 ml

Graduated cylinder, 100 ml

REAGENTS

Chromate solution, unknown (0.01 - 0.03 mol/l)

Hydrochloric acid, 0.0200 mol/l

Iron solution, unknown (in the range of 10^-4 mol/l)

1,10-phenanthroline, 1.00 x 10^-3 mol/l

Ascorbic acid, solid

Ammonium acetate, solid

Potassium chromate, 0.0200 mol/l

TITRATION OF UNKNOWN CHROMATE SOLUTION

A. Prepare acidified and non-acidified chromate solutions according to the the following procedure. Pipet 10.0 ml of 0.0200 mol/l K₂CrO₄ into a 250-ml beaker and dilute to 100 ml with distilled water. After uniform mixing, fill a spectrophotometric cell with the solution. Acidify the remainder of the solution in the beaker with 6 drops of concentrated HCl. Obtain the absorption spectra of the non-acidified (K₂CrO₄) solutions in the 440-520 nm region. Find the wavelength at which the difference in absorbances of the two forms of the chromium(VI) is the greatest. This is the optimal wavelength for the titration.

B. Pipet 10.00 ml of the unknown chromate solution into a 250 ml beaker and dilute to approximately 150 ml with distilled water. Titrate with 0.0200 mol/l HCl using the wavelength setting determined above in part A. Take absorbance measurements at 0.5 ml increments of titrant. When three or four successive constant readings are obtained you will have all the points you need to locate the end point graphically. Plot the data, and calculate the molarity of the chromium solution.

TITRATION OF UNKNOWN IRON SOLUTION

Pipet 25.00 ml of the unknown into a 250 ml beaker. Dilute to 150 ml with distilled water and add 0.1 g of ascorbic acid (or 0.1 g of hydroxylamine hydrochloride) to reduce any iron(III) to iron(II) and 1 g of ammonium acetate to adjust the solution to pH 7. Titrate with 1.00 x 10^-3 mol/l 1,10-phenanthroline as described above. A wavelength setting at 510 nm provides maximum sensitivity in measurement of the iron(II) chelate formed stoichiometrically as follows:

Fe²⁺ + 3 phen --> Fe(phen)⁺

QUESTIONS

1. Addition of titrant gives rise to a predictable dilution and diminution of absorbance. Would a calculated dilution correction for each plotted point increase the precision of the end point determination?

2. Estimate the precision of your results. How does this compare with the expected precision of 1-2% commonly observed for spectrophotometric measurements of absorbance?

PVZLG