Instrumental Methods of Analysis
SPECTROPHOTOMETRIC TITRATIONS OF CHROMATE AND IRON
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+ --> Cr2O72- + 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).
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.
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
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
Copied from a previous handout: 24 August 1998