News from MicroLab Inc.

Photometric Titrations Go Way Back...

See the article "Photometric Titrations," Arthur L. Underwood, J. Chem. Educ., 1954, 31 (8), p 394-397

This is an excellent review of the nature of photometric titrations: background concepts, accuracy and precision compared to volumetric methods, and a number of specific applications that might be of interest to the reader. There have been some significant advances in spectrometers since the article was published that make the photometric titration approach even more desirable and far less cumbersome than with older spectrometer designs.

FS 522 unit with stirred vial

Figure 1. Representation of the FS522 with stirred vial in the FASTSpec compartment during a photometric titration. The vial's scew cap can be replaced with a cloth cover during the titration to minimize external stray light. The red arrow indicates light from an LED to a photodiode.

Specifically,

the patented design of the built-in FASTSpec spectrometer on the 16 bit MicroLab FS522, with its range of 360-940nm and circular array of detectors that supports cylindrical sample vials, greatly facilitates the addition of titrant directly into the spectrometer vial while it remains in the sample compartment. This results in very high photometric reproducibility. And the cylindrical vial gives very efficient stirring to create a homogeneous solution quickly.

Photometric titration Ni with en abs 555

Figure 2. An example of a micromolar photometric titration using the FS522 built in FASTSpec spectrometer along with a prototype of the new Photometric Titration software package. The titration was carried out with replicate additions from a 10 uL auto-pipet. The vial was stirred throughout the experiment. Data can be exported easily to a spreadsheet where the pre-endpoint and post-endpoint data can be regressed to establish their intersection. The interpolated endpoint was 68.3 uL.

Some examples of photometric titrations that are suitable for use with the FS522 are illustrated below:

Titrations involving KMnO₄: Permanganate is suitable for a wide range of redox titrations under acidic conditions: C₂O₄^2-, Fe²⁺, As³⁺, Sb³⁺, V⁴⁺, H₂O₂, SO₃^2-, and NO₂^- can all be determined. End points are easily seen by the appearance of pink at the first drop of excess MnO₄^-. The intense color of the permanganate ion is an advantage in photometric titrations in that it allows high accuracy of end point determination on very small quantities of analyte and titrant because there is an abrupt change in slope of the absorbance vs. volume KMnO₄ at the end point. Extrapolations of the plot up to the end point and beyond the end point allow precise determination of the end point graphically, with <1% error on titrations involving only a few mL of KMnO₄ solution.

Titrations involving complex formation: Cu(II)-EDTA, Ni(II)-EDTA, mixtures of Fe(III) and Cu(II) with EDTA, mixtures of Bi(III) and Cu(II) with EDTA. Again, abrupt changes in slope in plots of absorbance vs. volume titrant allow for precise graphical determination of the end points on small samples.

Turbidometric titrations: some precipitation reactions lend themselves to being followed on the MicroLab FS522 unit, which has the ability to measure scattered light. Underwood suggests that Ni(II) with dimethylogyoxime; Pb(II) with citrate; and Cr(III), Al(III), and Fe(III) with phthalate can all be titrated turbiditmetrically with good accuracy and precision.

The titration data are obtained in a manner analogous to that used to develop the Beer's Law standard curve (see Newsletter Volume 1, Number 4), but the analyst enters volumes of titrant (or moles) from the keyboard instead of concentrations of standards. The titration curves that develop can be examined at any wavelength in real time with a click of the mouse.

Conventional acid-base titrations: following an indicator color change to determine the end point; following a colored acid with a strong base, eliminating the need for indicators; investigating indicator response against pH during a titration. See for example "Acid-Base Indicators: A New Look at an Old Topic,"Ara S. Kooser , Judith L. Jenkins and Lawrence E. Welch , J. Chem. Educ., 2001, 78 (11), p 1504.

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Determination of an Equilibrium Constant: "Photometric titrations" can also be used to study equilibrium constants for a reaction of the type D + X = DX in which DX absorbs light. An excellent description of a conceptually simple method that is suitable for reactions that do not go to completion is given in the article "Equilibrium constants from spectrophotometric data: Principles, practice and programming," Richard Ramette, J. Chem. Educ., 1967, 44 (11), p 647.

The Ramette method kind of got lost since he described not just the algebra but an outdated BASIC program to solve the equations. However, the math involved lends itself nicely to spreadsheet solutions, so it is probably time to give it a new look. The raw [concentration, absorbance] MicroLab data must be exported to a spreadsheet to determine the Keq, a trivial operation with the MicroLab software (File, Export as, Comma Separated Variable (CSV) for MS Excel etc).

Concentration/absorbance data (data taken from Ramette reference above). The MicroLab FASTSpec spectrometer could be used to do this experiment using the 470 nm LED.

The basic idea is that one guesses the molar absorptivity of the absorbing product (which is unknown since it is never formed quantitatively) to compute an equilibrium constant value for each data point. The molar absorptivity guess is iterated to minimize the standard deviation of the range of K_eq values computed for each data point. See Figure 3. The built in Excel analysis tool "Solver" can also be used to obtain the best fit. Feel free to contact the editor for a sample spreadsheet that illustrates this using the data shown and other data.

Figure 3. Spreadsheet results after data treatment.