The spetrophotometric determination of pH

Topics: PH, Chemistry, PH indicator Pages: 3 (493 words) Published: February 19, 2014

The Spetrophotometric Determination of pH

Instrumentation Lab

This experiment is designed to find the pH of an unknown buffer. We find this by adding acid/base indicator of known Ka then by using a spectrophotometric measurement device to measure the absorbance of the solution. Because of an overlap difference between the spectra for acid and base forms, the molar absorptivity for each form will be measured at it’s own wavelength. The relationship between the two forms of the indicator (Bromocresol Green) in an aqueous solution is described by the equilibrium:

Hln + H2O   H3O⁺ + In⁻

For which
Ka =
So, if [ln-] and [HIn] are known the calculation of [H3O⁺] and therefore pH can be found. Background
We are given that bromecresol green (indicator) has a ka = 1.6 x 10⁻⁵. We are also told that there is an overlap between the specta for acid and base form and we measure those separately. We then find the wavelength of maximum absorbance for both the acid and the base. We measure the absorbance of the unknown pH at these two same wavelengths, the concentrations of these two forms of the indicator in the solution are solved by using two equations. (see data) Experimental

First we prepared our solutions. Bromocresol green, 40.0 mg indicator to 500.0 mL to (0.0402g) BG with water, dissolved into beaker a little at a time to diluted in 500 mL flask. Then we used 4.0 mL HCl 0.5M diluted with 100.0 mL of water. Next for the base 0.4 M NaOH - we diluted 7 mL of 6 M NaOH with 100 mL of water. Transferred 25.00-mL of the bromocresol green indicator solution to two 100-mL volumetric flasks. To one we added 25mL of 0.5 M HCl; the other we added 25mL of 0.4 M NaOH, then mixed well. Next we got the absorption spectra for the acid and the conjugate-base forms of the indicator between 400 nm and 600 nm, using water as a blank. We recorded absorbance values at 10-nm...
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