CHM3120L

Introduction to Analytical Chemistry: Laboratory

 
 Spectrophotometric Determination of pH : Procedure

Bromocresol green.

• Dissolve 40.0 mg (to the nearest 0.1 mg) of the sodium salt of bromocresol green (720 g/mol) in water and dilute to 500 mL in a volumetric flask.

HCl, 0.5 M.

• Dilute about 4 mL of concentrated HCl to approximately 100 mL with water. (You can prepare this and the NaOH solution in a graduated cylinder, since you do not need to know the exact concentrations. Why do you not need the exact concentrations?)

NaOH, 0.4 M.

• Dilute about 7 mL of 6 M NaOH to about 100 mL with water.

Determination of Individual Absorption Spectra

1. Transfer 25.00-mL aliquots of the bromocresol green indicator solution to two 100-mL volumetric flasks.
2. To one, add 25 mL of 0.5 M HCl; to the other, add 25 mL of 0.4 M NaOH. Dilute to the mark with water and mix well.
3. Obtain the absorption spectra for the acid and conjugate-base forms of the indicator between 400 and 650 nm, using water as a blank.
4. Record absorbance values at 10-nm intervals routinely and as needed to define maxima and minima.
5. Evaluate the molar absorptivity for HIn and In- at wavelengths corresponding to their absorption maxima.

Determination of pH of an Unknown Solution

1. Transfer 25.00 mL of the stock bromocresol green indicator to the 100-mL volumetric flask containing 50.0 mL of the unknown solution, dilute to the mark with water, and mix well.
2. Measure the absorbance of the diluted solution at the wavelengths for which absorptivity data were calculated.