Introduction

Acid-base or pH indicators change color as the acidity or pH of the solution they are in changes. This allows them to be used to signal a change in the pH during a titration. Indicators themselves are weak acids. In this experiment we will use spectroscopic methods to determine the acid dissociation constant (K_a) (this is just an equilibrium constant) of the indicator bromcresol green (BCG).

The form of BCG that is present in strongly acidic solutions is designated by HIn. The form of BCG present in basic solutions is In^-. A solution of pH 4 or 5 will contain both forms of this indicator, HIn and In^-. The acid dissociation constant of the indicator, K_a, is independent of pH and is given by the expression:

K_a = 
 

Where:	HIn (aq) Û	H+(aq) + In- (aq)
	(yellow)	(blue)

H⁺ is the hydrogen ion concentration, and In^- and HIn are the concentrations of the basic and acidic forms of the indicator. The acid form the indicator HIn is yellow and the basic form of indicator In- is blue.

To determine K_a we must first measure the equilibrium concentrations of HIn and In^-. We will measure these equilibrium concentrations spectroscopically, but first we must consider the spectroscopic characteristics of HIn and In- . To do this we will take the absorption spectra of HIn and In- . We will measure how BCG absorbs as a function of wavelength (absorption spectra) in both strongly acidic and strongly basic solution. The acid solution will essentially contain only HIn, so the absorption spectra in acid solution will give the absorbance of only Hin

Likewise in basic solution, only In^- will be present so the absorption spectra in basic solution will show how the absorbance of In^- varies with wavelength.

We can relate the absorption to the molar concentration, C, by:
 

(1)

A = C ·

where is the extinction coefficient, which depends on the compound and the wavelength studied. e also varies greatly numerically with values of 0 for no absorbance at that wavelength to values of 10³ to 10⁶ for strong absorbances at that wavelength.

We will take 3 absorption spectra: 1 in acidic solution, 1 in basic solution and 1 in a buffer of pH of 4 or 5. The absorbance spectra in acid solution will allow us to calculate e at the maximum absorbance of . The wavelength for the maximum absorbance, l_max occurs at approximately 450 nm. The absorbance spectra in basic solution will allow us to calculate e for In^- at its maximum absorbancel ' _max . The l ' _max for In^- occurs at about 600 nm. The exact values for l _max and l ' _max will be determined from the absorption spectra. There is a complication: the basic form of the indicator In^- also absorbs at the wavelength where the acid form of the indicator HIn absorbs. In order to determine the correct contributions to the absorption spectra by both HIn and In^- we must calculate e for In- at l _max as well as at its own l ' _max . We also must look at the absorption of HIn at l ' _max .

To calculate the extinction coefficients we simply use equation (1), where the absorbances are obtained at l_max (about 450 nm) and l ' _max (about 600 nm) for both HIn and In- . Since in the acid solution only HIn is present, the molarity, C, for HIn is the concentration of the BCG initially in the solution. This is also true for the basic solution where only In- is present in the solution.

Once we have calculated the extinction coefficients at the maximum absorptions, we can consider a solution near the transition point (between yellow and blue) that has both HIn and In^- present. The strong acid and base solutions, of course, do contain both forms of the indicator. However, in strong acid solution, the is so small that it is insignificant. Since we now know the contribution of the e for both HIn and In- at the maximum absorptions, and we can get the absorption, A, off the spectrum, we can use these data to calculate the equilibrium concentrations of and . Now that we have the equilibrium concentrations we can calculate K_a of the indicator. Since we will have both HIn and In- present in the solution, we can not use equation (1). We must have an equation that takes into account that two absorbing species are present at both l _max and l ' _max.
 

(2)	A = In-+ HIn at l max	And
	A' = 'In-+ 'HInat l ' max.

We have a system of two equations and two unknown. A and A' are determined from the absorption spectra of BCG in the buffer at l_max and l ' _max respectively. The extinction coefficients _HIn and '_HIn were calculated using the data from the absorption spectra in acid solution and _In- and '_In- were calculated using the data from the absorption spectra in basic solution. We can use these two equations to calculate the concentrations of and .

Equipment
 

50 mL volumetric flasks	10 mL pipets
125 mL Erlenmeyer flasks	scanning UV-VIS spectrophotometer

Chemicals Student

Bromocresol Green solution (the concentration in mg/L will be on the flask),
 

0.4M sodium hydroxide solution,NaOH (aq)	0.4 M hydrochloric acid, HCl (aq)
pH 5.02 buffer solution	pH 4.00 buffer solution

Spill/Disposal

Mixture, NaOH & HCl : B1

Procedure

1. Preparation of the acidic and basic solutions of BCG:

Transfer about 30 mL of the BCG stock solution into a small Erlenmeyer flask. Clean a pipet; the last rinse should be with about 1 mL of the BCG solution. Pipet 10.00 mL of the BCG stock solution into a 50.00 mL volumetric flask. Add 10.0 mL (graduated cylinder is OK) of EITHER 0.4 M NaOH (aq) OR 0.4 M HCl (aq). ( Your instructor will assign you one or the other. The class will share the data obtained from the spectra.) Dilute to the mark with deionized water and mix. Rinse a clean Erlenmeyer flask with a few mLs of the solution you just prepared, and then transfer the solution to the Erlenmeyer. When you record the spectra for your solution, be sure to rinse the cuvette with the solution before filling the cuvette.

2. Preparation of BCG at pH 5.02 or 4.00:

In the following procedure, use the same rinsing techniques you used in preparing either the acid or base form of the BCG solution. Pipet 10.00 mL of the stock BCG solution into a 50.00 mL volumetric flask. Add 20.00 mL of EITHER pH 5.02 OR 4.00 buffer solution. Dilute to the mark with deionized water and transfer the solution to an Erlenmeyer flask. Dilution the buffer will not change the pH. Take an absorption spectra of the resulting solution. Each team will record TWO spectra, one from part (1) in strong acid or base and one from part (2) at pH of 5.02 or 4.00.

Disposal

All contentsof the flasks and cuvettes may be disposed of into the sink.