Method and Equipment

Equipment and Materials:

UV/Visible spectrophotometer

Quartz cell

Volumetric Flasks (25 and 250mL)

Automatic pipette

Deionized water

KCl

KH4 phalate

HCl

NaOH

Acetic acid

NaH2PO4

Na2HPO4

Bromophenol Blue

Bromothymol Blue

Bromocresol Purple

Methyl Red

Methyl Yellow

Methyl Orange

Method:

                To begin this experiment many solutions need to be made. First, create 0.2 M solutions of KCl, KH4 phalate, HCl, NaOH, acetic acid, NaH2PO4 and Na2HPO4. Then create stock solutions of the unknowns which in this case were Bromophenol blue, Bromothymol blue, Bromocresol Purple, Methyl Red, Methyl Yellow and Methyl Orange. To create the Bromophenol blue, the Bromocresol Purple stock solutions dissolve 0.1 g of the indicator in 0.75 mL of 0.2 M NaOH then dilute with deionized water to 250 mL. Then dissolve 0.1 g of Bromothymol blue in 0.8 mL of 0.2 M NaOH and dilute with deionized water to 250 mL to obtain the Bromothymol blue stock solution. To obtain the Methyl Red and Methyl Orange stock solutions dissolve 0.25 g of the indicator in 250 mL of deionized water. To make the Methyl Yellow stock solution dissolve 0.1 g in 250 mL of Ethanol. Filter each acid-base indicator stock solutions to ensure no solid remained. Next, make multiple buffer solutions for each indicator within their pH range. Bromophenol blue has a pH range of 3.60-4.80, Bromothymol has a pH range of 6.20-7.40, Bromocresol Purple has a pH range of 5.40-6.60, Methyl Red has a pH range of 4.60-5.80, Methyl Orange has a pH range of 3.20-4.40 and Methyl Yellow has a pH range of 2.80-4.20.

           Once the solutions are made analyze them using a UV/visible spectrophotometer. The lamps on the spectrophotometer must be turned on prior to analysis. To begin analysis, record a blank run by putting deionized water in the cell. Then run each buffer solution. Clean out the cell with the desired solution after each run. Before placing the cell into the cell holder clean the cell walls with a kimwipe to ensure no solution remains on the outside of the wall. Record the spectrum. Repeat this process with each buffer solution for each unknown indicator.  Calculations can then be performed to obtain the dissociation constant of the unknown.