CHEM 112-507 Spring 2014
Technical Abstract: Determination of Equilibrium Constant, Ka, and pKa Using Spectrophotometry
A useful measure in general chemistry is the pKa value; a pKa value reveals the acidity of given hydrogen atoms within molecules. In this experiment pKa of solution is determined from its measured absorbance.
Materials and Methods
To begin the experiment one must prepare the spectrophotometric samples. First, obtain a 150 mL beaker, a 250 mL beaker, four 50 mL beakers, a 5 mL serological pipet, four 1.0 cm path-length cuvettes. Once you have located these items, bring them back to the lab station and set them carefully out of the way. The other instrument needed is a Spectrovis-Plus machine, there are four located in the lab. Lab groups will rotate when they have finished the experiment. The last thing needed to perform this lab is pre filled burets, which are located on either side of the lab room. Now to begin, pour 50 mL of the sodium phosphate buffer solution with a pH of 6.84 into the 150 mL beaker. From here on out, the sodium phosphate buffer solution will be referred to as simply the buffer solution. Next, locate the indicator called bromothymol blue (0.04%) and add 20 drops to your 150 mL beaker. The solution should then appear green. Next, obtain a 5 mL serological pipet and thoroughly rinse it with the buffer solution, then discard the buffer solution into the 250 mL beaker. Now, use the pipet to distribute 5 mL of the buffer solution into three 50 mL beakers. Be sure that the 50 mL beakers have been cleaned are dried prior to this. Next, locate the three pre filled burets in the lab room. Find the buret labeled 1.0M HCl and add exactly 1.00 mL of HCl to just one of the three 50 mL beakers with buffer solution already in them. Mark this beaker with tape as “Yellow”. Next, find the buret labeled 1.0M NaOH and add 1.00 mL of NaOh to one of the two remaining 50 mL beakers. Mark this beaker with tape as “Blue”....