[agivente]

So for my final lab in general chemistry, my assignment is to measure the turbidity of potassium in a sample of seawater by creating a calibration curve from data obtained by a spectrometer. I am given the following chemicals to work with: potassium bromide, 1.0 M NaOH, and .005 M tetraphenylborate with a pH of 8.

I plan on making a stock solution of 500 mg potassium/L by dissolving .308 g of potassium bromide in 250 mL of water. Then, I will make seven different standard solutions by transferring 1, 2.5, 3.5, 5, 7.5, 10, and 12.5 mL of the 500 mg/L solution into a seven different 25 mL flask and diluting them to the mark with water.

My question is, how do I know how much NaOH and tetraphenylborate to add the the potassium solutions and the sea water sample before measuring absorbance? Where does pH come into play? I know that potassium and tetraphenylborate forms a precipitate when dissolved in water, but what is the purpose of NaOH?