Summerdreams

For a science experiment I am trying to measure the growth of algae grown in these test tubes. I have a spectrometer available and I have been told I can measure the growth of algae by using a spectrometer to help analyze the algae's density. However, I am unsure about the steps and what nm setting I should use. Can somebody please help?

I really need to be specific with the amount needed for my school's curriculum. I also was going to use HCL or NAOH but I'm worried that I would use too much and kill the algae. Also, I want to use the buffer solution so that it would stay at a specfic pH for the duration of my experiment. I can call the seller of the growth medium and finds its ingredients but I won't know until tomorrow. Lastly, do you think I could use the Henderson Hasselbach equation to answer my question or would I need another equation? Thanks.

My highschool science experiment is testing to see if there are differences in growth of algae if they are grown in growth mediums with different pH levels. The problem is that I am very confused on how to to calculate how many mL of phosphate buffer solutions I should use to change the growth medium to the desired pH. For example, How many mL of a .1 M phosphate buffer solution (pH 5) should I add to 15 mL of a growth medium (pH 7.8) to lower the growth medium's pH to the desired pH level of 5? I believe that I need the Henderson Hasselbalch but I am very confused with the math. Any help would be much appreciated. If math is shown then I will be able to calculate the mL of buffer solution needed for my other levels of pH.