Help with Henderson Hasselbalch equation?

[Summerdreams]

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.

[hypervalent_iodine]

Without knowing what the growth medium consists of, I think it's impossible to say. Do you have to specify an exact amount? In my limited experience in making growth media, I've always adjusted the pH by slow addition of HCl or NaOH to a beaker containing a stirring solution of the media with a pH probe in until it gets to where I need. Is there any reason you can't do it this way?

[Summerdreams]

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.

[hypervalent_iodine]

Depending on the buffer in the media and the pH you need, using HCl or NaOH should be fine. I don't think you can use the HH in the case of adding another buffer to something that is already buffered, since I assume you don't know the buffer or the HA / A - ratios already present. Can you not just ask for a bit more than you think you'll need and go from there?