Diatom Science Fair Project - i need help! (Oct/26/2005 )
What are your expected results? I have a good idea of what you should see, but I don't want to give it away to you and skew your experiment...
i don't know what results to expect... but i do know that my teachers don't want me "guessing" and just adding acid/base until i get the desired pH. i do know the molarity of the chemicals we have, but i will have to post it later (don't have it right here).
That may be a little more complex than you may think. I looked up on the Carolina web site and the media is indeed buffered initially to a pH of 7.8 I am not sure how you would calculate exactly how much acid or base you need in order to overcome the buffer when they do not mention (that I saw) of what the buffer consisted. Anyone have suggestions?
I don't know why your teachers would be unsatisfied with adjusting the pH of a buffered solution a drop at a time...that is how us 'real scientists' do it in the lab all the time.
You have to consider the impact on volume and whether that affects your experiment...but it seems that you are mostly looking for morphological changes? I would make, oh, say a 1:5 dilution of your concentrated acid and base and add a small drop at a time. the initial changes will take many more drops than the later changes, depending on the range of the buffer in the media. but, the thing is to be patient...add a drop, stir for a moment, take a pH. I don't see why this would not be considered valid by your teachers????
I don't think that I have calculated the exact # moles needed to make a pH adjustment since way back in college chem...anyone else do it that way?
Well I guess I will have to do it by guessing then. Any ideas where I should start? The acetic acid we have is 17.4 molar and the sodium hydroxide...I'm not really sure. A teacher told me to make a .01 molar solution using 250 mL water and .1 grams NaOH. She also told me to make a .01 molar solution of acetic acid. Do those sound too weak? Thanks.
wow, a 0.01M solution will likely take a very large number of drops to affect any change in pH
I would recommend a higher dilution
you could do a preliminary experiment if you have some excess media on hand. make a small dilution of your acid and your base (1:2 or 1:5). add a few tiny drops to a small amount of media and see what happens to the pH. If you overshoot too much, dilute 1:10 and try again.
for example, if you have a 1:5 dilution of your acetic acid, and you have to use 3 drops to get 5 mL of media down 0.5 pH points, then you can guess that it will probably take about 12 drops to get 20 mL of media to your desired pH...do you see how you could set this up?
and please note that I pulled these numbers out of the air and do not expect it to be exact! I was just offering an example...
If the teacher that told you to make those concentrations is the one giving you the grade you should probably do what they ask. Whether the solutions will be too weak or not depends on the strength of the buffer in the media. Maybe make an initial concentration 10x what she said, dilute that 1 part in 9 parts water to get the concentration she wants. That way if it is too weak you still have your higher concentration that you can use. Give it a shot and see if her suggestion works, it just might.
Aimikins, I am with you, I don't think I've calculated the molarity since college. I just use the drop method as it is easier, faster, and just plain makes sense! 
Well I made some dilutions.... I will let you know how things turn out!
Good luck, Corin! Let us know how it goes!
Yes, for pHing things your solutions are a bit weak. It means you will have to put a lot in. I would use 1 M at least, and maybe just some 0.1 M strength acid and base to get the last few decimal pH points in.
Corin I am envious of you doing stuff like this in high school. In the UK most people only get to do research in the final year of their undergraduate degree! (I cut to the chase, spending my holidays in research labs instead!)