How to approximate pH of lemon juice and water

[dayhiker]

I have a science fair question and thought this would be the best place to ask a chemistry question. My sons topic (given to him, not selected) is on the food chain. We looked and looked on the net and about the best we could come up with was acid rain. We're going to test the effects of acid rain on plant growth. We're about a week late starting so I need to just get close on pH instead of saying it's exactly X. Lemon juice has a pH of 2-3. If we go 1 part lemon juice and 1 part distilled water, will the pH be in the 4.5 to 5 range? If so, that's close to acid rain. We could do two with pure lemon juice, two with a mix, and two with distilled water. Sooo, any thoughts on mixing the water and lemon without the aid of pH strips?

Thanks.

 

[PaulR]

As I understand it, pH is a logarithmic scale, so naively you'd need a 100:1 dilution to raise pH by 2.0. Hopefully somebody less naive about chemistry than I am will give you a better answer.
--paulr

 

[JasonLion]

If the water is very pure, the PH won't change much at all. This happens because lemon juice is buffered (ie contains chemicals that resist changes in PH). See http://www.plantphysiol.org/cgi/reprint/21/4/522.pdf for some discussion. This is well beyond high school chemistry, and so probably not a good thing to get involved in for your project.

You could just say that small amounts of lemon juice were used to simulate far larger amounts of acid rain over a much longer time period, and otherwise ignore the PH difference.

 

[dayhiker]

If the water is very pure, the PH won't change much at all. This happens because lemon juice is buffered (ie contains chemicals that resist changes in PH). See http://www.plantphysiol.org/cgi/reprint/21/4/522.pdf for some discussion. This is well beyond high school chemistry, and so probably not a good thing to get involved in for your project.

You could just say that small amounts of lemon juice were used to simulate far larger amounts of acid rain over a much longer time period, and otherwise ignore the PH difference.

So don't do the pure LJ option and go maybe 1:4 on LJ:water?