Science Buddies: "Ask an Expert"

My son is doing his science fiar project on coffee acidity.

He has selected one type of coffee bean and has roasted it at differnt times

Green Coffee- not roasted
Yellow Coffee- 4 mins
1st Pop- 9 mins
2nd Pop- 13 mins

The next step is to grind and brew each of the coffee samples and measure the acidity level.

My question is in regards to PH test strips. The acidity of these may all be very similar since it is still all coffee. Where can I get PH test strips that are specific enough to show the difference in acidity levels?

tina!123B - I'm probably not the best expert on this topic, but I understand your problem. The Science Buddies references on pH experiments notes that black coffee has a pH of around 5. The variations that you need to measure may be very slight and difficult to test with universal paper pH strips. They seem to have a color resolution of about 1 pH. Things like swimming pool test kits don't indicate in this range. Specialized pH papers will have resolution to 0.1 pH, but probably not available except from laboratory supply firms. I noted that pet stores may have test strips for aquarium owners that might cover this range. The best solution might be an inexpensive electronic pH tester. I found several on eBay for less than $12 including shipment. At least these would be good indicators with resolution of 0.1 pH. Perhaps a pet store might carry something like this. Good luck.

Rick Marz

Long before pH meters were available, chemists used titration methods and typically used phenolphthalein as an indicator (clear in acid, pink in base)

pH test kits for pools are also very accurate around neutral (pH = 7).

With titration, you would create a known dilution of your sample (so that you can see the indicator color change instead of being hidden by the coffee color), add the indicator, then add small amounts of a known base solution until the indicator changes color. By knowing the dilution of your sample and the amount and pH of the base you added, you can calculate the pH of the sample.

Here is one explaination of how chemists do titrations to determin acidity:
http://www.digipac.ca/chemical/mtom/con ... ncurve.htm