acids and bases

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acids and bases

Postby lolie5 » Thu May 09, 2013 1:53 pm

My daughter wanted to set up an experiment to test her hypothesis that it would require more of a weaker acid to neutralize a set amount of a base (in this case baking soda). We used plain vinegar, cider vinegar and lemon juice. We had pH test strips with a range of 6.2 to 9.0. Unfortunately we don't have any way to determine which of the 3 acids is strongest. I can't find test strips in the appropriate range, and we don't have access to a pH meter. In some ways, what she is trying to do is similar to the project, "Measuring the amount of acid in vinegar by titration" which is posted on this site. However, we don't have access to the chemicals and equipment required for that, and I also think it is beyond her age level (5th grade). Do you have any thoughts on the best way to proceed with this? She could modify her hypothesis if need be. It can't be in the form of a question to investigate, however. She must have an actual hypothesis.
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Project Question: relative strength of common (weak) acids
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Re: acids and bases

Postby JasonS » Fri May 10, 2013 10:16 am

Hello lolie5,

Fun experiment. Yeah it is difficult to find pH strips in the lower acid range. Mainly because I believe that they target the consumer pool testing market (~pH7). But I did find a couple that you can order from Grainger (links below) which claim to be able to test down to 0 pH!

http://www.grainger.com/Grainger/MICRO- ... Pid=search
http://www.grainger.com/Grainger/MICRO- ... Pid=search

Another strategy is with some research find out the expected pH's of your substances. For example lemon juice should be around pH ~2. The vinegars will have a higher pH. Knowing that lemon juice is more acidic than vinegar you can proceed with testing your hypotheses.
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Re: acids and bases

Postby Craig_Bridge » Wed Jun 05, 2013 3:39 pm

An inexpensive pool test kit with Phenol Red will be yellow at pH < 6.8 and red at pH > 7.4. This makes it a great indicator for detecting when a titration reaches pH near 7.0, e.g. neutral. The least expensive pool test kits do two tests: free chlorine and pH and come with two reagents, one is phenol red for the pH test. Best to buy the kit from a reputable local pool store, one that won't sell you a year old kit as the reagents shelf life is affected by heat and light.

You can do 1:10, 1:100, 1:1000, and 1:10000 dillutions of your acid with distilled water to measure lower pH values using a drop of Phenol Red in the pool test kit and compare the color with the yellow pH color comparisons. pH is a log10 scale so a 1:10 dillution of an acid will measure 1.0 greater than the original solution, 1:100 will measure 2.0 greater, 1:1000 3.0 greater, 1:10000 4.0 greater. This will allow you to fairly accurately measure strong acid pH's below the point where phenol red turns bright yellow.

When dilluting acids, always add the acid to water. With very strong acids, always wear face protection, gloves, and wate/acid proof apron. BEWARE: the dilution process is exothermic - e.g. solution will get VERY HOT when using very strong acids so be sure that the container is securely held by something that can withstand the heat.

Pure lemon juice is around pH 2 which is about the lowest pH that is appropriate for experimentation with in the K-5 grade level. I would expect 1:10 and 1:100 dilutions of lemon juice to be measureable using phenol red, just subtract 1.0 for a 1:10 and subtract 2.0 for a 1:100 dillution to know what the original solution's pH was.

BEWARE: Acid base reactions can be violent. Baking soda acid reactions are going to give off a lot of gas products and they can be VERY messy. These reactions are often use to propel toy boats and rockets.
-Craig
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