My woes with testing TA

Yesterday I was testing pH and TA on a must and, AGAIN, I got low pH
(3.35) and low TA (0.4%). This had happened to me previously with
another batch. I know that my digital pH meter was OK because I
calibrated it, so I decided to take a look at the NaOH. This came from
a kit that I bought at the local winemaking supply shop. You know, one
of those boxes for $7 with a syringe, a cup, a small bottle of
phenoftalein, and a bottle of 0.2N NaOH. Except that instead of the
phenolftalein I use the pH meter to titrate to a pH of 8.2, and instead
of the syringe I use a pipette.

I know that NaOH gets weaker over time due to contact with air, but
that would mean that my measurements would likely be artificially high
instead of too low. So I decided to do an experiment. I dissolved 5 g.
of tartaric acid in 1 l. of water. I figured that my TA test should say
0.5%. I do the test and I read a TA of 0.35%. I do it again, this time
with the color indicator instead of the pH meter, and I get the same
result. Then I decided to try another bottle of 0.2N NaOH that I bought
at the same store, and this one didn't do anything! It might as well
have been water in the bottle.

So one bottle of sodium hydroxide gave me a value that was way too low,
and the other was useless. Has this happened to other people? Where can
I buy NaOH that is of good quality, and that has a date on the bottle?
I'd rather buy 1N instead of 0.2N so that it will keep better.

Thanks in advance for any suggestions.

Franco

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