ab ) wrote:
>Hi,

>When doing determinations using a rankin, you add mixed indicator to
>the Hydrogen peroxide in the pear shaped flask. You then titrate until
>an end point is reached (ie the colour changes). You then perform the
>rest of the procedure and re-titrate back to the same end point to
>work out your SO2 levels.

>My question is once you have completed your determination, if wish to
>perform a second determination, do you need to change the indicator /
>hydrogen peroxide / NaOH solution in the pear flask?

>I wonder because you are starting from the same point (ie having
>titrated until an end point is found)...

>Any info would be appreciated

>AB1

I dunno for sure, but I've wondered that same thing about doing
TA's. Why NOT re-use the same solution time after time? You're
starting at pH 8.2 anyway.

For SO2 measurements, though, I definitely throw out the solution
and start over. I can't defend that decision, but SO2 measurments
are so important that I don't want to take any chances.

I'll be interested to hear what other people say.

Dave
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Dave Breeden