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

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
************************************************** **************************
Dave Breeden

"ab" > wrote in message
m...
> 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?

In a commercial environment, we habitually reuse the solution up to half a
dozen times.

Definitely reuasable. The H2O2 is quite reactive and loses it's "trapping
capacity" rather quickly ie........don't save for another day.

"ab" > wrote in message
m...
> 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