titrating for acid

[Alexander Peckham]

I want to calculate the titratable acid (as Malic acid) in some
juice. I have 0.1M NaOH and am using a 25ml juice sample and
phenolphtalein indicator. I think The equation is:

Malic acid (g/l) = HaOH used (ml) * 0.268

is this correct?

Thanks. Alex

[Andrew Lea]

By my calculations, yes. Though I haven't had them checked by a grown-up
yet!

Andrew

[Alexander Peckham]

Thanks Andrew. I couldn't find this in your book but thought it was
there somewhere?

[Andrew Lea]

No I didn't put any specific titration details in the book because most
people buy proprietary titration kits which presumably come with
instructions. Malic acid is dibasic and has a molecular weight of 134.
Hence a 0.1N solution is 0.05M and contains 6.7 g/L. So each mL of 0.1N
NaOH is equivalent to 6.7g/l acid. If you titrate against 25 ml of
sample then the equation becomes 6.7/25 which is 0.268 as you say.

I know it is politically incorrect these days to refer to 'normality'
but I have always done it and for simple acid base titrations of this
sort it makes a lot of sense. Incidentally the use of phenolphthalein as
indicator gives an end point (AFAIR) of around pH 9; the actual
equivalence point is pH 8.1 (weak acid / strong base) which is what you
would titrate to with a pH meter. Phenolphthalein has been used for
around 100 years or so for this purpose and is cheap and cheerful and
good enough for me!

If you do this titration in cider, not juice, you need to boil off the
dissolved CO2 first.

Andrew

[Alexander Peckham]

Andrew, T

hank you for your thorough answer. It was very helpful to me and
will no doubt enhance the archive. I am a little embarrassed to ask
but how would you go about boiling off disolved CO2?

Alex

[Andrew Lea]

Alexander Peckham wrote:
> Andrew,
>
> Thank you for your thorough answer. It was very helpful to me and

> will no doubt enhance the archive. I am a little embarrassed to ask
> but how would you go about boiling off disolved CO2?
>
> Alex
>

It is as simple as it sounds, really. The cider is brought briefly to
the boil simply to expel the dissolved gas. Just shaking it *isn't*
sufficient. You can do it in a small pan on the stove or by pulsing it
in a microwave oven (but beware as if well saturated with gas it will
tend to erupt violently!). Then cool to room temperature before doing
the titration.

The reason for all this is partly because bubbles can interfere with the
pipetting, but more fundamentally because dissolved CO2 ('carbonic
acid') can noticeably increase the titre above that of the 'fixed acid'
(i.e. malic). However it is often forgotten about, even by those in
professional labs who should know better!!

Andrew

--
Wittenham Hill Cider Page
http://www.cider.org.uk

[Alexander Peckham]

I am still at the juice stage and am not using a kit so the above is
very helpful as my formal Chemistry was some time ago. I will boil as
suggested when I test the cider.

Thanks, Alex