The Art of Titration
CiderHead
titratiion. It's a simple question, but one that may be hard to answer
in writing. The question is this : how pink is pink?
The instructions on my test kit say that it should become a faint
permanent pink. However, I find that the solution gets a bit darker at
one point but not what I would call pink. The "pink" stage, as I see
it, is much further on, possibly even as far at 1.5% away from the
first slight change in colour. Consequently, I'm racked with doubt as
to whether I have an accurate measurement.
Any ideas anyone?
Cheers,
Martin
Claude Jolicoeur
> I have a question for those members who are experienced at acid
> titratiion. It's a simple question, but one that may be hard to answer
> in writing. The question is this : how pink is pink?
Sometimes, it may seem that it is turned pink, but by shaking well,
the colour disappears.
You may also add more phenolphtalein to obtain a redder colour once it
turns.
Claude
Jim K
point at which the color did not completely disappear after shaking.
Jim
Andrew Lea
>
> The instructions on my test kit say that it should become a faint
> permanent pink. However, I find that the solution gets a bit darker at
> one point but not what I would call pink. The "pink" stage, as I see
> it, is much further on, possibly even as far at 1.5% away from the
> first slight change in colour. Consequently, I'm racked with doubt as
> to whether I have an accurate measurement.
>
> Any ideas anyone?
If you are using phenolphthalein as indicator then 'permanent pink' is
the aim but may be obscured if the juice is already quite coloured. It
is normal to dilute out the pipetted juice with boiled distilled water
to make the colour change easier to see. This dilution does not affect
the final result; if anything it makes it easier to do. If you are are
doing a titration on fermenting or freshly fermented cider remember to
boil out the CO2 first or you will get erroneous high results.
Andrew
Andrew
--
Wittenham Hill Cider Pages
www.cider.org.uk
David Llewellyn
freshly fermented cider remember to boil out the CO2 first or you will get
How significant is the error likely to be? And could you just vigourously
shake the sample before titrating? Would this also apply to wine?
David L.
Andrew
Andrew
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Andrew Lea
> Andrew wrote: "......If you are are doing a titration on fermenting or
> freshly fermented cider remember to boil out the CO2 first or you will get
> erroneous high results....."
>
> How significant is the error likely to be? And could you just vigourously
> shake the sample before titrating? Would this also apply to wine?
Big enough to have registered an ISO 17025 non-compliance and a formal
customer complaint from a major cider company when I got it wrong! (Or
rather, the technician doing it got it wrong, but I was the manager
responsible!). Could be 0.1 - 0.2%; easily enough to push a carbonated
product out of spec.
Yes it applies to wine too. See Zoecklein's Wine Analysis and Production
which recommends holding 10 ml sample in a water bath at 60C for
'several minutes'. In my lab days we did not regard this or
ultrasonication as enough, boiling is safer. The technician in my
example did actually try to decarbonate by vigorous shaking which he
thought was sufficient but isn't.
David Llewellyn
product out of spec...."
When you say that Andrew, do you mean 1 - 2 g/l tartaric equivalent?
David L.
-----Original Message-----
From: cider-w...@googlegroups.com
[mailto:cider-w...@googlegroups.com] On Behalf Of Andrew Lea
Sent: 18 October 2010 16:47
To: cider-w...@googlegroups.com
Subject: Re: [Cider Workshop] The Art of Titration
Andrew
--
Andrew Lea
order. Might not matter too much in a home or hobby situation but if you
are working to a commercial spec it could cause problems as it did for
me. Anyway it's best to do it right, whatever.
There is much more to accurate TA titration than meets the eye if you
want to do a really pukka job to AOAC or ISO standards. For instance use
of fresh boiled cooled distilled water for dilutions to keep out CO2.
Then a pre-neutralisation step with a couple of ml of wine in 100 ml of
prepared water to be sure the dilution buffer is already at the
neutrality point (which is actually closer to pH9 than pH7), then add
the 5 ml test sample and re-titrate. This also helps to avoid the
drifting end point that Claude described which is due to more aerial CO2
dissolving over time. And of course the sodium hydroxide solution is not
stable for the same reason. When I was at Long Ashton we used
'soda-lime' tubes as protectors for the NaOH and it was standardised
frequently.
Good wine labs do all that stuff as routine. For home use you are
unlikely to need such accuracy /reproducibility.
Andrew
John Mott
> the aim but may be obscured if the juice is already quite coloured. It
> is normal to dilute out the pipetted juice with boiled distilled water
> to make the colour change easier to see. This dilution does not affect
> the final result; if anything it makes it easier to do. If you are are
> doing a titration on fermenting or freshly fermented cider remember to
> boil out the CO2 first or you will get erroneous high results.
that the addition of neutral water does not change the acidity of the
solution? Therefore, no adjustment required to the test results? Is
there a limit to how much water can be added?
John Mott
Andrew Lea
Yes. I do not know exactly how you are doing your titration, but
standard lab practice would be to dilute an accurate 5 ml of juice or
cider to 100 ml (ideally with freshly boiled distilled or deionised
water - certainly not tap water). Then add a few drops of phenophthalein
solution as indicator and titrate against standard NaOH solution (often
given as 0.0667N for wines because it makes the final calculation as
tartaric acid easier). This may not be quite the same as the 'kit'
procedure.
The reason dilution does not affect the result is because you are
neutralising *all* the acid groups and it does not matter what volume of
neutral water they are spread out in. You must of course pipette your
wine accurately in the first place though. I think one of the problems
with kits is maybe that they omit the dilution step for simplicity
(otherwise people need a supply of distilled water) but it actually
makes the job harder to do.
Wilf
Looking for some help regarding titration please. I'm tying myself up
in knots,
Like somebody else that that posted somewhere I've got the winemaster
kit with uses sulphuric acid which needs to be multiplied by 1.4 to
converst to malic acid (OK so far I hope).
As usual I made things more complicated that is probably necessary
however I didn't have much acid so I decided to used 2.5ml of juice
rather than the 5ml recomended. I diluted with 2.5ml boiled water and
proceeded as normal.
One of the readings for example is 3.5ml of sulphuric which equatest
to 3.5 ppt or 0.3%
The bit that I'm not sure about is whether I should be doubling the
0.3 before converting to malic becuase I diluted it or not
i.e. 0.3 *2 =0.6%
convert to malic 0.6* 1.4= 0.84% malic
or if the dilution of the juice doesn't affect the acid content is it
just 0.3*1.4??
I'm going around in circles.
Any help gratefully recieved.
Wilf
Andrew Lea
You have 2 confusions.
1. You are not adding sulphuric acid at any point. You are trying to
measure acid and so it is alkali (sodium hydroxide) that you are adding.
The result is expressed as 'the amount of alkali needed to neutralise so
much acid, expressed as sulphuric'.
2. You have only used half the amount of sample (i.e. acid) that the kit
is designed for. Hence only half the amount of alkali will have been
needed. So you need to multiply your result by 2 to get it back to what
it would have been for the full 5 ml. Then multiply by 1.4 to convert
sulphuric 'units' to malic.
Andrew
Wilf
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