Acid Titration Procedure for Ciders, Apple Juices and Cider Vinegar
You will need
- 0.1 M Sodium Hydroxide (NaOH) solution [0.1M potassium hydroxide (KOH) can also be used but is less usual]* #
- Phenolphthalein indicator in alcohol*
both items closely stoppered in a cool dark place.
- Various size syringes with volume markings (10 ml, 1 ml) and / or calibrated pipettes and a burette
- Glass flasks or Sterilin Pots of 50 - 100 ml capacity.
- Flexible bulb pipette
- Distilled water (optional).
Procedure (for juices and
If measuring a cider, first degas the sample by heating in a microwave oven for about 15-20 seconds until steaming hot, not boiling. Beware if the cider has a lot of gas in it it might fizz quite violently! Swirl vigorously and then cool to room temperature.
- Dispense 6.7 ml of sample
into a clean Sterilin pot. The easiest
way of doing this is to draw up an excess of the liquid into a 10 ml syringe, then
lining up the back of the plunger with the 10 ml mark. Dispense the contents into the pot until the
back of the plunger reads 3.3 ml. This
will give an exact 6.7 ml of liquid in the pot.. A calibrated pipette
can more simply be used for this, if available. (If distilled
water is available, the accurately dispensed sample can be diluted to a
larger volume eg 15 - 20 ml to make handling and observation
easier. But do not use tap water for dilution, since it will affect the result)
- Add two or three drops of indicator solution to
the pot, using the flexible bulb pipette.
- In a fresh clean and dry 10 ml syringe, take up 10 ml of 0.1M NaOH
solution, lining up the back of the plunger with the 10 ml mark as before. A burette can be used for this stage (steps 4-6).
- Slowly dispense NaOH
dropwise from this syringe in 0.5 ml steps into the pot, swirling after each
addition. Stop when the sample has turned a permanent pink. and remains so for at least 10 seconds
- Read the figure shown at the
back of the plunger, and subtract it from 10 to get the total volume
delivered. This volume is X ml (e.g. 10
- 4.5 = 5.5)
- The figure X represents the acid level in grams per litre of malic acid (e.g. 5.5). Divide X by ten to get the
acid in percent (e.g. 5.5 / 10 =
0.55% malic acid)
the same procedure, but using a 1 ml syringe in step 1 and
measuring just 0.60 ml of vinegar. Or dilute the vinegar exactly ten
times with distilled water before measurement and use 6 ml of diluted
- Omit step 7. The volume dispensed will represent the acid in %
acetic acid directly.
Should be 5% or greater.
out all items carefully with clean water (the syringes can be dismantled). Blot or shake dry if possible (tap water is
alkaline and may lead to inaccuracies if allowed to dry on to the equipment). Distilled water is best.
calculation formula for cider and juice is
% malic acid =
(Volume of 0.1 M NaOH) x 10 x 0.067 / (Volume of sample)
using an exact 6.7 ml of sample, the calculation can be avoided because
everything is in tens! If a different
volume of juice or cider is used, the formula will be needed.
alkali and indicator can be bought on eBay. You can also buy stronger
sodium hydroxide solutions eg 1M and dilute them accurately with
distilled or deionised water to the correct working strength. If you
want to make up your
own NaOH solution from solid pellets, buy the purest grade you can
(laboratory or analytical, not technical). Beware that the pellets are
hygroscopic and also absorb CO2 from the air, so they must be kept
tightly closed in a dry place or they will lose their strength.Dissolve
exactly 4 grams of NaOH (or 5.61 g of KOH) in 500ml of distilled
water and make up to exactly 1 litre to make an 0.1M solution. Beware
the solution will get hot! NaOH solid and solutions are very
corrosive and can cause irreparable damage to the eyes through
splashes. Wear a mask and goggles when you do this!
0.1M solution is not stable and will absorb carbon dioxide from the air
and will lose its strength, so the titration will become inaccurate. It
should be replaced with fresh solution at least annually, preferably
It is possible to use a calibrated pH meter to replace the phenolphthalein indicator. In this case, the end point is pH 8.2
#Note on nomenclature0.1M
is the correct chemical way to express things. M stands for 'molar' or
'one gram mole per litre'. In old fashoned literature, the term 0.1N
is often used, where N stands for 'normal'. It so happens that
for monobasic alkalis like sodium hydroxide, 0.1M and 0.1N are
just the same thing. So you can use them interchangeably here. But in a
lot of cases you can't. Beware.
Last modified by Andrew Lea December 2013.
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