Determination
of Titratable Acidity
Equipment (can be purchased together in a titration kit from
winemaking supplier):
300 ml beaker or large baby food jar
5 ml volumetric pipette
25 ml buret calibrated by 1/10 milliliters.
1% phenophthalein indicator solution
0.1 Normal (0.1N or N/10) sodium hydroxide (NaOH) solution
Rectangular base and clamp or buret support
Glass rod for stirring
Procedure
1. Fill the buret with 0.1 N NaOH solution.
2. Place 100 to 200 ml of recently boiled distilled water in 300 ml beaker
or other suitable glass container.
3. Add about 3 drops of 1% phenophthalein indicator solution*.
4. Add in increments (titrate) 0.1N NaOH from the buret to the distilled
water until you see a faint pink color. Stir after each addition.
5. Once the faint pink color remains after stirring, note the level of the
NaOH in the buret and record this value (read the NaOH level in the buret at
the bottom of the meniscus) or refill the buret to 25ml.
6. Add 5 ml of room temperature juice or wine to the distilled water
using a 5 ml pipette.
7. Add in increments (titrate) 0.1N NaOH from the buret to the distilled
water, stirring after each addition, until you see a faint pink end point in
a white wine sample. A red wine sample will change several shades of green
before it reaches the gray or otherwise murky colored (non-green,
pinkish-brown) end point*.
Note the volume of 0.1N NaOH you used to reach the end point
(subtract the end reading on the buret scale from the beginning amount that
you recorded just before running the juice sample).
Calculation
Tartaric acid is the main acid in grapes so the total acidity is
expressed as percent tartaric acid.
The formula used to calculate % Total Acidity is as follows:
Tartaric acid, g/100ml = (V) (N) (75) (100) = % T.
A.
(1000) (v)
Where V = volume of sodium hydroxide used to titrate to the
endpoint
N = normality of the sodium hydroxide (0.1 is recommended)
v = the volume of the juice or wine sample (5 ml is recommended)
If you use 0.1N NaOH and use a 5 ml sample of juice or wine as
recommended, the formula is simply
%T.A. = (V)(0.15) or literally the volume of 0.1N NaOH used to titrate to
the end point multiplied by 0.15.
Juice for wine should have acid levels between 0.6 - 0.9 grams/100 ml
tartaric (0.6 - 0.9% Total Acidity)
* If you have a pH meter, you can set the end point to 8.2 and then
titrate to the pH of 8.2 instead of
using phenolphthalein indicator.
