The majority of this page and the basis for the image has been sourced from Iland et al (2000), however their method for the titration has been modified to reflect common practice.
pH measurement is complex and will not be discussed here. Determination of the titratable acidity uses sodium hydroxide (NaOH) to neutralise all the titratable protons in the wine. Because the acids in wine are weak, the end point is not at pH7.0, but rather at pH8.2. The calculation expresses the titratable acidity of wine in terms of g/L of tartaric acid. It should be noted that the French use an end point of pH7.0, and express the TA in terms of g/L of sulphuric acid. To convert g/L sulphuric to g/L tartaric, multiply by . There is no direct correlation between the end points, however a rough conversion of a pH7.0 measurement to pH8.2 can be achieved by adding approximately 0.65g/L expressed as tartaric acid.
The tests require a calibrated pH meter and a burette with 0.1mL graduations.
For care of the pH meter, see instructions below. To calibrate the pH meter, buffers of pH4.0 and pH6.88are most suitable. Buffers with pH7.0 are unstable. Many pH meters have auto-calibrate features, however these depend on standard buffer strengths, which may be pre-set. Check the manual before use.
Caution!: Before carrying out pH measurements or determining the TA, it may be necessary to degas the wine (almost certainly so for current vintage wines). This can be achieved either by gently heating the wine, or by using a vacuum pump or venturi. If the heating method is used, the wine must then be cooled to 15-20°C before any other measurements are made.
Calibrate the pH meter as described in Equipment above. Rinse the pH probe with distilled water. Add approx 20-30mL to a 50mL beaker. Insert the pH probe, and gently swirl the beaker until the pH measurement stabilises. Note the pH. Remove the probe and wash with distilled water.
There are two main methods for measuring the titratable acidity. The method described here uses a 10mL wine sample and 0.1M NaOH, however there is a method which uses a 25mL wine sample and 0.33M NaOH - using this method, the volume of NaOH corresponds directly to the titratable acidity.
Do not allow the probe to stand in wine for any length of time, as proteins can coat the pH-sensitive glass and electrode. Keep the probe stored in potassium chloride (KCl) solution, which will last approximately one month. If the probe is not to be used for more than a month or so, clean and dry the probe, storing it with its protective cap in place.
To clean the electrode- initially stand the electrode in 0.1M hydrochloric acid (HCl) overnight, then rinse well with distilled water and stand in pH4.0 buffer overnight. If this does not work, then a commercial enzymatic cleaner will be required. The electrode should be soaked overnight in this solution. Clean all the parts of the electrode (including plastic shields etc) in the same solutions.
Before reassembling, it is likely that the electrode will need to be refilled with concentrated KCl. This may also be supplied in the form of a gel.
To prepare 1L of solution. Using a commercial solution of strength 1.0M: pipette 100mL of this solution into a 1L volumetric flask. Make to volume with distilled water.
Using solid sodium hydroxide: weigh 4g of NaOH. Dissolve in a small quantity of distilled water, and transfer to a 1L volumetric flask. Make to volume with distilled water.
Either way, the solution should be standardised with 0.1M hydrochloric acid (HCl). Sodium hydroxide absorbs carbon dioxide from the air to form sodium carbonate, therefore where possible the solution should be protected from contact with the air. To standardise with a pH meter, titrate 25mL of 0.1M HCl) with the prepared sodium hydroxide solution to an end point of pH7.0. The actual molarity of the sodium hydroxide can then be calculated from:
M(NaOH) = M(HCl) x Vol(HCl)/Vol(NaOH)
Storage and stability: Burette at room temperature with lime trap, 3 weeks. Sealed plastic container, 2 months.