Measuring Total Acidity (TA) using a pH meter and 0.2N sodium hydroxide (NaOH) is a standard professional method in winemaking. This approach is far more accurate than using color indicators, especially with red wines where color changes are difficult to see.
Based on our old youtube video, here is a step-by-step guide to Measure TA with a pH meter..
pH Meter: Calibrated with 4.0 and 7.0 buffer solutions: (Youtube video: 00:30)
Titration Stand & Burette: To hold and dispense the NaOH (titrent).
Reagent: 0.2N (normality) Sodium Hydroxide (NaOH): (Youtube video: 00:24)
Sample: 15 mL of wine: (Youtube video: 03:14)
Diluent (optional): 20 mL of distilled water, note: this is optional if using a 50ml beaker your electrode should fully submerge in the 15ml of wine sample and not need addition dilution (Youtube video: 03:51).
Magnetic Stirrer (Optional): Highly recommended for constant mixing: (Youtube video:05:09)
Calibrate the Meter: Rinse the electrode with distilled water. Calibrate using the 4.01 buffer first, then the 7.0 buffer: (Youtube video: 01:37). Ensure you do not touch the glass bulb with your fingers: (Youtube video: 01:09)
Prepare the Burette: Ensure the stopcock is closed. Fill the burette with the 0.2N NaOH and flush a small amount through to remove any air bubbles: (Youtube video: 04:51)
Record Initial Volume: Note the starting level of NaOH in the burette (e.g., 4.6 mL): (Youtube video: 05:59)
Measure Wine: Use a pipette to draw exactly 15 mL of wine and place it in a 50 mL beaker: (Youtube video: 03:22)
Add Water (Optional): Add 20 mL of distilled water to the beaker. This dilutes the color and provides enough volume for the pH electrode to be fully submerged (Youtube video:04:13)
Set Up Stirrer: Drop in a magnetic stir bar and turn on the stirrer to create a gentle vortex: (Youtube video: 05:34)
Submerge the pH electrode into the wine sample, ensuring it clears the stir bar and the burette tip: (Youtube video: 06:37)
Slowly drip the 0.2N NaOH into the sample while watching the pH meter: (Youtube video: 06:50)
Slow down the drip rate as the pH approaches 7.0 to avoid overshooting: (Youtube video: 07:43)
Stop at pH 8.2. This is the recognized endpoint for TA in wine. (Youtube video: 08:59)
Key: By using 0.2N NaOH with a 15ml wine sample, the volume of NaOH used (in mL) converts directly to grams per liter (g/L) of tartaric acid. You can use other reagents (ie. 0.1N) but you'll need a different volume sample and have to use a formula to calculate results. By using 0.2N and 15ml sample you greatly simplify the process.
Calculation Example:
Starting Burette Reading: 4.6 mL
Ending Burette Reading: 11.1 mL
Total Used: 11.1 - 4.6 = 6.5 mL
Result: 6.5 g/L Total Acidity. (Youtube video: 09:19)
Why the math works:
To show why the volume of 0.2N NaOH used for a 15mL sample translates directly to g/L of Tartaric acid, we look at the relationship between equivalents and concentration.
The calculation for Total Acidity (expressed as Tartaric Acid) is based on the principle that at the titration endpoint (pH\ 8.2), the milliequivalents of the base (NaOH) equal the milliequivalents of the acid.
The general formula for TA in g/L is:
TA\ (g/L) = \frac{V_{base} \times N_{base} \times eq.wt_{acid}}{V_{sample}}
Where:
V_{base}: Volume of NaOH used (in mL).
N_{base}: Normality of the NaOH (0.2N in this guide).
eq.wt_{acid}: Equivalent weight of Tartaric Acid (75\ g/eq).
V_{sample}: Volume of the wine sample (15\text{ mL} in this guide).
When you plug the specific values from the Valley Vintner method into the formula, the constants cancel out: