acid testing - 101winemaking.com free winemaking how to make wine for amateur winemakers

Retyped from Wine Analysis Bulletin, by Harold E Applegate, with permission from American Wine Society ,
http://www.americanwinesociety.org

Reagents: 0.1Normal (N/10) sodium hydroxide (NaOH)
1% phenolphthalein in 95% ethyl alcohol

1. Must or white wine: First clarify the must or wine by filtration through Whatman #1 filter paper or by centrifugation. The latter method is a fast and most efficient method for those who can afford a small clinical centrifuge. Add 5 ml of must or wine to 75 ml of boiling distilled water followed by 5 drops of phenolphthalein indicator. Agitate well and continuously. Titrate with N/10 NaOH to the faint pink endpoint stable for at least 1 minute.

2. Red Wine: Add 5 ml of red wine to 75 ml of boiling distilled water and with adding the indicator, titrate with N/10 NaOH until the solution is blue-green in color. Now add 5 drops of phenolphthalein indicator and complete the titration to the pink endpoint. The endpoint is very tricky and requires practice. One way to avoid the difficulty is to stir the red wine vigorously with a little bone charcoal and clarify as done initially. The resultant decolorized wine is then used as a white wine. Use the same equation that was used for white wine to calculate the result. (end of AWS excerpt)

For red wine (and white also) Tom will filter the juice or wine that he is taking the sample from through multiple layers of cheese-cloth, drawing his 5 ml sample with a pipet or a syringe. He uses a graduated beaker or a white interior container for the test. The use of boiled or hot distilled water drives off any CO₂ that may be in the wine/water. CO₂ present in the sample will give a false reading. CO₂ is less soluble in hot than cold and will go off.

The amount of water for the test is not important. 75 ml is fine for a white wine test, but for a red wine he uses 150 ml of water to dilute the color. Very dark red wine is almost impossible to do without a pH meter.

When titrating a red sample without a pH meter, Tom adds the 5 drops of phenolphthalein when his sample turns a tinge of green and holds for a while. The end point then is very near and is easier to see.

He uses the pH meter to determine the end point in red samples. If using a pH meter, color indicator is not even necessary. So he titrates and agitates the sample with the pH meter probe in it. The pH endpoint is 8.2. Isn't that wonderful? Plus we have a magnetic stirrer and a self-leveling buret that we picked up used from a lab. These tools definitely make the task easier. Tom and several of our winemakers use 8.2 as the end point when testing with use of the pH meter. 8.2 is the end point of the color indicator, phenolphthalein.

(I have in the past indicated the end point as 8.4, which was in text of Technology of Wine Making by Amerine, Berg and Cruess, AVI publishing Co. 1972. Our advance amateur winemakers, when asked, use 8.2, but don't know the original source of their information!)

To calculate the acid reading (tartaric) TA, multiply the amount of NaOH used by 0.15.

Acid test kit instructions that come with the titration kit:

The ideal total acidity of a finished wine ranges from about 0.6% to 0.8%, expressed as tartaric acid (TA).

A fresh juice should run 0.1 to 0.3% higher because some acidity is lost by the completion of the winemaking process (fermentation and cold stabilization).

This TA test will permit you to make prudent corrections with unbalanced musts or wines and to indirectly observe the progress of Malolactic fermentations as well as spoilage by vinegar production. Vinegar production is marked by an increase in acidity in the wine.

Equipment included in your kit: Buret with stand and rubber tubing, glass end and a clamp (pinchcock), 5 ml pipet, N/.10 sodium hydroxide, color indicator (phenolphthalein).

Also needed to run this test: distilled water, beaker or glass container, clear or white.

Procedure: For white wine or juice:

1. Fill the buret with sodium hydroxide. Run enough through the buret so that no air bubbles remain below the pinchcock or stopcock.

2. Using the pipet, add 5 ml sample of the wine or juice to test into the beaker or glass.

3. Add about 100 ml to distilled water to the wine or juice sample. The amount of distilled water used (more or less) will not change the result of the test. If using boiled tap water, please note: The pH of tap water may vary from pH7. If you use it instead of distilled water, you should use the same volume each time and run a blank test with just the water to see what correction you should make. The amount of NaOH used should be subtracted from the amount used in the test for the wine or juice. If the phenolphthalein turns the water pink before any NaOH is added, you won’t be able to use it.

4. Add about 5 drops of phenolphthalein indicator to the sample.

5. Place the wine-sample container under the buret. Record the sodium hydroxide (NaOH) level in the buret before starting. Begin running NaOH into the sample. This will cause it to turn pink. Upon stirring or swirling the sample, the pink color will disappear quickly. As you approach the end point the pink takes longer to fade. The end point is the first faint pink blush that won’t fade within 20 seconds.

6. Upon reaching the end point, record the level of NaOH in the buret. Subtract your first reading from this to determine the number of ml of NaOH. The milliliters of NaOH times (X) 0.15 equals the percent acidity expressed as tartaric acid. (Example: 4.8 ml X 0.15 = 0.72% acidity or TA)

Procedure for red wine or juice:

Follow steps 1, 2, and 3 as above for white wine.

Do NOT immediately add the phenolphthalein indicator as you would with a white wine or juice. The red pigments make it difficult to see the end point unless you first titrate the NaOH without the phenolphthalein until the color turns from red to a blue or green. Only then will you add the indicator and then continue to the first pink that persist. If the sample is deeply pigmented, you can dilute it with more distilled water, which does not affect the result of the test. A bright light under or behind the sample also helps.

Tom uses the pH meter to determine the end point in red samples. If using a pH meter, color indicator is not even necessary. So he titrates and agitates the sample with the pH meter probe in it. The endpoint is 8.2. Isn't that wonderful? Plus we have a magnetic stirrer and a self-leveling buret that we picked up used from a lab. These tools definitely make the task easier.