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THE PH METER
The strength of both acids and bases is measured by a yardstick known as pH, which is defined as the "common logarithm of the reciprocal of the hydrogen ion concentration" (don't worry about that, it won't be on the final exam; this mathematical legerdemain is required in order to yield a usable numeric range. Otherwise the number would have so many zeros as to be virtually incomprehensible). The value of pH ranges from 1 (highly caustic acid) to 14 (highly caustic base). Water - being neutral - lies in the middle with a pH of 7.0. The total amount of acid can also be measured as a percentage of the total volume of the must or wine, usually expressed in grams per litre (g/L), which is equal to parts per thousand, the measure used in industry. Total acid in this article means total titratable acid, or fixed acid.
We measure pH using a pH meter. These meters come in various configurations, but unless one is prepared to pay top dollar for a laboratory model, check at a Winemaking store or a scientific-supply firm for a vest-pocket styled model with digital readout. Typically, a red wine should show a pH of between 3.2 and 3.6, while a slightly lower value, representing a more acidic solution - between 3.0 and 3.2 - works well for whites.
You will need to calibrate a pH meter before each use; this is typically done by immersing the electrode in two solutions of known pH, one at pH 7.0 and the other at pH 4.0, and adjusting the reading on the meter until it reads accurately starting with the pH 7.0 standard. These solutions are made using distilled water and capsules of reagent, usually available from the same source as the meter. Since the standard solutions are not stable over time, they should be recreated each time the meter is put into use, unless the previous occasion was within several days. In addition, the adjusting mechanism on some meters is flimsy, for example, a countersunk screw made of soft metal that deforms after several uses, so the meter can become impossible to calibrate.
All pH meters should be stored with the electrode kept damp, preferably in a solution made for this purpose by the manufacturer. A reasonably-priced pH meter should be accurate to at least one decimal place, for example, able to distinguish 3.2 from 3.3. It doesn't need to be as good as proper lab equipment, but it should be perfectly adequate for us amateurs. Be careful to check the difference between accuracy and resolution, for example if the meter is accurate to +/=0.2 but displays resolution to 0.01 then the overall accuracy is 0.2
For rough measurements of pH, litmus paper is an alternative. This is a chemically-treated paper strip which changes colour when dipped in a wine, with the colour being an indication of the pH. One simply dips the paper in the wine, waits for a few seconds, then compares the ensuing colour with that on a chart, provided with the container. Matching up the colour with one on the chart indicates the pH with a reasonable approximation - but not the same as a pH meter. (thought by many to be useless in fact as they are too inaccurate)
It should be emphasised that litmus paper only gives approximate results, depending as it does on both the quality of the product and your own ability to discriminate slight differences in colour. However, as a fast means of determining changes in acidity during fermentation, and the direction in which the change is occurring, it is useful for a quick, rough-and-ready determination. For absolute accuracy, however, a pH meter is highly recommended when you are making the final determination.
The strength of both acids and bases is measured by a yardstick known as pH, which is defined as the "common logarithm of the reciprocal of the hydrogen ion concentration" (don't worry about that, it won't be on the final exam; this mathematical legerdemain is required in order to yield a usable numeric range. Otherwise the number would have so many zeros as to be virtually incomprehensible). The value of pH ranges from 1 (highly caustic acid) to 14 (highly caustic base). Water - being neutral - lies in the middle with a pH of 7.0. The total amount of acid can also be measured as a percentage of the total volume of the must or wine, usually expressed in grams per litre (g/L), which is equal to parts per thousand, the measure used in industry. Total acid in this article means total titratable acid, or fixed acid.
We measure pH using a pH meter. These meters come in various configurations, but unless one is prepared to pay top dollar for a laboratory model, check at a Winemaking store or a scientific-supply firm for a vest-pocket styled model with digital readout. Typically, a red wine should show a pH of between 3.2 and 3.6, while a slightly lower value, representing a more acidic solution - between 3.0 and 3.2 - works well for whites.
You will need to calibrate a pH meter before each use; this is typically done by immersing the electrode in two solutions of known pH, one at pH 7.0 and the other at pH 4.0, and adjusting the reading on the meter until it reads accurately starting with the pH 7.0 standard. These solutions are made using distilled water and capsules of reagent, usually available from the same source as the meter. Since the standard solutions are not stable over time, they should be recreated each time the meter is put into use, unless the previous occasion was within several days. In addition, the adjusting mechanism on some meters is flimsy, for example, a countersunk screw made of soft metal that deforms after several uses, so the meter can become impossible to calibrate.
All pH meters should be stored with the electrode kept damp, preferably in a solution made for this purpose by the manufacturer. A reasonably-priced pH meter should be accurate to at least one decimal place, for example, able to distinguish 3.2 from 3.3. It doesn't need to be as good as proper lab equipment, but it should be perfectly adequate for us amateurs. Be careful to check the difference between accuracy and resolution, for example if the meter is accurate to +/=0.2 but displays resolution to 0.01 then the overall accuracy is 0.2
For rough measurements of pH, litmus paper is an alternative. This is a chemically-treated paper strip which changes colour when dipped in a wine, with the colour being an indication of the pH. One simply dips the paper in the wine, waits for a few seconds, then compares the ensuing colour with that on a chart, provided with the container. Matching up the colour with one on the chart indicates the pH with a reasonable approximation - but not the same as a pH meter. (thought by many to be useless in fact as they are too inaccurate)
It should be emphasised that litmus paper only gives approximate results, depending as it does on both the quality of the product and your own ability to discriminate slight differences in colour. However, as a fast means of determining changes in acidity during fermentation, and the direction in which the change is occurring, it is useful for a quick, rough-and-ready determination. For absolute accuracy, however, a pH meter is highly recommended when you are making the final determination.
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