Joe - Actually, it sounds crazy because it is! It's a commonly held (even in some wine books) misconception that adding water doesn't affect the titration, it does. The good news is the error is fairly small and so usually doesn't critically affect results. The reason: the "end point" is taken by convention to be the pH at which phenolpthalein changes color, namely pH 8.2. If we titrated to neutrality, pH 7.0, what you say would be true. But think about it: if you titrated the water (let assume it's neutral, pH 7.0), you'd need to add some base (usually sodium hydroxide) to get it to pH 8.2. So how can you add water without affecting things? In fact, the more water you add without accounting for this, the more error you introduce. In fact, this tells you the correct technique to avoid the error: first titrate the water you're adding to the phenolpthalein end point, *then* add it to the wine and start your regular titration. As I said, if the endpoint of titration were pH 7.0, everything you've told Ron would be true - but it's not.
So it's not really correct to say adding water doesn't affect the results of a titration. The good news is that the error introduced isn't large unless a lot of water is used. pH is a logarithmic scale, so during titration it takes a lot of sodium hydroxide to get you from pH 3 to pH 4, about nine times less from pH 4 to 5, still less from pH
5 to 6 and so forth. Another way of looking at it: the pH swings rapidly near the end point. So the error introduced by ignoring the effects of the water is often not enough to screw up a titration, especially given the error inherent in the procedure.I hope I haven't confused you (or Ron!). As I've said, you can go on ignoring what the water does and it's very unlikely to cause a problem in your winemaking. Technically, though, it's not quite right: the results will be slightly different with and without the water.
- Mark W.