Hi fyliu,
It wasn't a complete sacrifice. I only titrate 15 ml of the 90ml juice I used for Brix measurements. I drank the rest
One advantage of using multi-fruit juice is that it averages out inhomogenieties in the distribution of sugar in a single fruit, and provides a good average reading of the tree as a whole. The UCR citrus collection does that for the brix and acid levels reported on it's website.
The wine kit came with solutions pre-mixed; They keep forever if tightly sealed. I'll run out of Sodium Hydroxide soon, though, but you can buy more either on-line, or at the local beer and winemaking store (Portland is big on microbrews, so such stores abound locally).
Or, you can simply buy some Roebic Crystal Drain opener -which is nearly 100% NaOh-and mix up your own 0.2N NaOh solution, if you're confident of your measuring skills and basic Chemistry. You need a good scale.
Simon, yes you can use a Ph meter to determine the acid level, with some kind of calibration or conversion factor which I don't remember. I found how to do so somewhere on-line when I was considering purchasing one, but am now too lazy to look again. I eventually decided to stick with what I've got- it works.
Perhaps a new kind of sourness App can be developed using the camera on your cell-phone to measure the shape of your mouth and the number of wrinkles around it as a kind of 'Puckerometer'
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