When my starsan goes cloudy too soon after mixing, I just add a bit of 85% phosphoric acid to drop the pH through the floor again.
We have soft(ish) tap water, very suitable for most brewing water, as is, with a proper Campden treatment to remove Chlorine. Although it has very little mineral content, my StarSan starts to get cloudy after a day or so, too. The pH remains well under 3.0 (around 2.7) so there's not always need to add acid simply because it gets cloudy.
IIRC, a fresh Starsan working solution in RO or distilled water has a pH of 2.50 or thereabout. I need to remeasure that or find my notes.
85% Phosphoric acid is crazy strong for routine use. I'd make a 10% strength dilution with some and use that. Much easier to measure 3.5 ml than .3 ml, and with a far smaller measuring error.
@Silver_Is_Money posted a thread with calculations to prepare Phosphoric Acid dilutions:
https://www.homebrewtalk.com/thread...ting-from-85-75-or-30-phosphoric-acid.674372/I've used his thread and calculations to make proper Phosphoric Acid dilutions that are much easier to work with than the 85% stock.
These measurements are from my own notes:
- Measure out 18.34 ml of 85% Phosphoric Acid.
- Or 30.97 gram. It's usually easier and more precise for homebrewers to weigh small amounts on a $15 precision scale, like the one used for brewing salts, than measuring small volumes accurately.
- Add the acid slowly (and quantitatively*) to ~100 ml distilled water (at 20°C/68F), while stirring. Stir well to homogenize.
- Top up to 250 ml with more distilled water, while slowly mixing/stirring.
- Using a volumetric flask will get you better accuracy than a wide beaker. Or weigh it.**
* Quantitatively means making sure all the acid measured or weighed makes it into your final solution. E.g. after adding the acid to the distilled water, rinse the measuring vial or container with a relatively small amount of more distilled water and add that to your volumetric flask or mixing vessel. Do that 3-5 times.
** In lieu of a volumetric flask, you could weigh out the
final volume on a (decent) scale. 250 ml of 10% Phosphoric Acid weighs 263.25 gram.
Using a scale with a precision of around 1-2 gram is fine. One you may already use for weighing hops, or in the kitchen for baking. Although there may be a total weighing error of 1-2 gram, it's only 0.4-0.8% of the total in this case, and very acceptable given the total volume weighed (263 gram).
IOW, due to the scale's inherent (in)accuracy and/or internal rounding you may end up with actually 265 gram instead of the intended 263 gram (which is what the scale displays). That would mean you'd have [
revised] 9.92% Phosphoric Acid on hand instead of 10.00%.
