Phosphoric acid is not really a ‘fuming’ acid, so it isn’t as dangerous as sulfuric or hydrochloric acids. It is a fairly strong acid though and demands careful attention at all times.
Using 10% or 85% is not much different from an acidification standpoint. I see that there is a concern over the large volume of 10% version compared to the 85% version. Remember, the only difference is water. In many respects, using the 10% version is more accurate and safer. (its more accurate since a minor error in measurement is magnified for the concentrated acid) But you will end up paying quite a bit more on a molecule to molecule basis. You do have to pay to ship all that extra water, you know.
So if the handling and dosing are a big concern, then using the 10% strength is the way to go. If you have some confidence in your ability to dilute a high concentration acid to a lesser concentration, then you can have the best of both worlds. Buy the high concentration acid and dilute it at home with distilled water to create that safer and easier to dose low concentration acid.
The formula for acid dilution is very simple:
M1 x V1 = M2 x V2
The M variable stands for molarity, but you can use any strength indicator like Percent also. The V variable is the volume.
So if you have 85% acid and want to create 100 mL of 10% acid, the formula would be:
85% x V1 = 10% x 100 mL
performing the algebra results in V1 = 11.76 mL. So you would add 11.76 mL of 85% acid to (100 - 11.76 mL = 88.23 mL) of distilled water to produce 100 mL of 10% acid.
If you are worried about the day to day handling, the dilution route is the way to go. That way you only need to get dressed out in a Hazmat suit a few times, instead of every time you brew. (OK, you don’t really need to wear a hazmat suit, but you do have to use extreme care and protection when handling strong or concentrated acids)
Enjoy!
