Is my ph meter broken?

[bgburdman9]

Bru'n water predicted I would need 13.97 ml of phosphoric acid to bring my pH down from 7.7 to 5.5 for sparging. I added 10 ml and my pH meter that I just calibrated is reading like 3.5. My mash pH was lower than expected with a smaller acid addition than it predicted also. I am thinking I may just need a new probe but what do you guys think?

 

[ajdelange]

Go to https://www.homebrewtalk.com/f128/ph-meter-calibration-302256/ and carry out the calibration, calibration check and stability check procedures. That will tell you if there is a problem with your electrode. How old is it? If it is getting old (a year or more) it may indeed be time to replace it but 2 year and longer lives are becoming common with the new electrode technology.

The amount of phosphoric acid required to acidify a given volume of water to a particular pH depends, of course, on the strength of the acid, the alkalinity of the water and the volume. The ability to predict the amount of phosphoric acid required depends on a accurate model for the behavior of both carbonic acid (which I am pretty certain Bru'n water has) and an accurate model for phosphoric acid as well about which I am not so certain as it is awfully easy, and not terribly inacurate, to assume that 1 molar acid is 1 normal in the range of pH's that we usually deal with as brewers.