ONLINE MEASUREMENT OF ACID CONCENTRATION

A. Jake,
The monitoring of acid concentration in a finishing line can be a tricky thing, because many of these baths aren't a straightforward solution of a single substance dissolved in water.

There are various ways to measure a solution.

pH will tell you the concentration of H+/OH- ions, which is the acidity or alkalinity, but it won't tell you anything about the conjugate base or acid material (e.g. the nitrate from nitric acid). For example, in a mix of two acids, you will know the total acidity but not how much of it is from one or the other acid.

Conductivity measures how well electricity flows through the solution, which is related to the amount of ions dissolved. But again, if it's a mix of multiple salts, acids, or bases, it only tells you the total, not how much of which one.

Density/specific gravity is just about how much a given volume weighs. A gallon of water with a lot of stuff dissolved in it weighs more than a gallon of pure water. Same as before, if there is an unknown amount of only one thing dissolved, the relation is easily found by taking readings from a series of solutions with known amounts of the same substance. With multiple items dissolved, it's much less certain.

Temperature is also a factor, since all of the above readings are altered depending on temperature. For a single solution you can take a reading, heat it up, and watch how that reading changes.

So how is any of that useful? Mainly, variables can be eliminated, accounted for, or disregarded. If you always take your measurements at the same temperature, then you don't have to worry about that as a variable. Even if you have a small temperature range, it's not likely to cause a large enough difference to matter. If you plan to take measurements over a wide temperature range, then you have to account for that by taking readings from your "known" standards across the same range.

As for having a mix of multiple substances, if the part you really care about is in there in a large amount, while the other things are in there in much smaller amounts, then you can approximate that the contribution to the reading from the other things is zero, and just pretend it's a straight-up solution of the single material you are trying to measure. This is not exact of course, but it's usually "good enough" in lieu of much more complex and expensive methods of solution analysis.

Put together some solutions of known concentrations across the range that you expect to see in your bath, take the readings, and see what trends develop and if they look like you will be able to relate readings from the live bath back to those concentrations on a reliable basis.