Users of chemical treatment baths often ask for controls that guarantee a steady process and also allow to maximize the utility of the bath. How can we measure that the bath is the same and how do we know when it should be replaced? Well, such guidelines are difficult to be had for a couple of reasons. First, parts are not standardized, and 1 cm2 of surface of a certain Nitinol part is likely different from 1 cm2 of surface of another part (even if we are able to measure the surface area). Then, in many cases the analytical tests that would be required are complex, require chemical expertise and/or training, and sometimes advanced instrumentation. We would like the simple pH measurement to do the trick but it can’t. An acid titration? More complicated and it would give better information; but still not very usable. A fluoride concentration measurement, using a fluoride ion selective electrode, is much better but the complexity of the measurement goes up significantly. Then, perhaps water concentration is important to measure in electropolishes – this needs a specialized Karl Fischer titrator with special reagents and careful maintenance. For dissolved metal (example: nickel and titanium) we need an ICP-OES or an equivalent spectrophotometer. So the pH measurement is easy and quick but not very useful, the other more useful controls require expensive instrumentation, expertise, maintenance, and each test takes longer time. This is the reality. Yes, this all can be done, but is it worth it? The answer so far seems to be “no” from many of our customers. And the adopted process (SOP) often describes the bath control like this: run 20 stents per liter and then replace with new (example).
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