Lou

I agree mostly with what the others say, but only up to a point. Saponifier solutions consist essentially of four components: water, an alkali (usually monoethanolamine or a near relative) and a water-soluble solvent mixture, such as glycols/diglycol ethers, etc., and a buffer. As the alkali is used up for saponifying
the carboxylic acids in the residues, so the quantity is diminished, but the pH hardly changes, because the solution is buffered to prevent attack of amphoteric metals. pH, as such, is therefore useless for determining the amount of MEA available. Titration will give a relative value, in the form of a straight-line
curve but the zero is not at the origin of the axes, also because of the buffering. However, it should not be forgotten that some of the saponification reaction products are also basic, which may make you think you have more saponifier than is really present. This means that absolute values are hairy. The function of
the solvent mixture is to dissolve soluble contaminants and to lower the surface tension. The amount of these present is not easy to determine on the workshop floor and the ratio of MEA:solvents will inevitably drift away from the optimum and even the solvent mixture itself will alter with differential evaporation and
bonding to contaminants. Replenishing saponifier baths is therefore fraught with the hazard that, sooner or later, things are going to get out of control.

The mechanics of titration are simple and easy but I suggest that you use a colour indicator with an end point of between 8,5 and 9.  Phenolphthalein is frequently used, with a range of about 8,2 - 10. A better one would be ethyl-bis(2,4-dimethylphenyl)acetate with a narrower range of 8,4 - 9,5. If you use a pH meter
for the work, then choose 8,7 as your end point. This will reduce (but not eliminate) errors due to the presence of basic saponified residues.

You could fully automate the titration and the addition of fresh concentrate to maintain the acid acceptance at a constant. This would not be horrifically expensive (a few thousand).

However, all this is a little theoretical, because no amount of simple tests will determine the cleanability of your saponifier solution with your particular flux residues. The simple answer is to try to change the solution just before it starts to give poor cleaning results. How? I don't really know, because there are
simply too many uncontrollable parameters.

By the way, there is an excellent book which concentrates on saponifier cleaning. One of the co-authors is Frank Cala who is very active on IPC committees, the other being an ex-colleague of his, Tony Winston. It is entitled "Handbook of Aqueous Cleaning Technology for Electronic Assemblies". There is also a chapter on
the subject in my "Cleaning and Contamination of Electronics Components and Assemblies". Both are from Electrochemical Publications. (Sorry for the plug!)

Brian

Lou Hart wrote:

> Technetters, how do you measure saponifier concentration in a board assembly cleaning machine?  We've been using a refractometer to measure degrees Brix, but this is appears to be a "legacy" method.  I've been hearing titration would be the way to do it.  Please comment.  Lou Hart QA manager, Compunetix 412-858-6184
>
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