Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 82 of 707

Fig. 1. Single-stage rinse. The literature gives advice concerning what dilution ratio is usually required in rinse tanks. A very general rule of thumb is that a dilution ratio of 1,000:1 is a good starting point in many cases. Then trial and error or a more rigorous statistical process control approach can be employed to get a more exacting answer. EQUILIBRIUM RINSING The best approach to understanding a complex phenomena usually is to begin with the obvious and build upon it. So, picture an empty mixing vat. Then envision adding to this vat 1 part of concentrated plating solution and 999 parts of water, and thoroughly stirring the mixture. The mixture will be 1 part plating solution per 1,000 total parts, or a 0.1% concentration. The next step in extending our understanding of rinsing is to comprehend what is known as an equilibrium process. Start with the same 1,000 parts of 0.1% mix- ture but, instead of leaving well enough alone, continue to add plating solution and water, in the ratio of 1:999, until the mixing vat overflows. Without going into elaborate proofs, recognize that things are in balance here. The vat contained a 0.1% solution and what is being added is a 0.1% solution, so what is overflowing must be a 0.1% solution also. Now, imagine that we had started this process of continuously overflowing the vat years ago, and that it has continued uninterrupted ever since. We are confi- dent that the overflowing rinsewater is and always has been of 0.1% concentra- 81

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