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ANALYSIS OF METAL FINISHING BATHS

A key to Quality, Productivity, Cost-Savings and a Cleaner Environment

Browse the resources listed on the left to see how important Analysis is in maintaining
high quality and maximising profitability in Metal Finishing operations.

ANALYSIS - HOW ? WHAT ?

So we have a sample of solution, maybe 100 ml or so, and we want to analyse it. How do we go about this ? Qualitative Analysis will hopefully tell us what species are present – but not in what concentrations. Quantitative Analysis will give us this additional information. Semi-quantitative Analysis is somewhere in between, giving us a rough idea of how much of a given species is present.

Choosing an Analytical Technique - Five Important Concepts We Need to Appreciate

Range - Sensitivity - Accuracy
Reproducibility - Interference

1. Range

An analytical method will only give good results within a certain concentration range. If the sample is too concentrated, we can easily dilute it by a known amount. If the sample is too dilute, it can be concentrated - but this is tedious and can result in loss of accuracy.

2. Sensitivity

The lowest concentration at which the analytical method gives satisfactory results. Thus the lower end of the Range limit.

3. Accuracy

Also referred to as Resolution. Is the answer 33 ? 33.4 ? 33.47 ? grammes/litre. Could the method distinguish between two solutions of concentration 33.4 and 33.5 gm/litre ? Don’t be deceived by an instrument with lots of numbers digitally displayed. The digital display has to show something – but the last digits could be quite meaningless.

4. Reproducibility

If we try to analyse, this morning, this afternoon, tomorrow, next week, next month, what is in fact the same solution, will the method give us the same answer ? This takes into questions concerning calibration and standards which we won’t discuss here.

5. Interferences

You might wish, for example, to analyse for cadmium. But maybe if there was also zinc present in the same solution, the method would not distinguish between them. So rather than giving the cadmium concentration, it would give you [cadmium] + [zinc] - a wrong answer. Sometimes we will be told that a method is vulnerable in this way. Other times, we’ll have to use our instinct and judgement, run a few tests to see if this is a problem.

Planning our Analysis Strategy

First of all, we have to decide what we are going to look for. We’re unlikely to want to analyse for gold in a nickel plating bath. Our first task will be to determine the concentrations of all known bath constituents, as set out by the Supplier. Looking for impurities - contaminating species – is more difficult because in many cases, we have to take a guess at what the contaminant might be. So one approach is to start using Qualitative Analysis. If our plated or other components have an obvious defect, we could try another approach. Consult David Luke’s Troubleshooting Charts for the particular process you are using. In many cases, the defective nature of the finished surface can give you a clue to the type of impurity present, causing the problem..