Metal Finishing Guide Book


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troubleshooting, testing, & analysis CONTROL, ANALYSIS, AND TESTING CHEMICAL ANALYSIS OF PLATING SOLUTIONS* BY CHARLES ROSENSTEIN TESSERA-ISRAEL, LTD., JERUSALEM, ISRAEL AND STANLEY HIRSCH LEEAM CONSULTANTS LTD., NEW ROCHELLE, N.Y. Plating solutions must be routinely analyzed in order to maintain the recommended bath formulation and to preempt the occurrence of problems related to improper levels of bath constituents. Contaminant levels in the solutions must also be monitored. Manufacturers of plating systems establish optimum specifications to ensure maximum solution efficiency and uniformity of deposits. The various factors that cause the concentrations of bath constituents to deviate from their optimum values are as follows: 1. drag-out; 2. solution evaporation; 3. chemical decomposition; and 4. unequal anode and cathode efficiencies. A current efficiency problem is recognized by gradual but continuous changes in pH, metal content, or cyanide content (see Table I). The techniques employed for the quantitative analysis of plating solutions are classified as volumetric (titrimetric), gravimetric, and instrumental. Volumetric and gravimetric methods are also known as ��wet�� methods. The analyst must select the method that is best suited and most cost effective for a particular application. The wet methods outlined here are simple, accurate, and rapid enough for practically all plating process control. They require only the common analytical equipment found in the laboratory, and the instructions are sufficiently detailed for an average technician to follow without any difficulty. The determination of small amounts of impurities and uncommon metals should be referred to a competent laboratory, as a high degree of skill and chemical knowledge are required for the determination of these constituents. Hull cell testing (see the section on plating cells elsewhere in this Guidebook) enables the operator to observe the quality of a deposit over a wide current density range. VOLUMETRIC METHODS When titrants composed of standard solutions are added to a sample that contains a component whose concentration is to be quantitatively determined, the method is referred to as a volumetric method. The component to be determined must react completely with the titrant in stoichiometric proportions. From the volume of titrant required, the component��s concentration is calculated. The simplicity, quickness, and relatively low cost of volumetric methods make them the most widely used for the analysis of plating and related solutions. Volumetric methods involve reactions of several types: oxidation-reduction, acid-base, complexation, and precipitation. Indicators are auxiliary reagents, which usually signify the endpoint of the analysis. The endpoint can be indicated 533

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