Metal Finishing Guide Book

2012-2013

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is taken. Solutions should be stirred before sampling. If there is sludge in the tank, the solution should be stirred at the end of the day and the bath allowed to stand overnight, taking the sample in the morning. Solutions should be sampled by means of a long glass tube. The tube is immersed in the solution, the thumb is placed over the upper open end, and a full tube of solution is withdrawn and transferred to a clean, dry container. The solution should be sampled at a minimum of 10 locations in the tank to ensure a representative sample. A quart sample is sufficient for analysis and Hull cell testing, and any remaining solution can be returned to its tank. STANDARD SOLUTIONS, REAGENTS, AND INDICATORS FOR WET METHODS Standard solutions, reagents, and indicators can be purchased ready-made from laboratory supply distributors. Unless a laboratory has the experience and high degree of accuracy that is required in preparing these solutions, it is recommended that they be purchased as prepared solutions. Preparations for all the solutions are given here to enable technicians to prepare or recheck their solutions. A standard solution is a solution with an accurately known concentration of a substance used in a volumetric analysis. Standardization of standard solutions requires greater accuracy than routine volumetric analyses. An error in standardization causes errors in all analyses that are made with the solution; therefore, Primary Standard Grade chemicals should be used to standardize standard solutions. Fig. 1. Test setup for determination of cathode efficiency. Use 500-ml beakers and 1 �� 2-in. brass cathodes. The anodes for the test solution should match that used in the plating bath. Use copper anodes for the coulometer. The strengths of standard solutions are usually expressed in terms of normality or molarity. Normalities of standard solutions and their equivalent molarities are listed in Table III. The methods to standardize all the standard solutions required for the analysis of plating and related solutions are listed in Table IV. Indicators are added to solutions in volumetric analyses to show color change or onset of turbidity, signifying the endpoint of a titration. The indicators required for all of the analyses and their preparations are listed in Table V. Analytical Grade chemicals should be used in preparing analytical reagents 544

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