Preparation and Analysis of a Copper Complex

Chemists frequently make a new compound and then analyze the new material in order to determine its formula. In this experiment you will prepare a copper-ammine complex, which is a coordination complex, and then you will analyze the complex for copper and ammonia. Your ultimate goal is to determine the copper-ammine ratio and the probable formula of the coordination complex and of the compound. 

The analysis of the copper-ammine complex for the ammonia, NH3, content will involve an acid-base back titration. In this analysis, you will react the complex with an accurately measured volume of standardized hydrochloric acid, HCl, solution. The volume of HCl solution should be sufficient to react with all the ammonia and leave some excess HCl. The excess HCl will then be back reacted with standardized sodium hydroxide, NaOH, solution. The difference between the number of moles of HCl added and the number of moles of excess HCl will give the number of moles of ammonia in your measured quantity of copper-ammine complex. For your information, you cannot titrate the ammonia directly with HCl because of problems with seeing the color changes of the appropriate indicator in the presence of the colored copper-ammine complex. 

The copper content of the complex will be determined by a disodium ethylenediaminetetraacetic acid (EDTA) titration. Disodium EDTA or EDTA, in short, is a hexadentate chelating ligand, a ligand which can potentially bond one copper ion at a maximum of six coordination sites. 

The copper-EDTA complex will only form when the hydrogen ion concentration of the solution is carefully controlled. Hence, it is important to carefully adjust and buffer the hydrogen ion concentration or pH of the solution before doing the EDTA titration. 

Preparation of Disodium Ethylenediaminetetraacetic Acid Salt Solution 

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