The Use of Ammonia as a Qualitative Test for Determining When Solid Copper Has Completely Precipitated from Aqueous Solutions Containing Copper(II) Ions

In this study, two methods were compared to efficiently determine the absence of copper ions from a copper(II) chloride dihydrate (37.274% copper by mass) solution after precipitation using solid magnesium. In one method, a solution color change from pale blue to colorless was used as indication of a completed precipitation of copper. In the alternative method, ammonia was added to aliquots of solution which indicated remaining copper ions with a royal blue complex. For both methods, magnesium turnings were added to a blue copper(II) chloride solution until it was determined, as indicated above, that all copper had precipitated from solution. Afterwards, any excess magnesium was reacted with concentrated hydrochloric acid. The solution was then filtered and the precipitate was washed, dried, and weighed. After ten trials per method, the average percent copper obtained, to a 95% confidence interval, was 34 ± 2% with ammonia testing and 22 ± 2% without it. An F-test indicated an equal variance between methods. The corresponding t-test value of 9.58, when compared to the two-tailed t-critical value of 3.20, indicated a statistically significant difference of data between the two methods