What's in Your Brass?

The copper solution from the brass and nitric acid reaction.

My AP Chemistry students just finished a fun spectroscopy lab to determine the amount of copper in a sample of brass shot.  This lab is a new twist on the classic "copper cycle" lab that AP Chemistry students have been doing for years.  The transformation from the classic copper cycle lab into a guided inquiry lab is part of the revised AP Chemistry curriculum which has a strong emphasis on application and inquiry-based problem.  (Experienced AP teachers, please don't start throwing things at your computer right now.  I know how much longer it takes to do these labs.)  We used the lab kit from Flinn Scientific called "Percent Copper in Brass:  AP Chemistry Big Idea 1, Investigation 2", publication #7643.

I added the nitric acid in the fume hood.

So we started with three pieces of brass shot.  We poured concentrated nitric acid (yes, the very nasty stuff) over the brass and watched as brown mustard gas was produced.  I want to take this moment to give a shout out to Tony who bought me a fume hood over the summer!  This lab would not be possible without it.   We lingered at the fume hood as we admired the beauty of this chemical reaction.

Eventually, once we all got our fill of watching copper oxidize, my kids went about the task of preparing standard solutions for a calibration curve that will allow them to determine just how much copper was in their brass sample.  The class worked cooperatively to prepare four solutions with concentrations ranging from 0.05 to 0.40 M of copper(II) nitrate.  Beer's law tells us that there is a linear relationship between the absorbance of a solution and the concentration.  No, there were no alcoholic beverages served in this experiment!   The quantity of the copper in the reaction mixture, and thus the brass shot sample, is calculated from the equation for the line in the calibration curve.  The data for this lab was so wonderful.  We got r-squared values of 0.99 and even a 1!

The reaction between copper and nitric acid is fun to watch.

After the reaction was done (it turns out we had to leave it over night to completely react), the only thing left to do was to dilute their reaction mixtures in the adorable 100-mL volumetric flasks and measure the absorbance of the solution.  So simple!  Three days of lab work led to the conclusion that the brass is around 70% copper, and that spectroscopy really isn't that bad!

One of the calibration curves from the lab, very nice.