Analysis of Silver in an Alloy

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Garner

Analysis of Silver in an Alloy

Mark Andrew Garner

Gareth Smith

January 28 - February 1, 2008

Aim and Outcome:

The purpose of this lab was to calculate the percent of silver in an alloy using gravimetric analysis. Through the procedure mentioned in this report, the percent of silver in an alloy of a U.S. Mint dime made before 1965 was 90.2% ± 0.05%.

Introduction: 

The alloy used for this experiment was a U.S. Mint ten-cent dime alloyed as silver and copper if made before the year 1965, after which mints began using copper and nickel instead. First, the dime piece was dissolved in nitric acid and the silver was precipitated as silver chloride. The precipitate was then filtered, washed, dried and weighed. To obtain the percent of silver in the alloy, the mass of silver chloride was compared to the mass of the original sample. Because the results are based on the mass of a product, the procedure is determined as gravimetric analysis.

Nitric acid (HNO) is required as an “oxidizing agent” because silver and copper are very non-reactive metals and cannot dissolve in hydrochloric acid or sulfuric acid. The nitrate ion (NO3¯) oxidized Ag(s) to Ag+(aq) and Cu(s) to Cu2+ (aq). As the nitric acid was reduced, the product became nitrogen monoxide (NO), a colorless gas that instantly reacts with oxygen in the air to produce nitrogen dioxide (NO2), an orange-brown gas. The following demonstrate the half reactions mentioned:

Ag(s) → Ag+(aq) + e-

Cu(s) → Cu2+ (aq) + 2e-

4 H+(aq) + NO3- + 3e- → NO(g) + 2H2O(l)

To separate the silver ions and the copper ions, the silver was precipitated as silver chloride (AgCl) by adding dissolved sodium chloride (NaCl) to the solution, which also created the product of copper (II) chloride (CuCl2). Theoretically, because copper (II) chloride is soluble in water and silver chloride is insoluble in water, the addition of chloride ions to the solution from the sodium chloride precipitated all the silver ions and none of the copper ions. The silver chloride precipitate was filtered from this solution. The following demonstrates the reaction for the precipitation mentioned:

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Ag+ + Cl- → AgCl(s)


Procedure:

Day 1

  • The mass of a piece of dime made before 1965 (silver-copper alloy) was measured using an analytical balance and recorded.
  • The dime/alloy was placed in a clean, group labeled 100-mL beaker.
  • While under a fume hood, a 10 mL graduated cylinder was used to pour 10 mL of 6M nitric acid into the 100-mL beaker containing the alloy.
  • To make sure the alloy had totally dissolved, the beaker containing the solution was placed on a hot plate
  • After calculating two times the amount of sodium chloride necessary to ...

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