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The method being used. I am currently carrying out a school project where I am determining the copper content of 1p coins. The coins used are British 1 pence coins dated before 1992 as after this year they were altered and no longer composed mainly of copper. In my experiment, the coin is placed in 16M nitric acid until dissolved and then titrated against sodium thiosulphate.

I have noticed that when the coin is dissolved in the nitric acid and made into a working solution with deionised water, a slight precipitate is formed and the solution appears cloudy which is not the case with the pure copper sample I have repeated the experiment with. The coins are made up of mainly copper with small amounts of zinc and tin so I am wondering whether this is causing the precipitate and affecting my results.

For both copper coins I have used, the percentage copper that I have calculated has been around 95% (online sources say coin is 97% copper). However when running the experiment with the pure copper sample my result was only 90% copper. While my results for the coins are fairly good, the result for the pure copper sample is far from ideal.

I will also add that the pure copper sample dissolved much more rapidly than the coin which could be a factor if the coin just simply isn’t fully dissolved.

I am looking for answers as to why this might be happening and any suggestions as to what could be changed.

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    $\begingroup$ Some precipitation of hydrated tin(IV) oxide? Copper and zinc should dissolve completely. $\endgroup$
    – Poutnik
    Commented Feb 27 at 9:56
  • $\begingroup$ Just a point on notation: I think you mean working solution here: " and made into a standard solution with deionised water". A standard solution is one of known concentration. You have the opposite. $\endgroup$
    – Buck Thorn
    Commented Feb 27 at 10:00
  • $\begingroup$ The textbook analysis of brass precipitates tin as a metastannic acid that was given an empirical formula. The procedure requires heating to dryness and redissolving copper and zinc. $\endgroup$
    – jimchmst
    Commented Feb 28 at 10:37
  • $\begingroup$ BTW, how clean is the surface of the coin? Could it be foreign matter (e.g., organic residue from handling)? $\endgroup$ Commented Feb 28 at 17:28

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