|MadSci Network: Chemistry|
Dear Brendan First, I am assuming that you have followed the qualitative analysis scheme, that the possible cations are: Mg, Ca, Sr, Ba. The scheme you have followed so far, should also enable you to complete the analysis and prove the presence of calcium carbonate. I also assume that you have already tested the sample and shown that it is a carbonate. These cations are quite similar, and the differences between them tend to be in degree only. There is, alas, no nice simple test which is specific for Ca only, that eliminates all other cations from consideration. Therefore in performing these tests, I strongly suggest running knowns [Ca, Sr, Ba, Mg] at the same time as your unknown. These tests will show the presence of Ca, but these other cations will sometimes interfere. The tests listed below may be different from those in your text, so they may supplement but not replace them. To a portion of your sample [and knowns of Ca and the other possible cations]add a little water, then dilute [[ca 5%] HCl dropwise until the bubbling stops and all the powder has dissolved. Now, all chemicals are in solution as chlorides. Then place the containers in an oven or steam bath, etc. to evaporate the water and excess acid. Redissolve the salts in a few cc of water. These are your TEST SOLUTIONS. Flame test: You need a length of Pt wire [preferably with a loop on the end]for the test. Also, a gas flame adjusted to yield a colorless flame. Pick up a drop of the solution with the wire,then insrt it into the flame slowly [to avoid spatter] then observe the color imparted to the flame by the dried salt. Ca is orange-red, Sr orange-yellow, Ba green. However, a trace of Na will show an intense yellow color,which will hide the other colors, unless you have a special filter. Oxalate test: Saturated ammonium sulfate solution is needed, and saturated ammonium oxalate solution. To a volume [say, 1 cc] of test solution, add the same volume of the amm. sulfate sol'n. Leave on the water bath for about ten minutes. Centrifuge, decant, or filter to remove any precipitate [which will be all the Sr and Ba and maybe a little Ca sulfate]. To the clear liquid add the same volume of amm. oxalate solution. Warm this in the steam bath then observe. Ca will form a white ppt. of the oxalate. Sulfate test: Microscope, glass slide, and 2 N sulfuric acid are needed. Evaporate a drop of the test solution to dryness on a microscope slide. Add a drop of weter to the dried residue, mix with a glass rod until all is dissolved. then ad one drop of the sulfuric acid. A white precipitate of calcium sulfate will form. These crystals have a very characteristic shape. Examine them under a microscope,compare them with knowns you have prepared in the same way. Ferrocyanide: Potassium Ferrocyanide solution [ferrO, not ferrI]about 13 % in water. Solid ammonium chloride. To about 1 cc of test solution, add about 0.2 grams of ammonium chloride, stir to dissolve. Add about ten drops of the ferrocyanide and stir. Ca will yield a white crystalline ppt. If much Sr is present, it will also form a ppt., but much more slowly. Charlie Crutchfield
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