Chrome – the element taking in Mendeleyev's table the place at number 24 in a secondary subgroup of the 6th group. Represents itself bluish-white metal. Is the major component in many grades of the alloyed steel, it is applied in a galvanics and also by production of heat resisting alloys, dyes. Compounds of chrome depending on the oxidation level of this element can show the main, amphoteric and acid properties.
Instruction
1. For example, the task is set for you: there are several samples with solutions of substances, and it is known that some of them contain compounds of chrome. It is required to define in what, and be at one to draw a conclusion what oxidation level is lame in these connections.
2. First of all it is necessary to consider that chrome can be present at solutions or in the form of Cr^3+ cation, or in the form of CrO4^2- or Cr2O7^2-anions. Begin with detection of a cation of chrome. For this purpose serially cast several drops of solution from each sample in a separate test tube and add several drops of alkali there.
3. If the sample contained Cr3^3+ cation, the friable, jellylike deposit of gray-green color instantly will drop out. Because slightly soluble hydroxide of Cr (OH) 3 chrome was formed. For example, according to such scheme: Cr2(SO4)3 + 6NaOH = 2Cr (OH)3 + 3Na2(SO4) 2. Color of a deposit can fluctuate from gray-green to gray-violet. It depends on what impurity contain in chrome salt.
4. So, you found a cation of chrome 3+. And how to define what samples contain CrO4^2- and Cr2O7^2-anions? Chromates of many metals are insoluble. It can use for carrying out high-quality reaction of definition. For example, flow to small quantity of each sample on several drops of solution of nitrate silver. That sample where the brown-red deposit is at once formed, contained CrO4^2-chromate ion. Because reaction of type leaked: Na2CrO4 + 2AgNO3 = 2NaNO3 + Ag2CrO4. Chromate of silver dropped out in a deposit.
5. Instead of expensive nitrate silver it is possible to use much cheaper salt of barium. Then the yellow deposit of BaCrO4 will drop out.
6. Well, and how to define Cr2O7^2-bichromate ion? First of all on characteristic orange color of solution. Also there is still a high-quality reaction: at careful stirring of a small amount of water solution of salt of a bichromate with ethyl air and peroxide of hydrogen, the radio layer will gain blue color.