![]() ![]() Isolating the solution and adding a small amount of Na 2CrO 4 solution to it will produce a bright yellow precipitate of PbCrO 4 if Pb 2+ was in the original sample ( Figure 17.11 "The Separation of Metal Ions from Group 1 Using Qualitative Analysis").Īs another example, treating the precipitates from group 1 cations with aqueous ammonia will dissolve any AgCl because Ag + forms a stable complex with ammonia: +. Because PbCl 2 is much more soluble in hot water than are the other two chloride salts, however, adding water to the precipitate and heating the resulting slurry will dissolve any PbCl 2 present. For example, the precipitated metal chlorides of group 1 cations, containing Ag +, Pb 2+, and Hg 2 2+, are all quite insoluble in water. Metal ions that precipitate together are separated by various additional techniques, such as forming complex ions, changing the pH of the solution, or increasing the temperature to redissolve some of the solids. As discussed in Chapter 6 "The Structure of Atoms", the other alkali metal ions also give characteristic colors in flame tests, which allows them to be identified if only one is present. A flame test on another original sample is used to detect sodium, which produces a characteristic bright yellow color. Wash the precipitate with a few drops of dilute HCl (prepare. (We cannot use the same sample we used for the first four groups because we added ammonium to that sample in earlier steps.) Any ammonia produced can be detected by either its odor or a litmus paper test. Test the transferred clear liquid for completeness of precipitation by adding 1 drop more 6 M HCl. We now take a second sample from the original solution and add a small amount of NaOH to neutralize the ammonium ion and produce NH 3. The only common ions that might remain are any alkali metals (Li +, Na +, K +, Rb +, and Cs +) and ammonium (NH 4 +). At this point, we have removed all the metal ions that form water-insoluble chlorides, sulfides, carbonates, or phosphates. ![]()
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