Qualitative Analysis Discussion

DISCUSSION:

Qualitative analysis is used to determine the presence of cations or anions present within a sample and their impurities. Information known, such as chemical and physical properties, about cations and anions can be used to determine what steps should be used to separate the sample into one, specific ion. Qualitative analysis involves forming and decomposing complex ions. Qualitative tests are very sensitive, allowing to detect a very small amount present in sample. Due to sensitivity, cleaning utensils and accurate observations are essential to results. Many basic principles are applied to determine the impurities or cations.

To determine which impurities are present, scheme or flow chart is followed. Scheme is used to help draw conclusions while keeping an accurate record of a place in the experiment. Each step will end with a supernatant and precipitate. At the end of the scheme, only one of the starting cations shall be present in the precipitate or solution. Confirmatory test are then carried out to confirm the present of the ion. When performing a confirmatory test, no other ions should be presence except inert ions.

Every reagent is used for a specific reason. If a reagent was removed from any step, results would not be the same. In this experiment, the 6M HCl is used to increase the H+ (lower pH) concentration in the solution. HCl dissolves insoluble carbonates, chromates, sulfates, ammonia complexes, and hydroxides that may be present within a solution. Cations such as Ag+, Hg22+, and Pb2+ ions will precipitate into insoluble chlorides. A high concentration of HNO3 is essential to lower pH and oxidize sulfide ions present in solution. The importance of NaOH is to precipitate insoluble hydroxide and increase pH. NaOH will form hydroxide complexes. Adjusting pH allows for ions to bind to other ions that will not occur at certain pH and become soluble.

A known sample containing seven ions was experimented with first. This set is used as a control for unknown samples for three and four ions. Knowing results help students recognize a positive result from a negative. In every step, there was a precipitate and supernatant liquid. This lead to students being able to determine which exact ion is present in solution.

A solution containing three unknown ions (#3) was then tested. Procedure was carried out accordingly to lab manual. Determined ions present in solution were Fe3+, Cr3+, and Cu2+. The confirmatory test for Fe3+ that was used was , K4Fe(CN)6 add to a precipitate containing Fe3+. A dark blue precipitate formed which is a positive result, Fe3+ is present in this solution. Continuing procedure, Cr3+ presence was confirmed in step 9 by dissolving the precipitate with HNO3 then adding 5 drops of diphenycarbazide (dcp) solution. The supernatant was yellow then a sudden change to pretty, dark pink/purple after the first drop confirmed chromium presence. The reason for the color change is because chromium is reduced and dcp is oxidized. Further, along in the procedure, another positive test revealed the