Qualitative Analysis Lab

Topics: Sodium, Potassium, Ion Pages: 5 (1545 words) Published: February 18, 2013
Casey Brown
Lab Partner: Samed Obeng
Qualitative Analysis Lab
Objective of Experiment:
The purpose of the experiment is to identify every cation present in an unknown solution by performing specific chemical tests. Chemical tests such as adding specific chemicals to make precipitates, adding strong and highly concentrated acids and bases, centrifuging the precipitate from the supernate, and performing a flame test are carried out to determine the presence or absence of the ions in each group. Experimental Procedure:

Separation of Group I Cations: Ten drops of the unknown solution C were placed into a small test tube. Four drops of 6 M HCl were added to the unknown solution. After being stirred, the yellowish unknown developed a white, cloudy precipitate. The solution was centrifuged and a white precipitate sunk to the bottom of the test tube. One drop of 6 M HCl was added to the solution making the clear yellow supernate cloudy, which showed that not all of the Group I ions precipitated. Two more centrifuges were performed until no more precipitate formed. The clear yellow supernate was decanted into a clean dry test tube. The yellowish white precipitate was rinsed with five drops of cold tap water and stirred. This solution was then centrifuged and added to the supernate in the other test tube.

Fifteen drops of tap water were added to the washed precipitate and the test tube was placed in a hot-water bath. The solution was stirred in the water bath for one minute. The now clear purplish liquid was centrifuged and there was no precipitate formed which confirmed the absence of Ag+. Three drops of 1 M K2CrO4 were added to the purple supernate, and a thick, cloudy yellow precipitate formed confirming the presence of Pb2+.

Separation of Group III Cations: Five drops of 2 M NH4Cl were added to the test tube containing the clear yellow supernate from group I. The solution was stirred. Then, seven drops of 15 M NH3 were added until the solution was basic and the litmus paper turned blue. The addition of NH3 caused a light-orange precipitate to form. The two additional drops of NH3 and the twenty drops of water created a reddish-brown precipitate and a clear supernate. The solution was stirred and centrifuged. The light blue supernate was decanted from the caramel brown precipitate. Twelve drops of 12 M HCl and five drops of 16 M HNO3 were added to the brown precipitate. The mixture was carefully stirred and turned into a clear yellow liquid. The test tube was heated in a hot-water bath for about forty-five seconds to dissolve any remaining precipitate. Ten drops of water were added to the solution and was centrifuged. There was no remaining precipitate, and the clear yellow supernate was divided into two separate test tubes. Two drops of 0.2 M KSCN were added to one of the test tubes containing the yellow supernate. The solution turned to a deep, blood-red which confirmed the presence of Fe3+.

After fifty drops of 6 M NH3 were added to the second test tube of the yellow supernate, the solution remained acidic and it only caused the solution to turn a paler yellow. Five drops of 15 M NH3 were added to make the solution basic by turning the litmus paper blue. The addition of the 15 M NH3 also caused an orange precipitate to form in the solution. The solution was centrifuged and decanted into a separate test tube to remove the precipitate. Four drops of dimethylglyoxime reagent was added to the solution which was then stirred. The solution turned a deep pink with the formation of a pink precipitate which confirms the presence of Ni2+.

Separation of Group IV Cations: Eight drops of 6 M CH3CO2H and one drop of 1 M K2CrO4 were added to the light blue supernate from the Group III procedure. The mixture was stirred and turned to a clear yellow color with no precipitate which confirms the absence of Ba2+.

Four drops of 15 M NH3 were added to the yellow supernate from the previous step to make it...
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