Qualitative Analysis Lab Essay

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 basic and turn the litmus paper blue. The solution retained its yellow color with no precipitate. Seven drops of 0.1 m K2C2O4 were added which caused the formation of a small amount of white precipitate. The small amount of precipitate was dissolved in ten drops of 6 M HCl. A small sponge was used to soak up a small amount of solution. Using a Bunsen burner, the solution soaked sponge was placed into the top of the blue flame. The flame was initially orange and then had sporadic bursts of red flame which confirmed the presence of Ca2+. A small amount of the original unknown solution C is absorbed by a section of sponge. A flame test is performed using a Bunsen burner. The solution soaked sponge produced an orange-yellow flame; however, the yellow was not very prominent. The solution did contain Na+, but not in a substantially large amount. 2 mL of original unknown solution C and 2 mL of 6 M NaOH were combined in an evaporating dish. Litmus paper was wetted with water and placed on the bottom of a watch glass. The litmus paper watch glass was placed over the evaporating dish. The litmus paper without extra heating immediately turned blue which confirms the presence of basic NH4+. Results: Table for Unknown C Pb2+| Ag+| Fe3+| Ni2+| Ba2+| Ca2+| NH4+| Na+| Yes| No| Yes | Yes| No| Yes| Yes| Yes| Discussion: The lead ion was found by making a supernate from water and the precipitate formed by mixing the unknown solution and 6 M HCl and decanting out the supernate. The unknown solution and HCl were added to a test tube and centrifuged to separate the precipitate from the supernate. The supernate was then decanted from the precipitate. Water was added to the precipitate and put into a hot-water bath. There was no precipitate formed after the solution was heated and centrifuged which confirmed the absence of the silver ion. The addition of K2CrO4 caused a yellow precipitate to form in the supernate. The formation of the yellow cloudy precipitate confirmed the presence of the lead ion. The iron ion was found by taking the supernate from group I and making a precipitate by mixing it with 2 M NH4Cl and 15 M NH3. The precipitate was mixed with 12 M HCl and 16 M HNO3 and heated in a hot water bath until the precipitate was completely dissolved. The supernate was then divided equally into two test tubes. One of the samples was combined with 0.2 M KSCN to form a blood-red precipitate which confirmed the presence of the iron ion. The second supernate sample was used in confirming the presence of nickel. 6 M NH3 was added to the supernate, but 15 M NH3 had to be used to make the solution basic. After centrifuging and decanting out the precipitate formed, the dimethylglyoxime reagent was added to the supernate and created a deep pink precipitate that confirmed the presence of nickel. The supernate from the group III sample was mixed with 6 M CH3CO2H and 1 M K2CrO4. There was no precipitate that formed which confirmed the absence of the barium ion. The calcium ion was found by using the supernate from the previous test and adding 15 M NH3 until the solution was basic. The precipitate was then centrifuged out and discarded. The addition of 0.1 M K2C2O4 created a small amount of white precipitate that suggested the presence of the calcium ion, but a flame test was conducted to confirm the presence of calcium. The expected red-orange flame produced by a solution soaked sponge confirmed the presence of calcium in the unknown solution. Ammonium ions were found by combining the original unknown solution and 6 M NaOH in an evaporating dish and testing if the fumes were basic. Moist litmus paper was stuck to the bottom of a watch glass which was then placed over the top of the evaporating dish. The litmus paper immediately turning blue proves the existence of ammonium ions. The sodium ion was found by performing a flame test on the original unknown solution. The original solution was added to a sponge which was put into a flame which showed a yellow-orange flame which shows the presence of sodium ions, but the intensity of the yellow did not suggest that there was a significant amount of sodium. The flame tests seem to be not as accurate since the sponge alone would burn an orange flame. Attention to the small bursts of reds and yellows in the dominate orange flame had to be highly focused since it was one of the only indications of which ions were present in the unknown solution. Conclusion: Through the formation of precipitates and supernates, the additions of acids and bases, the additions of highly concentrated solutions, and flame tests specific ions were separated out in order to determine the characteristics of the unknown solution. Unknown solution C was found in conclusion to the experiment to contain the cations lead, iron, nickel, calcium, ammonium, and sodium.