Experiment 30,31,32,33: Identification and Separation of Group A,B,C,D Cations
– Describe the types of reactions used (5 different kinds of reactions i.e hydrolysis, precipitation, etc.)
– Use 4-5 tests with results as examples for the above description
– Talk about the concept of qualitative analysis and seperation in relation to the experiments
The purpose of this lab is to qualitative analysis scheme to separate and identify Group A, B, C, D cations. The group A cations consists of: Pb2+, Hg22+, Ag+. These cations form insoluble chlorides where lead chloride is soluble in hot water, silver chloride is soluble in ammonia and mercurous chloride is reacts with ammonia. The group B cations consist of Sub B1: Bi3+, Fe3+, ZrO+, Mn2+ and Sub B2: Al3+, Cr3+. These cations are simultaneously precipitated as hydroxides or ozides that’re insoluble in ammonia solution and can only be separated by the addition of hydrogen perodice which dissolves B2 oxides and hydroxides. The group C cations consist of: Ba2+, Ca2+, Sr2+ and Co2+. These cations are precipitated by adding ammonium oxalate and nitric acid to destroy oxelate later on during the process to prevent interfering at later stage. HCl is added to the residue of oxides of group C cations to yield solution containing group C cations. The group D cations consist of: Mg2+, Ni2+, Cu2+ Zn2+ and Cd2+. These cations have hydroxides, oxides, and oxelates that are soluble.
The following are the types of reactions that are expected to be used in the experiments below and examples of these are also given and these are hydrolysis, precipitation, combustion, acid-base and decomposition. In hydrolysis water is used to break the strong union between molecules resulting in a weaker solution with a reduced pH. Precipitation is the reactions between chemicals which results in the formation of a solid or powder that can be clearly seen which is referred to as a precipitate. For example in the addition of nitric acid there is the formation of a grey precipitate. Combustion is when a compound is combined with oxygen through heating to form water and carbon dioxide. For example the addition of HCL and water and (NH4)2SO4 then after precipitate heated through the flame test. Acid-base is the reaction that takes place between an acid and a base and they end up neutralizing each other. Decomposition is the reactions through which one complex compound is separated into other simpler compounds.
In qualitative analysis is used to test for the occurrence of a certain substance in a solution, mixture or compound. One is able to tell whether is there or not there through known tests that are used to check for its presence. This process cannot assist one in telling the amount present in relation to the entire solution or given mixture. In these experiments we are able to separate certain known chemicals or compounds from the mixture through certain known tests which then tell as us about their presence.
For group A cations, HCl is added to the unknown solution, centrifuges and decanted. The precipitate is added water, boiled, centrifuged, decanted. The precipitate now is then tested with KI for the presence of lead ion. The precipitate is washed with water, added ammonia solution and if gray black precipitate present then mercury ion is present. Nitric acid is then added to the supernatant until it turns acidic and if silver ion is present white precipitate will be observed.
For group B cations, HCl and ammonia solution is first added. NaOH and hydrogen peroxide solution is added, centrifuged and decanted after HCl is added and heated. The supernatant is added with hydrogen peroxide solution and Alizarin Red as well to test the presence of zirconyl ion. The supernatant is added solid NaBiO3 to test the presence of manganese ion. KSCN is added to the supernatant to test the presence of Fe3+. The supernatant is added NaOH and SnCl2 to test the presence of bismuth ion. To test the presence of aluminium ion, ammonia solution, HCl and aluminion is added. To test the presence of chromium ion, hydrogen peroxide and HCl is added.
For group C cations, HCl and KNO2 is first added to the unknown solution to test the presence of cobalt ion. Then ammonia solution, (NH4)2CO3 is added, centrifuged, decanted. The precipitate is washed with water and centrifuged then decanted. Acetic acid is added until precipitate dissolves and deionized water is added to dilute K2CrO4 is added and the solution is added with HCl for flame test to test the presence of barium ion. The rest of the supernatant is added NH3 until basic, added (NH4)2CO3, centrifuged then decanted. Precipitate is washed with deionized water, centrifuged, decanted. Precipitate is added HCl and water and (NH4)2SO4, centrifuged and decanted. Supernatant is then put to flame test to test the presence of strotium ion. To test the presence of calcium ion, ammonia solution is added, centrifuged, decanted. The supernatant is put to flame test to test.
For group D cations, HCl, DMGH and ammonia solution is added until solution turns basic to test the presence of nickle ion. The solution is added ammonia solution and solid Na2S2O3, heated then cooled. HCl is added, centrifuged and decanted. Ammonia solution is added until acidic and K4Fe(CN)6 solution is added to test the presence of copper ion. NH3, solid Na2S2O3, HCl and K4Fe(CN)6 is added to test the presence of cadmium ion. To solution B, NH3 is added until basic and Na2HPO4 solution is added, centrifuged then decanted Precipitate is added deionized water and NaOH, centrifuged to test the presence of magnesium ion. To the rest of the supernatant, acetic acid is added and then K4Fe(CN)6 to test the presence of zinc ion.
Table for Group A Cations
Reactions Observations Results
Pb2+ : Add KI Yellow precipitate Lead ion present
Hg22+ : Add NH3 Grey precipitate, colorless solution Mercury ion absent
Ag+ : Add HNO3 White precipitate Silver ion present
Tablr for Group B Cations
Reactions Observations Results
Add HCl, NH3 Grey-blue solution, white precipitate
Add NaOH, H2O2, HCl, heat Colorless solution
ZrO2+: Add H2O2, Alizarin Red Yellow solution Zirconyl ion absent
Mn2+: Add NaBiO3 Yellow-green solution Manganese ion absent
Fe3+: Add KSCN Red-bronze solution Ferrous ion absent
Bi3+ : Add NaOH, SnCl2 Black precipitate, grey solution Bismuth ion present
Al3+ : Add NH3, HCl, Almuminiun Red precipitate Aluminium ion present
Cr3+ : Add H2O2, HCl Dark-blue-purple fast fading Chromium ion present
Table for Group C Cations
Reactions Observations Results
Co2+ : Add KNO2 Yellow solution Cobalt ion present
Ba2+ : Add NH3, (NH4)2CO3, Yellow solution, green-yellow flame Barium ion present
CH3COOH, K2CrO4, Flame Test
Sr2+ : Add NH3, (NH4)2CO3, HCl No precipitate, clear solution Strotium ion absent
Ca2+: Add NH3, (NH4)2C2O4 White precipitate, brick-red flame Calcium ion present
Tablr for Group D Cations
Reactions Observations Results
Ni2+ : Add DMGH, NH3 Grey-black precipitate Nickle ion absent
Cu2+ : Add NH3, K4Fe(CN)6 Maroon precipitate Copper ion present
Cd2+ : Add NH3, Na2S2O3, HCl Purple precipitate, clear solution Cadmium ion absent
Mg2+ : Add NH3, Na2HPO4 White precipitate, clear solution Magnesium ion present
Zn2+ : Add CH3COOH, K4Fe(CN)6 White precipitate, clear solution Zinc ion present
Table for Unknown Cations
Cations Present Unknown #
Group A – Pb2+ , Ag+ 25
Group B – Bi3+ , Al2+ , Cr3+ 2
Group C – Co2+ , Ba2+ , Ca2+ 15
Group D – Cu2+ , Mg2+ , Zn2+ 17
– Cite possible errors in the experiment
– Identify the unknowns and mention why they are what you think they are
In the group A cation unknown test, Pb2+ and Ag2+ are found to be present in unknown #25. This is found out because a white precipitate is formed at the test for Pb2+ and Ag+. Some errors may have occurred in this experiment such as not properly centrifuging the solutions thus leaving some precipitates in the solution and not being able to compare the colors with the positive control.
In the group B cation unknown test, Bi3+, Al2+, and Cr3+ are found to be present in unknown #2. This is found out because when NaOH and SnCl2 is added to the supernatant, a black precipitate forms which confirms the presence of Bi3+; when NH3, HCl and aluminion is added, a red precipitate forms which confirms the presence of Al3+; when H2O2 and HCl is added, a dark blue-purple by rapid fading occurs which confirms the presence of Cr3+. Some error may have occurred in these experiments such as not making sure that the solution has turned basic and acidic when ammonia solution is added before testing for the cations. Another source of error may be from cross-contamination of solutions that are added to the unknown solution which changes the colors of the supernatants and precipitates.
In the group C cation unknown test, Co2+, Ba2+ and Ca2_ are found present in unknown #15. This is found out because when KNO2 is added, a yellow solution with yellow precipitate is formed which confirms the presence of Co2+; when NH3, (NH4)2CO3, CH3COOH, and K2CrO4 is added and put to flame test, a yellow solution and green-yellow flame is produced which confirms the presence of Ba2+; when NH3, (NH4)2C2O4 is added, a white precipitate is formed and put to flame test to produce brick-red flame which confirms the presence of Ca2+. Some errors may have occurred in this experiment such as the assuming the colors are what one thinks it is when performing the flame test. Another source of error is not completely discarding washes which contaminated precipitates and thus giving a false change of colors.
In group D cation unknown test, Cu2+, Mg2+ and Zn2+ are found to be present in unkniwn #17. This is found out because when NH3 and K4Fe(CN)6 is added, a maroon precipitate forms which confirms the presence pf Cu2+; when NH3, Na2HPO4 and NaOH is added, a white precipitate clear solution forms which confirms the presence of Mg2+; when CH3COOH and K4Fe(CN)6 is added, a white precipitate clear solution forms which confirms the presence of Zn2+. Some errors may have occurred in this experiment such as not making sure that the solution has turned acidic or basic when NH3 is added and not cooling and heating the solution long enough to heat to dryness.
There are other additional errors that could have gone undetected. There is a possibility of the presence of contaminants that could have led to some undesired observations. When one is making observations there could be other colors that are not quite dominant showing the presence of other compounds but as it is one is likely to only see the colors that are very dominant. There are cases where the solutions is required to be quite clear this would be different to different people hence we might end up having different concentrations because of the color aspect if the experiment were to be done by different people.
– Describe about the purpose of the lab and mention if it has been met/successfulÃ‚Â
– ID the unknowns
– Major possible errors in the experiment, and how to improve them next time
The purpose of this experiment which is to identify and separate Group A, B, C, D cations have been achieved and the cations in the unknowns have all been identified. The major error that may have occurred in all the experiments is cross-contamination of solutions which will influence and change the colors of the cations being observed and will provide false data. A way to improve this experiment is to repeat the whole procedure at least twice and to consistently compare observations with the positive known control. The experiment can be done by different people and the average results can then be taken meaning that we will have increased the degree of accuracy.