: Qualitative Analysis For Identification Of Metal Cations 475d4v

LAB REPORT CHM475 INORGANIC CHEMISTRY

Experiment Number: 2 Title: Qualitative Analysis for Identification of Metal Cations

INTRODUCTION Based on their chemical and physical behaviour, analytically, metal cations can be classified into five groups as listed below; i) ii) iii)

iv)

v)

Analytical Group 1 Cations Ag+, Hg2+, Pb2+ Form precipitate when reacted with HCl Analytical Group 2 Cations As3+, Bi3+, Cd3+, Cu2+, Hg2+, Pb2+, Sn4+ Form precipitate when reacted acidic H2S Analytical Group 3 Cations Co3+, Fe2+, Mn2+, Ni2+, Zn2+, *Al3+, Cr3+ Form precipitate when reacted with H2S in NH3 *Al3+ is separated as Al(OH)3 Analytical Group 4 Cations Ba2+, Ca2+, Mg2+, Sr2+ Form precipitate when reacted with (NH4)2CO3 or NH4HPO4 Analytical Group 5 Cations Na+, K+ Form soluble salts when reacted with (NH4)2CO3 or NH4HPO4

OBJECTIVE The objective of this experiment is to perform qualitative analysis to identify metal cations.

METHOD Procedure A 1. 2 drops of 6 M HCl were added to 1 mL of the solution 2. The suspension was centrifuged for 1 minute due to presence of chloride precipitate. 3. 1 additional drop of 6 M HCl were added to the clear supernatant to check for complete precipitation. 4. The supernatant was centrifuged again for 1 minute due to the presence of precipitate. 5. The supernatant was decanted in a clean test tube. 6. The supernatant was saved for further testing.

Procedure B

1. 6 M of NH3 were added drop wise to the solution until it is basic. 2. The sample was placed in a boiling water bath for 3 minutes in order to remove excess NH3. 3. The volume was adjusted to 2 mL with distilled water and 10 drops of 6 M HCl were added. 4. 12 drops of thioacetamide were added to the solution and stirred. 5. The sample was heated in a boiling water bath for 10 minutes in order to allow hydrolysis of thioacetamide to occur which ended up producing H2S. 6. The sample was centrifuged for 2 minutes due to presence of sulfide precipitate and the supernatant was decanted in a clean test tube. 7. The supernatant was saved for further testing.

Procedure C 1. 3 additional drops of 1 M thioacetamide were added to the supernatant and the supernatant was heated for 5 more minutes to complete precipitation. 2. The supernatant was centrifuged and decanted into a clean test tube due to presence of precipitate. 3. The volume of the solution was adjusted to 3 mL with distilled water and 10 drops of 6 M HCl and 10 drops of 6 M NH3 were added. 4. 6 M NH3 was added drop wise until the solution is basic. 5. 5 additional drops of 6 M NH3 and 12 drops of 1 M of thioacetamide were added and the mixture was stirred thoroughly. 6. The sample was heated in boiling water for10 minutes. 7. The supernatant was centrifuged and decanted due to the presence of sulfide precipitate. Zn2+ was present in precipitate form as Zn(OH)2. 8. The supernatant was saved for further testing.

Procedure D

1. 6 M NH were added drop wise to the sample until it is basic. 2. 10-15 drops of NH were added to buffer the solution. 3. 20-30 drops of (NH ) CO were added to the sample and was stirred. 4. The test tube containing the precipitate of carbonate was transferred to warm water bath 3

3

4 2

3

of 70°C - 80°C for 2-3 minutes.

5. The sample was centrifuged and was decanted. 6. The precipitate was saved for further tests. Procedure D (i)

1. The precipitate was washed with 2 mL of distilled water and centrifuged and the wash water was decanted.

2. 6 M of acetic acid was added drop wise and was stirred to dissolve the precipitate. 3. A few more drops 6 M acetic acid was added until it is acidic. 4. 6 drops of 1 M K CrO were added to the solution and stirred. 5. The solution was centrifuged due to the presence of precipitate. 6. The supernatant was saved for further testing. 2

4

Procedure D (ii)

1. 10 drops of 1 M K C O were added to the solution and stirred for about 10 minutes. 2. The solution was centrifuged due to presence of oxalate precipitate. 2

2

4

RESULT AND ANALYSIS 1. Addition of HCl to the initial sample Mixture Known

Observation White Precipitate

Balanced Chemical Equation

Unknown I

-

-

+¿ ¿ +¿+ HCl → AgCl(s) + H Ag¿

2. Addition of H2S under acidic condition Mixture Known

Observation Black Precipitation

Balanced Chemical Equation

Unknown I

Black Precipitation

+¿ 2+¿+ H 2 S → Cu S(s) + H ¿ Cu¿

+¿ ¿ 2+¿+ H 2 S → Cu S(s) + H ¿ Cu

3. Addition of H2S under basic condition Mixture Known

Observation White Precipitation

Unknown I

White Precipitation

Balanced Chemical Equation

+¿ 2+¿+2 N H 3 +2 H 2 O → Zn(OH )2 (s) +2 NH 4 ¿ Zn¿ +¿ ¿ 2+¿+2 N H 3 +2 H 2 O → Zn(OH )2 (s) +2 NH 4 ¿ Zn

4. Addition of CO32Mixture Known

Unknown I

Observation White Precipitation

White Precipitation

Balanced Chemical Equation 2−¿ → XCO3 ( s) 2+¿+ CO3¿ X¿

X =Ba , Ca, Mg , Sr

2−¿ → XCO3 ( s) ¿ 2+¿+ CO3 ¿ X

X =Ba , Ca, Mg , Sr

5. Addition of CrO42Mixture Known

Observation White Precipitation

Balanced Chemical Equation

Unknown I

-

−¿

Mixture Known

Observation Brown precipitate

Balanced Chemical Equation

Unknown I

-

−¿

2−¿ → BaCrO 4 ( s) ¿ 2+¿+CrO 4 ¿ Ba

6. Addition of C2O422−¿ → CaCr 2 O 4 (s ) 2+¿+C 2 O 4¿ Ca¿

DISCUSSION This experiment was carried out in order to identify the metal cations by qualitative analysis. Unlike quantitative analysis, qualitative analysis focuses on identifying the components (ions) that present in the sample and the amount of the components present does not have any significance. Ideally, there are chemical tests that could be used to identify the individual ion without interference of other ions. Unfortunately, there are often complications. For instance, the formation of yellow precipitate upon addition of S2- often indicates the presence of Cd2+ in the solution. However, this reaction often masked when there are Pb2+ or Cu2+ ions present as it will react with S2- instead and will form black precipitate. Hence, before testing for any particular ion, another ions that might have interfered must be eliminate first as to avoid the “masking” effect. The elimination of ions in a solution can be accomplished by adding a precipitating agent that will selectively react with a specific ion in the solution and not react with the other ions present in the solution. The solid (precipitate) formed then can be removed through centrifugation and decanting. However, due to the fact that several ions may behave similarly, the separation of individual ions in complex mixture may be impossible. Instead, a group ions with similar property will be separated from a larger mixture and the solid formed will then be further separated and reacted in order to confirm the identity of the ions. From qualitative analysis, the component inside the known mixture and unknown mixture can be determined. Firstly, first reaction involves the addition of hydrochloric acid to the initial sample. For the known mixture, there are formation of white precipitate which indicates the presence of Ag+ ion in the mixture solution. However, in unknown 1 mixture, there are no formation of white precipitate. Hence, there are no Ag+ ion presence in the mixture solution.

Next, the second reaction is the formation of sulfide precipitate which is the product of reaction between Cu2+ with H2S under acidic condition. This formation of sulfide precipitate can be observed in the known mixture. Hence, indicating the presence of Cu2+ ions in the known mixture. Same goes to the unknown mixture 1, where there are formation of black coloured precipitate, thus, evidently shows that there are also Cu2+ ions in the unknown mixture 1. Then, the third reaction is the reaction to the supernatant of the mixture upon addition on H2S in basic condition. This reaction will lead to the formation of white precipitate which is Zn2+ ions that have reacted with H2S and Zn(OH)2 will precipitate out. This reaction occurred in both, known mixture and unknown mixture 1. This indicates the presence of Zn2+ ions in both of the mixtures. Next, the fourth reaction occurred due to addition of CO32- into the supernatant from the third reaction. If cations from Analytical Group IV are present in the mixture, it will react with the carbonate ion, resulting in the formation of white precipitate. However, the actual ion present cannot be singularly determined, the precipitate will be used later to identify the type of ion present more accurately. For Known Mixture, there are formation of white precipitate and same goes to unknown 1. Hence, both mixture contain cations from analytical group IV. However, in the supernatant, it is believed that there are Na+ or K+ ions. Then, the fifth reaction is the reaction that is required to narrow down the type of ions present in the past precipitate from the fourth reaction. It is the addition of CrO42- into the dissolved acetic acid. For the known mixture, there are formation of white precipitate, hence, indicating that there are Ba2+ ions in the known mixture. Whilst for the unknown mixture 1, there is no presence of precipitate, hence, there are no Ba2+ ions in unknown mixture 1. Furthermore, the sixth reaction is to determine the presence of Ca2+ or Mg2+ ions in known mixture and unknown mixture 1. The supernatant were reacted with K2C2O4 and if there are dark brown precipitate, it is proved that there are Ca2+ ions and if there are no precipitate formed, the available ion inside the mixture is Mg2+. There are dark brown precipitate in known mixture, hence, there are Ca2+ ions inside the known mixture. However, in unknown mixture 1, there are no formation of precipitate, hence, there are no Ca2+ ions in unknown mixture 1, but Mg2+ ion are present in unknown mixture 1. The weakness of the method in this experiment would be the mixture was not mixed thoroughly or was stirred using a contaminated glass rod, resulting in increase in contaminants which can disrupt the results. Adding too many precipitating agent also can interfere with the result as it can cause undesired side reaction. Improper separation and washing of precipitate is also one of the most common error that might have occurred. Next, heating of solution must never be boiled in the microcentrifuge tube may react with the substance or melt due to elevated temperature. Hence, heating must be carried out in the test tube.

CONCLUSION In the known mixture, there are Ag+ ions, Cu2+ ions, Zn2+ ions, Ba2+ ions and also Ca2+ ions whereas in the unknown mixture 1, there are Cu2+ ions, Zn2+ ions and Mg2+ ions.

REFERENCES 1. http://www.docbrown.info/page13/ChemicalTests 2. Murov, S. and Stedjee, B., 2004. Experiments and Exercises in Basic Chemistry. (6th Edition) New Jersey: John Wiley & Sons, Inc., 3. Greco, T.G., Rickard L. H. and Weiss G.S., 2002. Experiments in General Chemistry: Principles and Modern Applications. (8th Edition) New Jersey: Prentice Hall