OBJECTIVES

The main objectives of this experiment are :

 To understand the rationale and the procedure behind the separation for various cations.
 To determine cation present in the unknown solution using established schemes.

INTRODUCTION

This experiment is carried out on a semi-micro scale. Very small quantities of reagents are used.
Cleanliness and a great deal of care are necessary to obtain good results. While going through the
steps analysis, keep a copy of the appropriate flow chart available for reference. The flow chart
will help to give the ‘total picture’ of where each analysis is and where it is heading. Read the
directions carefully, and understand the theory for each step. Don’t just follow directions ‘cook
book’ style, but make an effort to understand the chemical principles behind he procedures.

Heating Solutions

Frequently it will be necessary to heat a solution to speed up a reaction. Do not heat small test
tubes over Bunsen burner flames. A sudden steam will cause the solution to shoot out of the test
tube. Instead, heat test tubes in a boiling water bath. A good idea is to set up this water bath
when beginning work in the lab because it may take time to heat the water bath to the appropriate
temperature.

Stirring Solutions

Each time a reagent is added to a test tube, the solution needs to be stirred. It is important to mix
the solutions at the top and the bottom of the test tube. A stirring rod that is flattened at the
bottom can be used as a plunger to effectively mix solutions in a narrow test tubes.
Separating Solids from Solutions

Centrifuge the solutions so that the solid settles at the bottom of the test tubes. Don’t forget to
counterbalance the test tubes in the centrifuge with similar test tubes holding equivalent volumes
of liquid . let the centrifuge spin for about 30 seconds. Usually the supernatant liquid ( the liquid
above the precipitate ) can be decanted from the precipitate.

1. Never fill centrifuge tubes to its maximum capacity. Keep liquid levels at least 1 cm from
the top.
2. Label all centrifuge tubes before inserting to avoid mix-up.
3. Places the tubes in a symmetrical fashion, the objective being to keep the rotor balanced.
Fill all tubes to the same height.
4. Follow manufacturer’s directions.
5. If only one tube needs to be centrifuged, achieve balance by inserting an additional tube
(labeled as a blank) containing the same volume of liquid.

Washing Precipitates

It is almost always necessary to wash precipitates to free them from ions that might cause
confusion in later steps. To do this, add 1 or 2 ml of water to the precipitate, stir, centrifuge and
discard the wash water. Sometimes the directions will require a specific reagent in the wash
water.

Checking the pH

To check the pH of a solution, put a piece of litmus paper or Ph paper on a clean glass plate or
watch glass. Dip the stirring rod into the solution in the test tube, and touch the stirring rod to the
paper. Do not dip the test paper into the test tube. This may cause some of the indicator dye to
dissolve in the solution, and the indicator color may confuse subsequent tests.

Safety Precautions

All chemical are toxic and irritating to body tissue. Work with care and wear goggles. Wash
hands thoroughly with soap and water before leaving the laborotary
APPARATUS

- Small test tubes - Spatula

- Test tube rack - Stirring rod
- Small test tubes stopper - Distilled water bottle
- Test- tubes holders - Glass rod
- Hot plate - Dropping pipette
- Centrifuge - Litmus paper
- 250 mL beakers - vial containing unknown solution

CHEMICALS

- 0.05 M silver nitrate AgNO3(aq)

- 0.05 M copper (II) nitrate Cu( NO3)2(aq)
- 0.05 M nickel nitrate Ni( NO3)2(aq)
- 0.05 M iron (III) nitrate Fe( NO3)2(aq)
- 6 M HCl(aq)
- 6 M HNO3(aq)
- 6 M NH3(aq)
- 6 M H2SO4(aq)
- 6 M NaOH(aq)
- 6 M CH3COOH(aq)
- 0.1 M K4[Fe( CN)6]
- 0.1 M KSCN
- Unknown solution
PROCEDURE
OBSERVATION

Qualitative analysis of Cations

Step Procedure Known solution

(Be specific) Results Conclusion

1 6 M HCL White precipitate Ag+cation presence

formed

2 6 M NH3 White precipitate Ag+cation presence

+ formed in acidic
6 M HCL solution

3 6 M NaOH Brown precipitate Presence of Cu2+ or

formed Fe3+

4 6 M H2SO4 Clear-blue solution Cu2+cation presence

+ turn red-brown
NH3 precipitate

5 6 M H2SO4 Deep- red solution Fe3+cation presence

+ formed
0.1 M KSCN

6 1ml dimethyl glyoxin Rose-red precipitate Ni2+cation presence

formed
 QUESTIONS

1. Write out the balanced net ionic equations for all the observations in the analysis of
the known cationic solution. Indicate the color of the ions and precipitate underneath
the chemical species.

Step Cation Colour Balanced chemical equation

White
1 Ag+ precipitate 2Ag+ +2HCl → 2AgCl + H2

White
2 Ag+ precipitate AgCl + 2NH4OH → Ag(NH3)Cl + 2H2O
Ag(NH3)2Cl + 2HCl → AgCl + 2NH4Cl

Brown
3 Cu2+ or Fe3+ precipitate FeCl3 + 3NaOH → Fe(OH)3 + 3NaCl
CuCl2 + 2NaOH → Cu(OH)2 + 2NaCl

Red-brown
4 Cu2+ precipitate Cu(OH)2 + H2SO4 → CuSO4 + 2H2O
CuSO4 + NH4OH → NH4SO4 + Cu(OH)2
Cu(OH)2 + 2CH3COOH → Cu(CH3COO)2
+ 2H2O
2Cu(CH3COO)2 + K4[Fe(CN)6] →
Cu2[Fe(CN)6) + 4KCH3COO

Deep-red
5 Fe3+ precipitate 2Fe(OH)3 + 3H2SO4 → Fe2(SO4)3 + 6H2O
 Rose- red
6 Ni2+ precipitate Ni2+ + 2C4H8N2O2 → Ni(C4H8N2O2)2

2. What is the precipating reagent for silver (Ag+) ? Would a solution of NaCl works
as well? Why or Why not ?

Cl2+ is the precipitating reagent for the (Ag+), a solution NaCl would also works as well, since it
provides the chloride ion in solution
DISCUSSION

This experiment revolved around the ability to develop and use strategies in order to identify
four cations from an aqueos solution. The use of a centrifuge is also an important concept to
this lab, because the centrifuge will aid in the the removal of the precipitated ions by separating
the pallet from the supernatant and the identification of the cations, overall the knowledge of
solubility rules, especially the solubility of salts in aqueous solution, is also an important concept
of this lab.

The separation process began with the solution containing the cations. First step was the
addition of HCl. Adding HCl is necessary because it will aid the separation of Ag+ from Fe3+,
Cu2+, and Ni2+ . the addition of HCl aids in the separation of said cations , because the reaction
that occurs when HCl is added produce chlorides of the cations., so a white precipitate will form

The Fe3+ ion is readily identified in a dilute nitric acid solution through the blood red color of its
thiocyanate complex. A large excess of reagent should be avoided.

Fe3+ (aq) + SCN- (aq) → Fe(SCN)2+(aq)

The very distinct deep blue color of the copper ammonia complex can be used to identify Cu2+.
This identification can be carried out to presence of other cations which form either colorless
ammonia complexes or white precipitates. Cu2+ forms a very insoluble reddish-brown
hexacyanoferrate (II)

2Cu2+(aq) + Fe(CN)4-(aq) → Cu2Fe(CN)6(s)

Nickel (II) is the of the easiest cations to identify. Ni2+ forms rose-red precipitate with
dimethylglyxomine in a buffered acid solution. A bright red precipitate is positive test for Ni2+.
CONCLUSION

In conclusion, qualitative analysis of cations Ag+, Fe3+, Cu2+ and Ni2+ can be determined by
performing this experiment through the procedure that have been stated using the suitable
precipitating reagent that suitable for each cations. We successful achieved the objective of this
experiment.

REFERENCES

 Experiment 2, qualitative analysis of cations Ag2+, Fe3+, Cu2+ and Ni2+, lab manual
inorganic chemistry, laboratory experiments for inorganic chemistry.
 Cotton, F.A. and Wilkinson, G. (1995) Basic Inorganic Chemistry
 www.wiredchemist.com
 https://www.chemguide.co.uk