General Examination of the Sample Mixture
The given mixture is described in detail in the laboratory notebook (e.g. the colour of solutions and precipitates). After examining it the mixture is mixed thoroughly by shaking in a test tube closed with a bung. Other possible observations (e.g. CO2↑) are written down. The pH of the solution is measured with a pH-paper. There cannot be CO32–– or S2–-ions present in an acidic solution. Samples are always taken from the mixture by pipetting from a freshly shaken test tube.
Solubility Tests of the Sample Mixture
Cation analysis is performed on a solution made from the sample mixture. So if the original sample mixture contains precipitates, solubility test are performed in the following order, until a solvation reagent is found. In the tests, only a small amount of the sample mixture is used, mixed thoroughly and heated.
1. H2O
2. 2 M HCl
3. conc. HCl
4. 2,5 M HNO3
5. conc. HNO3
6. Aqua regia (1 part conc. HNO3 + 3 parts conc. HCl)
Solubility can also be tested in NaOH (e.g. dissolving PbSO4↓), NH3, Na2S2O3 (dissolving AgCl↓, AgBr↓ and AgI↓) and KI (dissolving HgI2↓). Insoluble matter (e.g. BaSO4↓) can be made into a soluble form by melting it with a mixture of solid Na2CO3 and K2CO3 in a crucible. From the cooled flux, anions are dissolved with water and then cations with acid.
Alkali Salt Solution Preparation and detection of ammonium ion
Anion analysis is done before cation analysis, because knowledge of the anions usually helps in the cation analysis. Most of the time anion analysis can be done with the original mixture. However, If the original mixture does not dissolve in the acid that is used for the anion analysis (in reasonable volume), the removal of cations is necessary. If necessary, a so-called alkali salt solution is prepared by boiling the original mixture combined with Na2CO3. This causes the cations of groups 1.-4. and Mg2+ to precipitate as carbonates, hydroxides, and oxides and NH4+ is removed as NH3. If no precipitate is formed, the mixture usually contains no other cations except NH4+, Na+ and K+.
3 ml of the original mixture or an appropriate amount of solid sample and 3 ml of water is placed in a boiling tube. 200-300 mg of anhydrous Na2CO3 (the size of a piece of macaroni, pH=10) is added. The solution is heated while stirring every 10-20 minutes. Water is added if necessary. At the beginning of boiling a moist pH-paper is held above the boiling tube. If the mixture contains NH4+-ions, the pH-paper turns blue from the NH3 that is released. If this occurs, the boiling is continued until all NH4+ has evaporated. The alkali salt solution is separated from the precipitate with a centrifuge and water is added to make the total volume of the solution 3 ml. The precipitate is also preserved!
Reaction With sodium hydroxide and detection of ammonium ion
0.5 ml of sample mixture is placed in a test tube, 0.5 ml of 2 M NaOH is added (solution is made basic) and then mixed. While heating in a water bath, a wetted pH paper is placed over the test tube at about 1 cm height from the mouth of the tube. If NH4+ ion is present, the pH paper turns blue due to liberated NH3 gas. A precipitate formed in the test tube indicates the presence of one or more cations other than NH4+, Na+ and K+. The opposite interpretation is not as straight forward as some other ions may be in solution under these conditions. The behavior of each cation with sodium hydroxide can be found here. The presence of anions of weak acids makes cations less soluble.
The experiment can be repeated by using a slightly larger sample, dissolving it and adding 0.1 M NaOH dropwise. Performing the experiment like this may help in determining whether one or more cations are precipitated. The colors of the precipitates, the precipitation pH and possible dissolution in excess hydroxide may give clues on the identity of the cation(s).
Reaction With Sulfuric Acid
A small amount of the original sample mixture is made into a solution in a test tube (solubility tests). 2,5 M H2SO4 is added to the solution and then the solution is heated, causing potential Pb2+, Ba2+ and Sr2+ (in very concentrated solutions also Ca2+) to precipitate as white sulfates.
Flame Test and Detecting Sodium
A nichrome wire is cleaned by submerging in conc. HCl solution and then heating in the flame of a Bunsen burner. A clean wire does not colour the flame. A cleaned wire is dipped in a mixture of conc. HCl and the original sample mixture (does not need to be a solution), brought into the flame and the colour of the flame is examined.
|
Na |
Yellow (invisible through cobalt glass) |
|
|
K |
Violet (pink through cobalt glass) |
|
|
Ca |
Brick red |
|
|
Sr |
Crimson |
|
|
Ba |
Pale green |
|
|
Cu |
Blue-green |
|
|
Pb, Bi, Sb, As |
Pale |
|
Sodium is detected with a flame test!
Detecting Potassium
1. The flame test of the alkali salt solution is examined through cobalt glass. Potassiums weak reaction in the flame test can also be seen from the original sample mixture if it does not contain ions that mask it with their stronger reactions.
2. A drop of HClO4 is added in to a drop of the alkali salt solution (or original mixture soluble in acid), and a drop of ethanol. If white KClO4↓ is formed, the analysis contains the K+ ion. If alkali salt solution is not used, it must be taken into account that ammonium ion can give the same reaction.
Reduction of Potassium Permanganate
5 drops of the alkali salt solution is placed in a test tube and made acidic with 2,5 M H2SO4. A drop of 0,02 M KMnO4 solution is added to the solution. If the MnO4– ion is reduced immediately, the analysis contains some of the following anions: S2-, Br–, I– and AsO33−. If the colour did not disappear, the solution is heated for a few minutes. Thus the potential oxalate ions (or sometimes also Cl–ions) presence causes the reduction of the MnO4– ion. If the colour of the MnO4– ion did not disappear, the listed anions cannot be present in the analysis. If original mixture is used, the precipitate that remain after adding sulfuric acid is not a concern but Hg(I), Fe(II), Sb and very slowly Cr will reduce MnO4– ion.
If the above test was done with alkali salt solution it should also be repeated using the carbonate precipitate created in the preparation of the alkali salt solution. Place carbonate precipitate in a test tube, add 1 ml of 1 M H2SO4 and heat. Any potential remaining insoluble precipitate is centrifuged from the solution. A drop of 0,02 M KMnO4 solution is added to the solution and the solution is heated. The potential disappearance of the colour of the MnO4− ion is written down.
Oxidation of Diphenylamine
In a test tube 1 drop of the untreated sample or alkali salt solution is added and made acidic with a few drops of conc. H2SO4. Then 1 drop of a diphenylamine sulfuric acid solution is added. If the analysis contains the NO3– or AsO43- ion, the solution turns deep blue. If there is no change in colour, these anions cannot be present in the analysis. The opposite interpretation is not as useful because at least Br–and I– can cause the colour change by activating atmospheric oxygen. If the untreated sample mixture is used, remaining precipitate after addition of sulfuric acid is not a concern. However, many cations can induce the oxidation reaction.