Although this kind of analysis has been superseded by instrument techniques which have made application of the periodic table and division into groups obsolete, it is still useful to realise how much work the first analytical chemists had to do to rationalise quantitative analysis of anions and cations, and how this enabled them to come up with a preocedure, albeit a detailed and tedious one, for identifying the majority of the most common anions and cations.
Having even a superficial knowledge of quantitative analysis is essential in that it helps form the kind of mindset necessary for an analytical approach. It also provides a response to the first basic question we ask ourselves when faced with an unknown sample: what elements make up this sample?
Qualitative analysis consists in the analysis of the cations and anions found in the samples under examination.
Different methods have been designed based on the different chemical / physical characteristics of the analyte to be identified.
Let’s take a mixture containing, for example:
Ag+ , Pb++ , Hg2++ , Al+++ , Fe+++ , Cr+++ , Mn++
There is no reactive that reacts in one specific way with the various ions, in fact the anions and cations interfere with each other.
In practice, the ions are not separated individually but into groups, based on their behaviour with regard to specific reactives, called group or selective reactives, ones that induce the same reaction in all the elements belonging to that group. This is why this kind of analysis is termed systematic analysis.
To carry out a complete analysis you need a good knowledge (theory) of all the reactions that anions or cations can be involved in, and you need to acquire good manual skills (practice) to avoid making mistakes.
Systematic research into cations using wet analysis involves dissolving the substance in an appropriate solvent first. Before this however, we need to check for the presence of certain substances like complex cyanides, silicates and silicon dioxide which interfere with the analysis, getting rid of them if necessary.
If the initial sample is a solid, it is also necessary to transform it into a solution so a step by step approach is required: cold water, hot water, acidulated water, aqua regis (nitric acid and hydrochloric acid usually in a volume ratio of 1:3) and sulphur nitric attack (nitric acid and sulphuric acid in a volume ratio of 1:3 and hot).
Solubility tests are carried out on small amounts of substance (about 30 mg in a test tube) first at room temperature, then with warm water, then with diluted followed by concentrated HCL (Hydrochloric acid), then with diluted followed by concentrated nitric acid (HNO3) and finally with aqua regis if necessary (HCl c + HNO3 c 3:1 V/V).
These tests are very useful because they enable us to exclude, with the help of the salt solubility table, the presence of specific analytes.
if the sample is soluble in cold water it presupposes there are no insoluble compounds like AgCl, AgBr, etc present. It does not, however, preclude the possibility that there are Ag+ compounds present in the form of AgNO3, soluble in H2O;
If we imagine, on the other hand, that the sample is soluble in hot water, then the sample contains analytes with an average Kps (solubility equilibrium) value; a typical example is that of lead chloride (PbCl2);
If the sample dissolves completely when treated with HNO3, any analytes present are those which are soluble in acids like ferric hydroxide (Fe(OH)3) and ferrous hydroxide (Fe(OH)2) and reducing ones which are not attacked by acids like vintage sulphides like NiS and CoS.
Once the sample has been turned into a solution, systematic analysis can be got under way. We start by adding the first reactive, chloric acid, which leads to the precipitation of the first four cations: Ag+ , Pb++ , Hg2++, in the form of chlorides and WO3 which belong to the sixth periodic group.
After adding the group reactive, the next step is the digestion of the precipitate (if necessary), which contributes to the growth of the precipitate and thus facilitates the next step, which is filtration of the precipitate. The precipitate is filtered using filter paper and the solution used for analysing the next group.
The precipitate is rinsed in water containing a suitable electrolyte. (In general, ammonium chloride is used to prevent loss through dissolving and it is only at this point that the procedure for analysing the individual analytes begins.
Hot water is added to the precipitate to help dissolve the lead chloride because, as it has a Kps value of 10-4 it can only be dissolved by raising the temperature. At this point the precipitate is filtered and the lead recognition test carried out. When the solution is cooled down, the white lead chloride reprecipitates in the form of siliceous needles; alternatively, sodium chromate can be added and then yellow lead chromate can be seen.
Unlike cations, there is no systematic analysis for ions, so this means they need to be searched for using wet analysis.
Analysis of anions starts with a basic Na2CO3 (sodium carbonate) solution which serves to eliminate any colour interference from the presence of iron, manganese or nickel. The solution is then analysed for the presence of reducing anions and oxidising anions.
Part of the solution is acidified with H2SO4 (sulphuric acid) and heated to about 60 – 70°C so that excess carbonates (CO32-) and bicarbonates (HCO3) are eliminated in the form of CO2 (carbon dioxide). Potassium permanganate (KMnO4), a powerful oxidising agent, is then added to the sulfuric acid solution which makes it turn violet. In a reducing environment, the solution loses its colour.
3MnO42- + 4H+ → 2MnO4- + MnO2 + 2H2O;
which could indicate the presence of reducing ions like:
I-; Br-, S2-; SO32-; S2O52-.
Search for oxidising anions
Hydrogen sulphide is added to part of the solution and this is a strong reducing agent. The formation of a sulphur precipitate indicates the presence of reducing anions. HI (hydrogen iodide) can also be added to the solution and in the presence of oxidising molecules it reduces to I2 which turns the solution yellow. As an alternative, starch sauce can be added which is colourless in the presence of I- blue with I2.
These tests are based on the fact that Na2B4O7 • 10 H2O (Borax) in its molten state reacts with various metal compounds to form glass substances which are a specific colour whereas the borax bead itself is colourless. Once the bead has formed it is brought into contact with the substance under examination and then heated over the flame again. Many salts decompose to produce the respective oxide which then colours the bead.
Na2B4O7 • 10 H2O = Na2B4O7 + 10 H2O
Na2B4O7 • 10 H2O = 2 NaBO2 + B2O3→DECOMPOSITION OF BORAX OVER THE FLAME (colourless)
Cr2(SO4)3 = Cr2O3 + 3 SO3
Cr2O3 + 3 B2O3 = 2 Cr(BO2)3 (green coloured).
The borax bead test consists in melting some borax crystals into a small bead which a few crystals of the compound under question can then attach to. The bead is then heated in the Bunsen flame, first in the reducing region and then in the oxidising region. The ions present in the compound lend the beads a specific colour thus testifying to their presence.
Source: Wikipedia Territorio Scuola
These tests depend on the property typical to many salts, especially those in groups I and II, of changing the colour of the Bunsen flame. The colour derives from the emission of light radiation which is produced when the excited valence electrons in the atom move from one orbital to another with a higher level of energy. The only atoms or ions that can produce coloured compounds are those with an external electronic configuration which allows for movement of the electrons.
The wave length of the radiation which is emitted is specific to the individual element.
Flame tests and borax bead tests have been superceded since the advent of instrumental analysis. These instruments can be found in any modern laboratory and have rendered the old tests obsolete. However, it is important to understand the historical significance of finding analytical solutions to complex chemical problems like determining what elements are contained in an unknown sample and in what quantities. The main instrumental methods employed today for determining metal elements are: Atomic Absorption Spectrometry, AAS and Inductively Coupled Mass Spectrometry (ICP-MS).
13. Mohr method
14. Vohlard method