Qualitative analysis of important elements from the point view of biology and toxicology

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Medical fields use a whole range of analytical methods in their specialized laboratories (clinical biochemistry, molecular biology, etc.).

By chemical analysis we mean a series of operations that are carried out with the analyzed (examined) material in order to find out:

  1. its chemical composition – qualitative analysis , evidence of individual components
  2. amount of individual components – quantitative analysis


Qualitative analysis almost always precedes quantitative analysis. An average sample is taken from the examined material, which is assessed by preliminary analysis in the dry way (appearance, color of the sample, behavior during pyroreaction – annealing in the flame, coloration of the flame, etc.) and in the wet way (evidence of chemical compounds in aqueous solutions).

Most inorganic compounds in aqueous solutions dissociate into ions – positively charged cations and negatively charged anions. For the evidence of individual ions (cations, possibly anions) in solutions, precipitation analytical reactions are most often used, which are actually a double exchange (conversion) of ions to form a slightly soluble product of the given ionic compound, which is excluded from the solution as a white precipitate. colored precipitate. The other ions remain mostly unchanged in the (mother) solution above the precipitate. However, slightly soluble ionic compounds are not absolutely insoluble in water, it is only a matter of quantitative differences in their solubilities, which can be considerable, see approximate solubilities of ionic compounds , or solubility products of ionic compounds. Tabulated solubility values ​​of inorganic compounds are given in most chemical tables. Reactions in which colored, mostly soluble, less dissociated coordination compounds, complexes , are also used to a large extent to prove individual ions .

Various systematic procedures are then used to determine individual cations and anions in mixed samples, using reactions with group reagents that divide individual ions into analytical classes. Specific ions are determined by special targeted reactions after the previous gradual elimination of other cations and anions. The most well-known analytical qualitative procedure for the determination of cations is the sulfane (formerly hydrogen sulfide) procedure. However, these systematic procedures are laborious and tedious and are currently being replaced by looser and more purposeful combinations of group and selective reactions. The analysis adapts to the individual nature of the sample.

Group reagents[edit | edit source]

Very often used group reagents, which are used for the rapid determination of cations, are: hydrochloric acid HCl, sulfuric acid H2SO4, saturated solution of hydrogen sulfide H2S in an acidic environment, white ammonium sulfide (NH4)2S, alkaline hydroxides NaOH , KOH, ammonia NH3 , NH4OH, ammonium carbonate (NH4)2CO3 , alkaline iodide KI, hydrolytic reactions (reducing the acidity of the solution by diluting it with water, adding sodium acetate).

Group reagents for detecting anions are: barium salt solution Ba(NO3)2 , silver salts AgNO3 , oxidation reaction with MnO4 or I2 , reduction reaction with I- .

Determination of cations[edit | edit source]

According to the hydrogen sulfide procedure, cations are divided into 5 analytical classes:

I. class[edit | edit source]

Ag+, Pb2+, Hg22+. The group reagent is HCl . The chlorides of these cations are insoluble in water, as are their sulfides, see tab. solubility products .

II.A class[edit | edit source]

Cu2+, Hg2+, Bi3+ . The group reagent is hydrogen sulfide H2S from an acidic environment (a small concentration of S2- is enough to precipitate the sulfides of these cations), see Ks . The chlorides of these cations are soluble in water. Sulfides are insoluble in dilute acids, in ammonium hydroxide NH4OH and in yellow ammonium sulfide (NH4)2Sx .

II.B class[edit | edit source]

As3+. Sulfides are soluble in yellow ammonium sulfide (NH4)2Sx .

III. class[edit | edit source]

Zn2+, Fe2+, Fe3+. The group reagent is white ammonium sulfide, (NH4)2S (its complete dissociation guarantees a high concentration of S2- necessary for the precipitation of sulfides of these cations, see Ks ). Sulfides are already soluble in dilute acids, they are split into hydrogen sulfide and the corresponding metal hydroxide by hydrolysis.

IV. class[edit | edit source]

Ca2+, Ba2+. The group agent is ammonium carbonate (NH4)2CO3 . The sulfides of these cations are soluble in water, the carbonates form white precipitates that dissolve in dilute and strong acids, while carbon dioxide gas escapes.

V. class[edit | edit source]

Mg2+, Li+, Na+, K+, NH4+. There is no group reagent, each cation is tested with special reactions, and the colored flame reactions of some cations might be used.

Determination of anions[edit | edit source]

In a similar way to cations, anions are divided into three analytical classes according to their reactions with barium and silver salts, and then according to the reactions and behavior of the resulting compounds with other reagents. Some literature mentions the division of anions into 4 classes (SO42-, F- form a separate class).

I. class[edit | edit source]

CO32-, PO43-, SO32-, CrO42-, SO42-, F-. They are precipitated with both barium and silver nitrate .

II. class[edit | edit source]

Cl-, I-, CN-, S2-, HS-, NO2-. They are precipitated with silver nitrate .

III. class[edit | edit source]

NO3-, ClO4-, MnO4-. They do not precipitate with any group reagent.

Solubility product[edit | edit source]

We derive the solubility product from the relationship for equilibrium constants , which characterize the equilibrium of chemical reactions in heterogeneous systems. In precipitation reactions in aqueous solutions used in qualitative analysis, an equilibrium is established between unreacted ions in the solution above the precipitate and the solid phase of the precipitate. E.g. for the slightly soluble compound (precipitate) Ag2CrO4 , arising according to the chemical equation

Cu2+ + 4NH3 ⇔ [Cu(NH3)4]2+

applies to the steady state


Since during precipitation the solution of the insoluble compound is saturated and the substance concentration of the insoluble compound is constant, the product of the equilibrium constant and the concentration of the insoluble compound will also be constant.

We get a relationship

In general, therefore, the solubility product K s is given by the product of the equilibrium concentrations of ions in the solution above the precipitate, multiplied by the stoichiometric coefficients of the given chemical reaction. The solubility products of a number of substances are tabulated.

According to the solubility product, we can assess and calculate the solubility of substances under different conditions, it depends on temperature, pH and the presence of other foreign ions in the solution. This can be used in qualitative analysis, it can be influenced according to the need of the analytical procedures of the precipitation of poorly soluble compounds for the purpose of their better separations and evidence.

Complex compounds[edit | edit source]

Complex compounds , coordination compounds (complexes) can be molecules or ions that contain a central particle, atom or ion to which ligands are bound by a coordination covalent bond . Central particles are usually atoms or ions of transition elements with unoccupied valence orbitals that can accept free electron pairs, they are electron acceptors . The cores of the d and f -elements are the best, the p and the s -elements are the worst. Ligands ( electron donors ) can be anions, e.g. Cl- – chloro, Br- – bromo, CN- – cyano, OH- – hydroxo, or even neutral molecules that have an atom with a free electron pair, e.g. H2O – aqua, NH3 – ammine, NO – nitrosyl, CO – carbonyl. The maximum number of monovalent ligands around the central particle is called the coordination number of the compounds, it is most often the number 6, 4, 8, 2. Coordination compounds can contain a complex cation, anion, or both. The nomenclature of these compounds is discussed in  [1].

Complex compounds are mostly water-soluble, less dissociated and, due to coordination bonds, differ from their original components in color and solubility. They are therefore widely used in analytical chemistry. Chemical equilibrium is established in solutions of complex compounds, e.g.

Cu2+ + 4NH3 ⇔[Cu(NH3)4]2+

In this case it applies

Kk is the stability constant of the complex; the larger the value of Kk , the more stable the complex and vice versa.

Complex compounds also include chelate complexes ( chelates ). In these complexes, the ligand is usually an organic compound that can simultaneously occupy several coordination sites around the central atom and contains several free electron pairs. A chelating agent is an organic substance that provides at least two free electron pairs to form a dative bond. Some of these reagents are used in analytical chemistry in titration determinations and otherwise, e.g. chelating agent EDTA (ethylenediaminetetraacetic acid and its salts), biuret and other. A number of chelating agents are used in medicine for acute poisoning by the cations of some divalent and trivalent metals, to bind them and remove them from the body. Physiologically significant are also the chelate structures of many enzymes , as well as eg hemoglobin , chlorophyll and other biological pigments .

Examples of some inorganic compounds important in medicine and toxicology[edit | edit source]

Ag silver colloidal silver, bactericidal effects, disinfection of wells, Sagen, amalgams
Al(OH)3 aluminum hydroxide antacid
As2O3 arsenic oxide higly toxic
BaSO4 barium sulfate use in radiology, intestibar
CaCl2 calcium chloride it is given in case of calcium deficiency
CaSO4 calcium sulfate gypsum
CO carbon monoxde toxic gas, gas poisoning
CO2 carbon dioxide blood buffering system, carbonated drinks
FeI2 ferrous iodide dsadsagiven in case of iron deficiency
HCO3- bicarbonate anion blood buffering system
H3BO3 boric acid pine water, disinfectant, component of eye drops
H2O2 hydrogen peroxide disinfectant solution
HCl hydrochloric acid contained in gastric juices
Hg Quicksilver stomatology, amalgamy
Hg2Cl2 mercuric chloride calomel, measuring electrodes in pH measurement
HgCl2 mercuric chloride highly toxic
KCl potassium chloride given in case of potassium deficiency
KCN potassium cyanide highly toxic, cyankali
KI potassium iodide iodization of drinking water and table salt
KMnO4 potassium manganate strong oxidizing agent, disinfectants
KNO2 potassium nitrite antidote for cyanide poisoning
Li2CO3 lithium carbonate psychoactive drug (mood stabilizer)
Mg(OH)2 magnesium hydroxide antacid
MgSO4 magnesium sulfate laxative effects, laxatives
Na2HPO4 sodium hydrogen phosphate blood and urine buffering system
NaCl sodium chloride saline
NaClO sodium hypochlorite disinfectant, Savo
Na2CO3 sodium carbonate water softener (soda), acidity regulator in food
NaF sodium fluoride water fluoridation, toothpaste, toxic in higher doses
Na2PO3F sodium phosphate-fluoride toothpastes
NaH2PO4 sodium dihydrogen phosphate blood and urine buffering system
NaHCO3 sodium hydrogencarbonate blood buffering system, antacid, baking powders
NH4Cl ammonium chloride in the preparation Salnatrex with a salt-free diet, salmiak - dry cells
NH4HCO3 ammonium bicarbonate baking powders
(NH4)2SO4 ammonium sulfate it is used to desalt proteins
Pb3O4(2PbO+PbO2) lead oxide toxic, rust, primer paint
SO2 sulfur dioxide toxic gas, air pollution
TiO2 titanium dioxide titanium white
ZnO zinc oxide component of some ointments

Links[edit | edit source]

References[edit | edit source]

  1. HIRŠOVÁ, Danuše. Chemické názvosloví. Základní pravidla českého, tradičního latinského a mezinárodního latinského lékopisného názvosloví. 2. edition. Praha : Univerzita Karlova v Praze, Nakladatelství Karolinum, 2004. ISBN 80-246-0761-1.

Sources[edit | edit source]

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