Classical qualitative inorganic analysis is a method of
analytical chemistry
Analytical chemistry studies and uses instruments and methods to separate, identify, and quantify matter. In practice, separation, identification or quantification may constitute the entire analysis or be combined with another method. Separati ...
which seeks to find the
elemental
An elemental is a mythic being that is described in occult and alchemical works from around the time of the European Renaissance, and particularly elaborated in the 16th century works of Paracelsus. According to Paracelsus and his subsequent fo ...
composition of inorganic compounds. It is mainly focused on detecting
ion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...
s in an
aqueous
An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, or sodium chloride (NaCl), in water would be re ...
solution
Solution may refer to:
* Solution (chemistry), a mixture where one substance is dissolved in another
* Solution (equation), in mathematics
** Numerical solution, in numerical analysis, approximate solutions within specified error bounds
* Soluti ...
, therefore materials in other forms may need to be brought to this state before using standard methods. The solution is then treated with various
reagents to test for
reactions
Reaction may refer to a process or to a response to an action, event, or exposure:
Physics and chemistry
*Chemical reaction
*Nuclear reaction
*Reaction (physics), as defined by Newton's third law
*Chain reaction (disambiguation).
Biology and me ...
characteristic of certain ions, which may cause color change,
precipitation
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
and other visible changes.
Qualitative inorganic analysis is that branch or method of analytical chemistry which seeks to establish the elemental composition of inorganic compounds through various reagents.
Physical appearance of inorganic salts
Detecting cations
According to their properties,
cation
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
s are usually classified into six groups.
Each group has a common reagent which can be used to separate them from the solution. To obtain meaningful results, the separation must be done in the sequence specified below, as some ions of an earlier group may also react with the reagent of a later group, causing ambiguity as to which ions are present. This happens because cationic analysis is based on the
solubility product Solubility equilibrium is a type of dynamic equilibrium that exists when a chemical compound in the solid state is in chemical equilibrium with a solution of that compound. The solid may dissolve unchanged, with dissociation, or with chemical reacti ...
s of the ions. As the cation gains its optimum concentration needed for precipitation it precipitates and hence allowing us to detect it. The division and precise details of separating into groups vary slightly from one source to another; given below is one of the commonly used schemes.
1st analytical group of cations
The ''1st analytical group of cations'' consists of ions which form insoluble
chloride
The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride salts ...
s. As such, the group reagent to separate them is
hydrochloric acid
Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid
Acid strength is the tendency of an acid, symbol ...
, usually used at a
concentration
In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: '' mass concentration'', ''molar concentration'', ''number concentration'', an ...
of 1–2
M. Concentrated HCl must not be used, because it forms a soluble complex (
4">bCl4sup>2−) with
Pb2+. Consequently, the
Pb2+ ion would go undetected.
The most important cations in the 1st group are
Ag+,
Hg, and
Pb2+. The chlorides of these elements cannot be distinguished from each other by their colour - they are all white solid compounds. PbCl
2 is soluble in hot water, and can therefore be differentiated easily. Ammonia is used as a reagent to distinguish between the other two. While AgCl dissolves in ammonia (due to the formation of the complex ion
3)2">g(NH3)2sup>+), Hg
2Cl
2 gives a black precipitate consisting of a mixture of chloro-mercuric amide and elemental mercury. Furthermore, AgCl is reduced to silver under light, which gives samples a violet colour.
PbCl
2 is far more soluble than the chlorides of the other two ions, especially in hot water. Therefore, HCl in concentrations which completely precipitate Hg and Ag
+ may not be sufficient to do the same to Pb
2+. Higher concentrations of Cl
− cannot be used for the before mentioned reasons. Thus, a filtrate obtained after first group analysis of Pb
2+ contains an appreciable concentration of this cation, enough to give the test of the second group, viz. formation of an insoluble sulfide. For this reason, Pb
2+ is usually also included in the 2nd analytical group.
This group can be determined by adding the salt in water and then adding dilute hydrochloric acid. A white precipitate is formed, to which ammonia is then added. If the precipitate is insoluble, then Pb
2+ is present; if the precipitate is soluble, then Ag
+ is present, and if the white precipitate turns black, then Hg is present.
Confirmation test for Pb
2+:
:Pb
2+ + 2 KI → PbI
2 + 2 K
+
:Pb
2+ + K
2CrO
4 → PbCrO
4 + 2 K
+
Confirmation test for Ag
+:
:Ag
+ + KI → AgI + K
+
:2Ag
+ + K
2CrO
4 → Ag
2CrO
4 + 2 K
+
Confirmation test for Hg:
:Hg + 2 KI → Hg
2I
2 + 2 K
+
:2 Hg + 2 NaOH → 2 HgO + 2 Na
+ + H
2O
2nd analytical group of cations
The ''2nd analytical group of cations'' consists of ions which form acid-insoluble
sulfides. Cations in the 2nd group include: Cd
2+,
Bi3+,
Cu2+,
As3+, As
5+,
Sb3+, Sb
5+, Sn
2+, Sn
4+ and Hg
2+. Pb
2+ is usually also included here in addition to the first group. Although these methods refer to solutions that contain sulfide (S
2−), these solutions actually only contain H
2S and
bisulfide
Bisulfide (or bisulphide in British English) is an inorganic anion with the chemical formula HS− (also written as SH−). It contributes no color to bisulfide salts, and its salts may have a distinctive putrid smell. It is a strong base. Bisul ...
(HS
−). Sulfide (S
2−) does not exist in appreciable concentrations in water.
The reagent used can be any substance that gives S
2− ions in such solutions; most commonly used are
hydrogen sulfide
Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. The unde ...
(at 0.2-0.3 M),
thioacetamide
Thioacetamide is an organosulfur compound with the formula C2 H5 N S. This white crystalline solid is soluble in water and serves as a source of sulfide ions in the synthesis of organic and inorganic compounds. It is a prototypical thioamide.
...
(at 0.3-0.6 M), addition of hydrogen sulfide can often prove to be a lumbersome process and therefore sodium sulfide can also serve the purpose. The test with the sulfide ion must be conducted in the presence of dilute HCl. Its purpose is to keep the sulfide ion concentration at a required minimum, so as to allow the precipitation of 2nd group cations alone. If dilute acid is not used, the early precipitation of 4th group cations (if present in solution) may occur, thus leading to misleading results. Acids beside HCl are rarely used. Sulfuric acid may lead to the precipitation of the 5th group cations, whereas nitric acid oxidises the sulfide ion in the reagent, forming colloidal sulfur.
The precipitates of these cations are almost indistinguishable, except for
CdS
The compact disc (CD) is a digital optical disc data storage format that was co-developed by Philips and Sony to store and play digital audio recordings. In August 1982, the first compact disc was manufactured. It was then released in Octo ...
, which is yellow. All the precipitates, except for
HgS, are soluble in dilute nitric acid. HgS is soluble only in
aqua regia, which can be used to separate it from the rest. The action of ammonia is also useful in differentiating the cations. CuS dissolves in ammonia forming an intense blue solution, whereas CdS dissolves forming a colourless solution. The sulfides of As
3+, As
5+, Sb
3+, Sb
5+, Sn
2+, Sn
4+ are soluble in yellow
ammonium sulfide
Ammonium hydrosulfide is the chemical compound with the formula .
Composition
It is the salt derived from the ammonium cation and the hydrosulfide anion
An ion () is an atom or molecule with a net electrical charge.
The charge of an e ...
, where they form
polysulfide
Polysulfides are a class of chemical compounds containing chains of sulfur atoms. There are two main classes of polysulfides: inorganic and organic. Among the inorganic polysulfides, there are ones which contain anions, which have the general formu ...
complexes.
This group is determined by adding the salt in water and then adding dilute hydrochloric acid (to make the medium acidic) followed by hydrogen sulfide gas. Usually it is done by passing hydrogen sulfide over the test tube for detection of 1st group cations. If it forms a reddish-brown or black precipitate then Bi
3+, Cu
2+, Hg
2+ or Pb
2+ is present. Otherwise, if it forms a yellow precipitate, then Cd
2+ or Sn
4+ is present; or if it forms a brown precipitate, then Sn
2+ must be present; or if a red orange precipitate is formed, then Sb
3+ is present.
:Pb
2+ + K
2CrO
4 → PbCrO
4 + 2 K
+
Confirmation test for copper:
:2 Cu
2+ + K
4 6">e(CN)6+ CH
3COOH → Cu
2 6">e(CN)6+ 4 K
+
:Cu
2+ + 2 NaOH → Cu(OH)
2 + 2 Na
+
:Cu(OH)
2 → CuO + H
2O (endothermic)
Confirmation test for bismuth:
:Bi
3+ + 3 KI (in excess) → BiI
3 + 3 K
+
:BiI
3 + KI → K
4">iI4:Bi
3+ + H
2O (in excess) → BiO + 2 H
+
Confirmation test for mercury:
:Hg
2+ + 2 KI (in excess) → HgI
2 + 2 K
+
:HgI
2 + 2 KI → K
2 4">gI4(red precipitate dissolves)
:2 Hg
2+ + SnCl
2 → 2 Hg + SnCl
4 (white precipitate turns gray)
3rd analytical group of cations
The ''3rd analytical group of cations'' includes ions which form hydroxides that are insoluble even at low concentrations.
Cations in the 3rd group are, among others:
Fe2+, Fe
3+,
Al3+, and
Cr3+.
The group is determined by making a solution of the salt in water and adding
ammonium chloride and ammonium hydroxide. Ammonium chloride is added to ensure low concentration of hydroxide ions.
The formation of a reddish-brown precipitate indicates Fe
3+; a gelatinous white precipitate indicates
Al3+; and a green precipitate indicates Cr
3+ or Fe
2+. These last two are distinguished by adding sodium hydroxide in excess to the green precipitate. If the precipitate dissolves, Cr
3+ is indicated; otherwise, Fe
2+ is present.
4th analytical group of cations
The 4th analytical group of cations includes ions that precipitate as sulfides at pH 9. The reagent used is
ammonium sulfide
Ammonium hydrosulfide is the chemical compound with the formula .
Composition
It is the salt derived from the ammonium cation and the hydrosulfide anion
An ion () is an atom or molecule with a net electrical charge.
The charge of an e ...
or Na
2S 0.1 M added to the ammonia/ammonium chloride solution used to detect group 3 cations.
It includes:
Zn2+,
Ni2+,
Co2+, and
Mn2+.
Zinc
Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...
will form a white precipitate,
nickel
Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow to ...
and
cobalt
Cobalt is a chemical element with the symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. The free element, pr ...
a black precipitate and
manganese
Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...
a brick/flesh colored precipitate.
Dimethylglyoxime
Dimethylglyoxime is a chemical compound described by the formula CH3C(NOH)C(NOH)CH3. Its abbreviation is dmgH2 for neutral form, and dmgH− for anionic form, where H stands for hydrogen. This colourless solid is the dioxime derivative of the dike ...
can be used to confirm nickel presence, while
ammonium thiocyanate
Ammonium thiocyanate is an inorganic compound with the formula NH4SCN. It is the salt of the ammonium cation and the thiocyanate anion.
Uses
Ammonium thiocyanate is used in the manufacture of herbicides, thiourea, and transparent artificial resin ...
in ether will turn blue in the presence of cobalt. This group is sometimes denoted as IIIB since groups III and IV are tested for at the same time, with the addition of sulfide being the only difference.
4th analytical group of cations
The ''4th analytical group of cations'' includes ions which form sulfides that are insoluble at high concentrations. The reagents used are H
2S in the presence of NH
4OH. NH
4OH is used to increase the concentration of the sulfide ion, by the common ion effect - hydroxide ions from NH
4OH combine with H
+ ions from H
2S, which shifts the equilibrium in favor of the ionized form:
: 2 +
: +
: +
They contain
Zn2+,
Mn2+,
Ni2+ and
Co2+
5th analytical group of cations
Ions in 5th analytical group of cations form
carbonate
A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate g ...
s that are insoluble in water. The reagent usually used is
(NH4)2CO3 (at around 0.2 M), with a neutral or slightly basic pH. All the cations in the previous groups are separated beforehand, since many of them also form insoluble carbonates.
The most important ions in the 5th group are
Ba2+,
Ca2+, and
Sr2+. After separation, the easiest way to distinguish between these ions is by testing flame colour: barium gives a yellow-green flame, calcium gives brick red, and strontium, crimson red.
6th analytical group of cations
Cations which are left after carefully separating previous groups are considered to be in the sixth analytical group. The most important ones are
Mg2+,
Li+,
Na+ and
K+. All the ions are distinguished by flame color: lithium gives a red flame, sodium gives bright yellow (even in trace amounts), potassium gives violet, and magnesium, colorless (although magnesium metal burns with a bright white flame). Magnesium can also be distinguished from other cations in this group by adding sodium hydroxide to drive the pH to 11 or higher, which selectively precipitates Mg(OH)
2.
Detecting anions
1st analytical group of anions
The ''1st group of anions'' consist of
CO,
HCO,
CH3COO−,
S2−,
SO,
and
NO. The reagent for Group 1 anions is dilute hydrochloric acid (HCl) or dilute sulfuric acid (H
2SO
4).
* Carbonates give a brisk effervescence with dilute H
2SO
4 due to the release of CO
2, a colorless gas which turns
limewater milky due to formation of CaCO
3 (
carbonatation
Carbonatation is a chemical reaction in which calcium hydroxide reacts with carbon dioxide and forms insoluble calcium carbonate:
:Ca(OH)2CO2->CaCO3H_2O
The process of forming a carbonate is sometimes referred to as "carbonation", although t ...
). The milkiness disappears on passing an excess of the gas through the lime water, due to formation of Ca(HCO
3)
2.
* Acetates give the vinegar-like smell of CH
3COOH when treated with dilute H
2SO
4. A blood red colouration is produced upon addition of yellow FeCl
3, due to formation of
iron(III) acetate
Ferric acetate is the acetate salt of the coordination complex e3O(OAc)6(H2O)3 (OAc− is CH3CO2−). Commonly the salt is known as "basic iron acetate". The formation of the red-brown complex was once used as a test for ferric ions.
Struct ...
.
* Sulfides give the rotten egg smell of H
2S when treated with dilute H
2SO
4. The presence of sulfide is confirmed by adding
lead(II) acetate
Lead(II) acetate (Pb(CH3COO)2), also known as lead acetate, lead diacetate, plumbous acetate, sugar of lead, lead sugar, salt of Saturn, or Goulard's powder, is a white crystalline chemical compound with a slightly sweet taste. Like many other l ...
paper, which turns black due to the formation of PbS. Sulfides also turn solutions of red
sodium nitroprusside
Sodium nitroprusside (SNP), sold under the brand name Nitropress among others, is a medication used to lower blood pressure. This may be done if the blood pressure is very high and resulting in symptoms, in certain types of heart failure, and d ...
purple.
* Sulfites produce SO
2 gas, which smells of burning sulfur, when treated with dilute acid. They turn acidified K
2Cr
2O
7 from orange to green.
* Thiosulfates produce SO
2 gas when treated with dilute acid. In addition, they form a cloudy precipitate of
sulfur
Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...
.
* Nitrites give reddish-brown fumes of NO
2 when treated with dilute H
2SO
4. These fumes cause a solution of
potassium iodide
Potassium iodide is a chemical compound, medication, and dietary supplement. It is a medication used for treating hyperthyroidism, in radiation emergencies, and for protecting the thyroid gland when certain types of radiopharmaceuticals are u ...
(KI) and
starch
Starch or amylum is a polymeric carbohydrate consisting of numerous glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants for energy storage. Worldwide, it is the most common carbohydrate in human diets ...
to turn blue.
2nd analytical group of anions
The ''2nd group of anions'' consist of
Cl−,
Br−,
I−,
NO and
CO. The group reagent for Group 2 anion is concentrated sulfuric acid (H
2SO
4).
After addition of the acid, chlorides, bromides and iodides will form precipitates with
silver nitrate
Silver nitrate is an inorganic compound with chemical formula . It is a versatile precursor to many other silver compounds, such as those used in photography. It is far less sensitive to light than the halides. It was once called ''lunar causti ...
. The precipitates are white, pale yellow, and yellow, respectively. The silver halides formed are completely soluble, partially soluble, or not soluble at all, respectively, in aqueous ammonia solution.
Chlorides are confirmed by the ''
chromyl chloride
Chromyl chloride is the inorganic compound with the formula CrO2Cl2. It is a reddish brown compound that is a volatile liquid at room temperature, which is unusual for transition metal complexes.
Preparation
Chromyl chloride can be prepared by th ...
test''. When the salt is heated with K
2Cr
2O
7 and concentrated H
2SO
4, red vapours of chromyl chloride (CrO
2Cl
2) are produced. Passing this gas through a solution of NaOH produces a yellow solution of
Na2CrO4. The acidified solution of Na
2CrO
4 gives a yellow precipitate with the addition of
(CH3COO)2Pb.
Bromides and iodides are confirmed by the ''layer test''. A sodium carbonate extract is made from the solution containing bromide or iodide, and
CHCl3 or
is added to the solution, which separates into two layers: an orange colour in the or layer indicates the presence of Br
−, and a violet colour indicates the presence of I
−.
Nitrates give brown fumes with concentrated H
2SO
4 due to formation of NO
2. This is intensified upon adding copper turnings. Nitrate ion is confirmed by adding an aqueous solution of the salt to FeSO
4 and pouring concentrated H
2SO
4 slowly along the sides of the test tube, which produces a brown ring around the walls of the tube, at the junction of the two liquids caused by the formation of .
Upon treatment with concentrated sulfuric acid, oxalates yield colourless CO
2 and CO gases. These gases burn with a bluish flame and turn lime water milky. Oxalates also decolourise KMnO
4 and give a white precipitate with CaCl
2.
3rd analytical group of anions
The ''3rd group of anions'' consist of
SO,
PO and
BO. They react neither with concentrated nor diluted H
2SO
4.
* Sulfates give a white precipitate of BaSO
4 with BaCl
2 which is insoluble in any acid or base.
* Phosphates give a yellow crystalline precipitate upon addition of HNO
3 and
ammonium molybdate Ammonium molybdate can refer to:
*Ammonium orthomolybdate, (NH4)2MoO4
*Ammonium heptamolybdate, (NH4)6Mo7O24, usually encountered as the tetrahydrate
*Ammonium phosphomolybdate, (NH4)3PMo12O40
*Ammonium tetrathiomolybdate, (NH4)2MoS4 this chemical i ...
.
* Borates give a green flame characteristic of
ethyl borate when ignited with concentrated H
2SO
4 and ethanol.
Modern techniques
Qualitative inorganic analysis is now used only as a
pedagogical
Pedagogy (), most commonly understood as the approach to teaching, is the theory and practice of learning, and how this process influences, and is influenced by, the social, political and Developmental psychology, psychological development of le ...
tool. Modern techniques such as
atomic absorption spectroscopy
Atomic absorption spectroscopy (AAS) and atomic emission spectroscopy (AES) is a spectroanalytical procedure for the quantitative determination of chemical elemlight) by free atoms in the gaseous state. Atomic absorption spectroscopy is based o ...
and
ICP-MS
Inductively coupled plasma mass spectrometry (ICP-MS) is a type of mass spectrometry that uses an inductively coupled plasma to ionize the sample. It atomizes the sample and creates atomic and small polyatomic ions, which are then detected. It is ...
are able to quickly detect the presence and concentrations of elements using a very small amount of sample.
Sodium carbonate test
The sodium carbonate test (not to be confused with sodium carbonate extract test) is used to distinguish between some common metal ions, which are precipitated as their respective carbonates. The test can distinguish between copper (Cu), iron (Fe), and calcium (Ca), zinc (Zn) or lead (Pb). Sodium carbonate solution is added to the salt of the metal. A blue precipitate indicates Cu
2+ ion. A dirty green precipitate indicates Fe
2+ ion. A yellow-brown precipitate indicates Fe
3+ ion. A white precipitate indicates Ca
2+, Zn
2+, or Pb
2+ ion. The compounds formed are, respectively,
copper(II) carbonate
Copper(II) carbonate or cupric carbonate is a chemical compound with formula . At ambient temperatures, it is an ionic solid (a salt) consisting of copper(II) cations and carbonate anions .
This compound is rarely encountered because it is di ...
,
iron(II) carbonate
Iron(II) carbonate, or ferrous carbonate, is a chemical compound with formula , that occurs naturally as the mineral siderite. At ordinary ambient temperatures, it is a green-brown ionic solid consisting of iron(II) cations and carbonate anions ...
,
iron(III) oxide
Iron(III) oxide or ferric oxide is the inorganic compound with the formula Fe2O3. It is one of the three main oxides of iron, the other two being iron(II) oxide (FeO), which is rare; and iron(II,III) oxide (Fe3O4), which also occurs naturally ...
,
calcium carbonate,
zinc carbonate
Zinc carbonate is the inorganic compound with the formula ZnCO3. It is a white solid that is insoluble in water. It exists in nature as the mineral smithsonite. It is prepared by treating cold solutions of zinc sulfate with potassium bicarbonat ...
, and
lead(II) carbonate
Lead(II) carbonate is the chemical compound with the chemical formula . It is a white solid with several practical uses, despite its toxicity. It occurs naturally as the mineral cerussite.
Structure
Like all metal carbonates, lead(II) carbonate a ...
. This test is used to precipitate the ion present as almost all carbonates are insoluble. While this test is useful for telling these cations apart, it fails if other ions are present, because most metal carbonates are insoluble and will precipitate. In addition, calcium, zinc, and lead ions all produce white precipitates with carbonate, making it difficult to distinguish between them. Instead of sodium carbonate,
sodium hydroxide may be added, this gives nearly the same colours, except that lead and zinc hydroxides are soluble in excess alkali, and can hence be distinguished from calcium. See qualitative inorganic analysis for the complete sequence of tests used for qualitative cation analysis.
See also
*
Flame test
A flame test is an analytical procedure used in chemistry to detect the presence of certain elements, primarily metal ions, based on each element's characteristic flame emission spectrum (which may be affected by the presence of chloride io ...
*
Bead test
References
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Analytical chemistry