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Methodical pointing

June 6, 2024

Methodical instruction

for the students of pharmaceutical faculty

LESSON 1 (practical – 6 hours)

THEME: First group of cations (acid-basic classification of cations). Theory of strong electrolytes.

AIM: To acquaint oneself with the regulations of work and prevention of accidents in the laboratory of analytical chemistry, to study an apparatus and basic receptions of qualitative semimicroanalysis, to learn classification of cations on groups. To learn the characteristic reactions of cations: Na⁺, K⁺, NH₄⁺ and conditions of their implementation.

PROFESSIONAL MOTIVATION OF STUDENTS

Analytical chemistry occupies one of major places in trade education of pharmacist. As science she consists of qualitative and quantitative analysis. A qualitative analysis can be executed iumber of different ways. Normatively-analytical document on substances and medications contain an obligatory test “Identification”. He foresees the conduction of qualitative reactions on cations, anions or functional groups which are in the molecule of organic matter. A study of qualitative reactions, study of technique of their conduction is a primary stage of education of future pharmacist.

The cations of first analytical group – K⁺, Na⁺, NH₄⁺ enter in the composition of medicinal preparations in the different prepared forms. Pharmacist–analyst must be able to execute an analysis for confirmation of qualitative composition of preparation.

PROGRAM OF SELF-PREPARATION

1.The regulations of the work and prevention of accidents in an analytical laboratory.

2.Basic concepts of chemical analysis: specific and selective reactions, chemical reagent and demands to him, sensitivity of chemical reaction (determined minimum, minimum concentration, maximum dilution).

3.Methods of implementation of analytical reactions are after the amount of the explored substance.

4.Technique of execution of major operations of semimicroanalysis: heating, evaporation, precipitation, centrifugation, filtration, washing of precipitate.

5.Qualitative analysis: fractional and systematic.

6.Types of classifications of cations.

7.Acid-basic classification. Qualitative reactions of cations of first analytical group.

8.Basic concepts of theory of strong electrolytes: ionic force of solution, activity, concentration, coefficient of activity of ion and middle coefficient of activity.

10. A law of preservation the masses, his application in analytical chemistry.

11. Constants of equilibrium: thermodynamics, concentrations, conditional.

TESTS AND REAL-LIFE SITUATIONS FOR SELF-ASSESSMENT

1. As a result of reaction of ions of ammonium with sodium heksanitrocobaltat (III) appears precipitate formula of which:

A. (NH₄)₂Na[Co(NO₂)].

B. (NH₄)₂Na[Co(NO₃)₆].

C. (NH₄)Na[Co(NO)₆].

D. (NH₄)Na[Co(NO₂)₆].

E. (NH₄)₂Na[Co(NO₂)₆].

2. As a result of reaction of ions of ammonium in a track’s quantity with the reagent of Neslera, solution acquires fa … colour:

A. Will remain transparent.

B. Black.

C. Yellow.

D. Brown.

E. Red.

3. Pharmacopea’s reaction of potassium ions with of sodium hidrogentartratic accompanies of appears of sediment which has … color:

A. Black.

B. White.

C. Grey.

D. Blue.

E. Green.

1. Calculate the molarity (molar concentration) and normality for a solution of sulfuric acid, if he contains 5.88 g of H₂SO₄ in 300 mL of solution.

2. Calculate how many grammes of potassium hydroxide contains in 200 mL of his solution with the normality 0,1 mol/L.

3. Calculate how many grammes of SO₄^2- ions contains at a 1L solution with the concentration of this ions 0,2 mol/L.

4. At the of microcrystalloscopices detection of Ca²⁺ ions as CaC₂O₄ a reaction is succeeded already with 0,001 mL of solution of CaCl₂ with the normality 0,002 mol/L. Calculate the determined’s minimum for this reaction.

5. Maximum dilution for the reaction of K⁺ ions with solution of NaHC₄H₄O₆ (formation of precipitate of KHC₄H₄O₆) even 1:1000. Why the even least molar concentration of equivalent (normality) of solution of KCl, at what K⁺ ions can be discovered this reaction?

6. Calculate an activity of ions potassium and ion of sulfate in solution of potassium sulfate with a concentration (molarity) 0,1 mol/L.

7. Calculate the middle activity coefficient of sodium hydroxide in solution with a his concentration (molarity) 0,1 mol/L and compare him to the experimentally found size 0.764. Calculate the real constant (concentration’s constants) of dissociation of acetate acid in a presence of potassium chloride with his molarity 0,100 mol/L.

ANSWERS TON THE SELF-ASSESSMENT

Tests: 1. E., 2. C., 3. B.

Tasks: 1., 2., 3., 4., 5., 6., 7. .

REFERENCES:

1. The lecture’s material.

2. David Harvey. Modern Analytical Chemistry // www.mhhe.com

METHOD OF IMPLEMENTATION OF PRACTICAL WORK

To the major operations of semi microanalysis belongs: heating, evaporation, sedimentation, centrifugation, filtration, washing of sediment.

Heating. As known, a lot of reactions take place at heating. To heat of solution which is in little test tubes, directly on the opened flame it is impossible as a liquid practically will always be thrown out a pair. They are heated on a water-bath. The term ‘water bath’ means a bath of boiling water, unless water at some other temperature is indicated in the text. An alternative form of heating may be employed providing that the required temperature is approximately maintained but not exceeded.

Evaporation. The evaporation of solutions for the concentration or complete delete of water conducts on the water-bath in the cups of porcelains.

Sedimentation. In the qualitative analysis the sedimentation often conduct in centrifugation’s test tubes. In a test tube place the necessary amount of drops of the explored solution and, if it is needed heat on a water-bath, whereupon add the necessary amount of drops of reagent a pipette. In connection with that in little test tubes which apply in semi microanalysis, solutions are mixed badly, it is necessary immediately after their uniting to mix maintenance of test tube a glass stick. It is yet better to mix after addition of every drop a reagent. It is instrumental in more slow growth of crystals of sediment, which are then formed large, better move away from solution at centrifugation, and having a less surface, less adsorb related matters from solution.

If sediment which fell out can form colloid solution, for the best coagulation usefully after addition a reagent a few minutes to heat maintenance of test tube on a water-bath.

Types of Analytical Classifications of Cations

Group

Group reagent

Cations

Acid-basic

There are not

K⁺, Na⁺, Li⁺, NH₄⁺

ІІ

HCl

Ag⁺, Pb²⁺, Hg₂²⁺

Chlorides, insoluble in water and acids

ІІІ

H₂SO₄

Ca²⁺, Sr²⁺, Ba²⁺

Sulphates, insoluble in water, acids and bases

NaOH

Al³⁺, Zn²⁺, Cr³⁺, Sn²⁺,Sn⁴⁺, As⁺³, As⁺⁵

Hydroxides with amphoteric properties

NaOH

Mn²⁺, Mg²⁺, Fe²⁺, Fe³⁺, Sb³⁺, Sb⁺⁵, Bi³⁺

Hydroxides insoluble in ammonia and bases

NH₃

Cu²⁺, Co²⁺, Cd²⁺, Ni²⁺, Hg²⁺

Hydroxides soluble in ammonia with ammonia complexes formation

Sulphide

There are not

K⁺, Na⁺, Li⁺,NH₄⁺

ІІ

(NH₄)₂CO₃ + NH₃ + NH₄Cl

Ca⁺², Sr⁺², Ba⁺², Mg⁺²

Carbonates insoluble in water

ІІІ

(NH₄)₂S + NH₃ + NH₄Cl

Mn⁺², Fe⁺², Fe⁺³, Co⁺², Ni⁺², Zn⁺², Cr⁺³, Al⁺³

Sulphides insoluble in water, ammonia but soluble in НСІ

H₂S + HCl

Cu⁺², Cd⁺², Hg⁺², Sn⁺², Sn⁺⁴, As⁺³, As⁺⁵, Sb⁺³, Sb⁺⁵, Bi⁺³

Sulphides insoluble in НСІ

HCl

Ag⁺, Pb⁺², Hg₂⁺²

Chlorides, insoluble in water and acids

Ammonia-phosphate

There are not

K⁺, Na⁺, NH₄⁺

ІІ

(NH₄)₂НРO₄ + NH₃

Ca⁺², Sr⁺², Ba⁺², Mg⁺²Li⁺, Mn⁺², Fe⁺², Fe⁺³, Cr⁺³, Al⁺³Bi⁺³

Phosphates insoluble in water and ammonia

ІІІ

Nа₂НРO₄

Cu⁺², Co⁺², Cd⁺², Ni⁺², Hg⁺², Zn⁺²

Phosphates soluble in ammonia with ammonia complexes formation

НNO₃

Sn⁺², Sn⁺⁴, As⁺³, As⁺⁵, Sb⁺³, Sb⁺⁵

Oxidize to compounds with the highest oxidation numbers

HCl

Ag⁺, Pb⁺², Hg₂⁺²

Chlorides insoluble in water and acids

FIRST ANALYTICAL GROUP OF CATIONS

(K⁺, Na⁺, NH)

Almost all salts to Potassium, Sodium and ammonium well water-solubles. Therefore cations of K⁺, Na⁺, NH₄⁺ not have a general reagent which would be capable synchronously to obtain of precipitate with them as insoluble in water compounds. The first analytical group of cations differs from cations of other groups.

All salts of cations of the first group have not a colour, and their compounds can have colour, only when they will form compounds together of colour anions (for example, CrO₄^2–, Cr₂O₇^2–, MnO₄^–, and others). The salts of Potassium and of Sodium are persistent during of heating. The ammonium salts are not persistent during of heating, especially these salts which have obtained together with volatile acids (HCl, HNO₃, H₂CO₃ and other).

Characteristic reactions of ions of K⁺

Sodium hexanitrocobaltate (III) of Na₃[Co(NO₂)₆] (pharmacopeia’s reaction). This complex compound with the ions of K⁺ ieutral or acetic-acid medium forms yellow crystalline precipitate of double salt of K₂Na[Co(NO₂)₆]:

2K⁺ + Na⁺ + [Co(NO₂)₆]^3- = K₂Na[Co(NO₂)₆]¯.

Alkali metals hydroxides interfere of this reaction, because lay out a reagent, as a result darkly-brown precipitate of Co(OH)₃ is selected:

[Co(NO₂)₆]^3– + 3OH^– = Co(OH)₃¯ + 6NO₂^–.

If there are strong acids, complex anion is also laid out:

[Co(NO₂)₆]^3– + 6H⁺ = Co³⁺ + 6HNO₂.

It should be remembered that Na₃[Co(NO₂)₆] is not stability, his colour can change on rose (color of ions of Co²⁺), and such reagent caot apply for the detection of ions of K⁺.

The ions of NH₄⁺ interfere with the exposure of ions of K⁺, as from Na₃[Co(NO₂)₆] form sediment, similar to in color sediment which appears at presence of to Potassium.

Implementation of reaction. To the drop of the explored solution (pH 5-7) add 2-3 drops solution of Na₃[Co(NO₂)₆]. If a reaction of solution is acidic, that it is necessary to add Sodium acetate for linkage of ions of H⁺. If there are ions of K⁺, yellow precipitate appears.

Sodium hydrotartratic of NaHC₄H₄O₆ (pharmacopeia’s reaction). This compound with ions Potassium hydrotartratic forms in a neutral medium white crystalline precipitate Potassium hydrotartratic:

K⁺ + HC₄H₄O₆^–® KHC₄H₄O₆¯.

Precipitate is soluble in mineral acids and alkalis. Solubility of precipitate increases at heating. Precipitate KHC₄H₄O₆ forms on the rubbing of wall-side of a test tube a glass stick and cooling (refregeration).

This reaction conduct with tartratic acid in a presence Sodium acetate (Pharmacopeia of Europe).

Implementation of reaction. To 3-4 drops of the explored solution add 3-4 drops solution of NaHC₄H₄O₆ and rub the wall-side of test tube a glass stick. If a reaction of solution is acidic, that it is necessary to add Sodium acetate for linkage of ions of H⁺. If there are ions of K⁺, white precipitate forms.

Microcristaloscopic Reaction. Ions of K⁺ forms the black cubic crystals with the reagent of Na₂PbCu(NO₂)₆:

2K⁺ + PbCu(NO₂)₆^2–® K₂PbCu(NO₂)_6.

A reaction must run in a neutral medium; the ions of NH₄⁺ interfere with its conducting.

Implementation of reaction. Drop of the explored solution put on the glass and evaporate on water-bath. After that the dry remain is cooling, adds a drop of reagent. If there are ions of K⁺, that black or brown cubic crystals appear (fig. 1).

Flame test Potassium (pharmacopeia’s reaction). Salts of Potassium paint flame in a fleeting pale-violet color.

Implementation of reaction. A platinum wire moistens with solution HCl and heat in a flame. A clean platinum wire moistens with solution of salt K⁺and heat in a flame. If there are ions to Potassium, flame paints in a fleeting pale-violet color.

Characteristic reactions of ions of Na⁺

Potassium hexahydroxostibatе (V) K[Sb(OH)₆] (pharmacopeia’s reaction). This complex compound with the concentrated solutions of salts of Sodium in a neutral or weak basic medium white precipitate forms:

Na⁺ + [Sb(OH)₆]^–® Na[Sb(OH)₆] ¯.

Precipitate Na[Sb(OH)₆] forms on the rubbing of wall-side of a test tube a glass stick and cooling (refregeration). Precipitate does not appear in a strongly basic medium. This reagent is not stability in a acidic medium:

[Sb(OH)₆]^–+ H+ ® H[Sb(OH)₆]

H[Sb(OH)₆] ® HSbO₃¯ + 3H₂O.

Precipitate does not form in dilute solutions.

Precipitation is slowed in presence of nitrate ions. The ions of NH₄⁺, Mg²⁺, Li⁺ interfere.

Implementation of reaction. To 3-4 drops of the explored solution add 3-4 drops solution of reagent, the internal wall-side of test tube rubs a glass stick. If a reaction of solution is acidic, that it is necessary to add Potassium acetate for linkage of ions of H⁺. If there are ions of Na+, white precipitate appears.

Microcristaloscopic Reaction. Ions of Na⁺ forms the octahedral pale-yellow crystals with the reagent Zn(UO₂)₃(CH₃COO)₈:

Na⁺ + Zn(UO₂)₃×(CH₃COO)₈+ CH₃COO^– + 9H₂O ® NaZn(UO₂)₃×(CH₃COO)₉×9H₂O¯.

A reaction must run in a neutral or acetic medium; the ions of Ag+, Hg₂²⁺, Sb^(III), PO₄^3–, AsO₄^3– interfere with its conducting.

Implementation of reaction. Drop of the explored solution put on the glass and evaporate on water-bath. After that the dry remain is cooling, adds a drop of reagent. If there are ions of Na⁺, that octahedral pale-yellow crystals appear (fig. 2).

Flame test Sodium (pharmacopeia’s reaction). Salts of Sodium paint flame in a persistent yellow color.

Implementation of reaction. A platinum wire moistens with solution HCl and heat in a flame. A clean platinum wire moistens with solution of salt Sodiumand heat in a flame. If there are ions to Sodium, flame paints in a persistent yellow color.

Metoxyphenilacetic acid (pharmacopeia’s reaction). This compound with ions Sodium forms white precipitate in presence tetramethylammonium hydroxide:

Implementation of reaction. To 5-6 drops of the explored solution add 10-12 drops of reagent Metoxyphenilacetic acid in the solution tetramethylammonium hydroxide and cool in icy water.

If there are ions of Na⁺, white precipitate forms.

Characteristic reactions of ions of NH₄⁺

Solutions of alkalis (NaOH or KOH) (pharmacopeia’s reaction) during heating with solutions of salts an ammonia is selected an ammonia:

NH₄⁺ + OH^– = NH₃ +H₂O.

Implementation of reaction. 3-5 drops of the explored solution place in test tube and add some drops of NaOH or KOH. Phenolphtalein’s paper moisten a water and keep its above test tube. If there are ammonia ions, that Phenolphtalein’s paper will be rose-colour.

Potassium tetraiodomercurate (II) K₂[HgI4] in KOH (reagent of Nessler) This compound with ions NH₄⁺ forms red-brown precipitate in presence Potassium hydroxide:

NH₄⁺+ 2[HgI₄]^2– + 4OH^– = [NH₂Hg₂O]I¯ + 7I^– + 3H₂O.

The ions of Fe3+, Bi3+, Cu2+, Cd2+, Ag+, Pb2+, As (V) interfere with its conducting.

This reaction is sensible and it application for determination of “tracks” quantity of the ammonium or ammonia. Then solution is painted in yellow.

Implementation of reaction. To 1-2 drops of the explored solution add some drops of Nessler’s reagent. If there are ions of NH₄⁺, red-brown precipitate forms or solution appears yellow colour.

Sodium hexanitrocobaltate (III) of Na₃[Co(NO₂)₆] (pharmacopeia’s reaction). This complex compound with the ions of NH₄⁺ ieutral or acetic-acid medium forms yellow crystalline precipitate of double salt of (NH₄)₂Na[Co(NO₂)₆]:

2 NH4+ + Na+ + [Co(NO2)₆]^3- = (NH4)_2Na[Co(NO2)₆]¯

Systematic analysis of cation’s mixtures of the first analytical group

1. Determination ions of NH₄⁺with NaOH or Nessler’s reagent.

2. If there aren’t NH₄⁺-ions, that the ions of K⁺ and Na⁺ determinate in two separate portions of the explored solution.

3. If there are NH₄⁺-ions, that them extract (heat with NaOH to dry remain a few minutes). Then it cool and dissolve in some drops of distilled water (verification is with the reagent of Nessler). If there aren’t NH₄⁺-ions, that the ions of K⁺ determinate in this solution.

4. Use 3 for determination of Na⁺, but KOH necessary to add for extraction NH₄⁺-ions.