Qualitative determination of ions table. Application of ionometric methods in analysis. a) Determination with sodium sulfide

Qualitative analysis methods are based on ionic reactions, which allow the identification of elements in the form of certain ions. During the reactions, sparingly soluble compounds, colored complex compounds are formed, oxidation or reduction occurs with a change in the color of the solution.

For identification through the formation of sparingly soluble compounds, both group and individual precipitants are used. NaCl serves as group precipitating agents for Ag + , Pb 2+ , and Hg 2+ ions; for cations Ca 2+, Sr 2+, Ba 2+ - (NH 4) 2 CO 3, for ions Al 3+, Cr 3+, Fe 3+, Fe 2+, Mn 2+, Co 2+, Ni 2 + , Zn 2+ - (NH 4) 2 S.

There are many organic and inorganic reagents that form precipitates or colored complexes with cations (table):

Volatile metal compounds color the burner flame in one color or another. Therefore, if you add the substance under study on a platinum or nichrome wire into a colorless burner flame, the flame will color in the presence of certain elements in the substance, for example, in the following colors: bright yellow (sodium), violet (potassium), brick red ( calcium), carmine red (strontium), yellow-green (copper, boron), pale blue (lead, arsenic).

Anions are usually classified according to their salt solubility or redox properties. So many anions (SO 4 2 -, SO 3 2 -, CO 3 2 -, SiO 3 2 -, F -, PO 4 3 -, CrO 4 2 -, etc.) have a group reagent BaCl 2 in a neutral or slightly acidic environment , since barium salts and these anions are slightly soluble in water. The group reagent in the HNO 3 solution for the ions Cl - , Br - , I - , SCN - S 2 - , ClO - , 4 - and others is AgNO 3 . As for cations, there are reagents for certain anions (table):

The classification of anions according to their redox properties is given in the table:

Chemical identification of a substance is based mainly on the reactions of precipitation, complexation, oxidation and reduction, and neutralization, in which a colored precipitate forms, a change in the color of the solution, or the release of gaseous substances.

Let's imagine this situation:

You are working in a laboratory and have decided to conduct an experiment. To do this, you opened the cabinet with reagents and suddenly saw the following picture on one of the shelves. Two jars of reagents had their labels peeled off and safely remained lying nearby. At the same time, it is no longer possible to determine exactly which jar corresponds to which label, and the external signs of the substances by which they could be distinguished are the same.

In this case, the problem can be solved using the so-called qualitative reactions.

Qualitative reactions These are reactions that make it possible to distinguish one substance from another, as well as to find out the qualitative composition of unknown substances.

For example, it is known that cations of some metals, when their salts are added to the burner flame, color it a certain color:

This method can only work if the substances being distinguished change the color of the flame differently, or one of them does not change color at all.

But, let’s say, as luck would have it, the substances being determined do not color the flame, or color it the same color.

In these cases, it will be necessary to distinguish substances using other reagents.

In what case can we distinguish one substance from another using any reagent?

There are two options:

• One substance reacts with the added reagent, but the second does not. In this case, it must be clearly visible that the reaction of one of the starting substances with the added reagent actually took place, that is, some external sign of it is observed - a precipitate formed, a gas was released, a color change occurred, etc.

For example, it is impossible to distinguish water from a solution of sodium hydroxide using hydrochloric acid, despite the fact that alkalis react well with acids:

NaOH + HCl = NaCl + H2O

This is due to the absence of any external signs of a reaction. A clear, colorless solution of hydrochloric acid when mixed with a colorless hydroxide solution forms the same clear solution:

But on the other hand, you can distinguish water from an aqueous solution of alkali, for example, using a solution of magnesium chloride - in this reaction a white precipitate forms:

2NaOH + MgCl 2 = Mg(OH) 2 ↓+ 2NaCl

2) substances can also be distinguished from each other if they both react with the added reagent, but do so in different ways.

For example, you can distinguish a sodium carbonate solution from a silver nitrate solution using a hydrochloric acid solution.

with sodium carbonate hydrochloric acid reacts with the release of a colorless, odorless gas - carbon dioxide (CO 2):

2HCl + Na 2 CO 3 = 2NaCl + H 2 O + CO 2

and with silver nitrate to form a white cheesy precipitate AgCl

HCl + AgNO 3 = HNO 3 + AgCl↓

The tables below present various options for detecting specific ions:

Qualitative reactions to cations

Qualitative reactions to anions

Anion	Impact or reagent	Sign of reaction. Reaction equation
SO 4 2-	Ba 2+	Precipitation of a white precipitate, insoluble in acids: Ba 2+ + SO 4 2- = BaSO 4 ↓
NO 3 −	1) Add H 2 SO 4 (conc.) and Cu, heat 2) Mixture of H 2 SO 4 + FeSO 4	1) Formation of a blue solution containing Cu 2+ ions, release of brown gas (NO 2) 2) The appearance of color of nitroso-iron (II) sulfate 2+. Color ranges from violet to brown (brown ring reaction)
PO 4 3-	Ag+	Precipitation of a light yellow precipitate in a neutral environment: 3Ag + + PO 4 3- = Ag 3 PO 4 ↓
CrO 4 2-	Ba 2+	Formation of a yellow precipitate, insoluble in acetic acid, but soluble in HCl: Ba 2+ + CrO 4 2- = BaCrO 4 ↓
S 2-	Pb 2+	Black precipitate: Pb 2+ + S 2- = PbS↓
CO 3 2-		1) Precipitation of a white precipitate, soluble in acids: Ca 2+ + CO 3 2- = CaCO 3 ↓ 2) The release of colorless gas (“boiling”), causing cloudiness of lime water: CO 3 2- + 2H + = CO 2 + H 2 O
CO2	Lime water Ca(OH) 2	Precipitation of a white precipitate and its dissolution with further passage of CO 2: Ca(OH) 2 + CO 2 = CaCO 3 ↓ + H 2 O CaCO 3 + CO 2 + H 2 O = Ca(HCO 3) 2
SO 3 2-	H+	Emission of SO 2 gas with a characteristic pungent odor (SO 2): 2H + + SO 3 2- = H 2 O + SO 2
F −	Ca2+	White precipitate: Ca 2+ + 2F − = CaF 2 ↓
Cl −	Ag+	Precipitation of a white cheesy precipitate, insoluble in HNO 3, but soluble in NH 3 ·H 2 O (conc.): Ag + + Cl − = AgCl↓ AgCl + 2(NH 3 ·H 2 O) = ) Didn't find the answer to your question? Look here Lesson-study "Morphology Russian Church Slavonic dictionary online List of manual operations in 1 from 8