Qualitative Analysis: Dry Test, Borax Bead Test, Microcosmic Salt Bead Test – Procedure, Reactions, Practice Problems and FAQ

How can alkali metal ions be identified in a lab setting?

On the provided samples, we may do a flame test, and based on the distinctive colour of the flame, we can determine which specific alkali metal or group 1 metal is present. Lithium, for instance, gives the flame a crimson red hue; sodium, a yellow hue; potassium, a violet or lilac hue; rubidium, a reddish-violet hue; and caesium, a blue hue. Similar to this, there are other ways to determine what basic and acid radicals are present in the given salt. The dry test is one of the methods that help us in identifying the cation and anion present in the given salt.

Let's learn more about the dry tests without further ado!

TABLE OF CONTENTS

Qualitative Analysis
Borax Bead Test
Borax Bead Test – Chemical Changes
Microcosmic Salt Bead Test
Microcosmic Salt Bead Test – Chemical Changes
Sodium Carbonate Bead Test
Sodium Carbonate Bead Test – Observations
Practice Problems
Frequently Asked Questions – FAQ

Qualitative Analysis

Qualitative analysis is the process of determining a substance's nature and the identities of its constituents.

Qualitative analysis of inorganic salts means the identification of cations and anions present in the salts.

Qualitative analysis is carried out through reactions that are easily detectable to human senses such as sight and smell.

A dry test involves noting the appearance and physical properties, such as colour, odour, solubility, etc. of the salt.

Examples: Heating of dry salt, flame test, borax bead test, microcosmic salt bead test, charcoal cavity test, etc.

Borax Bead Test

Borax: It is one of the most important compounds of boron.

Formula: ${N a}_{2} B_{4} O_{7} . 10 H_{2} O .$ . Practically, borax is present as the tetranuclear unit ${[B}_{4} O_{5} (O H)_{4}] 2 -$ . So, the formula of borax is correctly represented as ${N a}_{2} {[B}_{4} O_{5} (O H)_{4}] . 8 H 2 O ⬚$

Structure:

Appearance: White crystalline solid

This test is used for coloured salts because borax reacts with the corresponding metal salts to form metal borates and show their characteristic colours.

Procedure:

A loop is made at the one end of the platinum wire and borax powder is taken in it. The platinum wire containing the borax powder is heated in the hottest part of the flame until it becomes red hot.

The borax powder swells up due to the loss of water of crystallisation and shrinks on forming a transparent, colourless bead that consists of boric anhydride and sodium metaborate.

The hot bead is dipped in the salt sample, heated in the flame and the colour of the bead is observed for both the non-luminous flame as well as in the luminous flame while it is hot and when it is cold.

Borax Bead Test – Chemical Changes

On heating borax, it loses its water of crystallisation and forms sodium metaborate and boric anhydride.

${N a}_{2} B_{4} O_{7} . 10 H_{2} O (s) \to {N a}_{2} B_{4} O_{7} (l) + 10 H_{2} O (l)$

${N a}_{2} B_{4} O_{7} (s) \to 2 N a B O_{2} (s) + B_{2} O_{3} (s)$

Treatment with metal salt:

$C o S O_{4} (s) + B_{2} O_{3} (s) \to C o (B O_{2})_{2} + {S O}_{3}$

Blue colour bead $C o (B O_{2})_{2}$ (cobalt metaborate) formation takes place.

The colour of the borax bead for different metals is given below.

Metal	Oxidising Flame		Reducing Flame
Metal	Hot	Cold	Hot	Cold
Cr	Yellow	Green	Green	Green
Mn	Violet	Violet	Colourless	Colourless
Fe	Yellowish brown	Yellow	Green	Green
Co	Blue	Blue	Blue	Blue
Ni	Violet	Reddish brown	Grey	Grey
Cu	Green	Blue	Colourless	Opaque

Microcosmic Salt Bead Test

Microcosmic salt: ${N a (N H_4) H P O_4}_$ (Ammonium Sodium Phosphate). This salt is termed as microcosmic salt.

Microcosmic Salt Bead Test is used for coloured salts because microcosmic salts react with corresponding transition metal salts to form metal orthophosphate and show their characteristic colours. The procedure is the same as the borax bead tets.

Microcosmic Salt Bead Test – Chemical Changes

On heating, the microcosmic salt loses its water of crystallisation and forms metal metaphosphate which is transparent and liberates ammonia gas.

${N a (N H_{4}) H P O_{4}}_{.} 4 H_{2} O (s) \to N a P O_{3} (s) + N H_{3} (g) + 5 H_{2} O (a q)$

Treatment with metal oxide.

$C u O (s) + N a P O_{3} (s) \to N a C u {P O}_{4} (s)$

Blue coloured bead of NaCuPO4 is formed.

To identify halides (X-, X can be Cl, Br & I)

$N a C u {P O}_{4} + 2 N a X \to {N a}_{3} {P O}_{4} + 2 N a X$

Cl-→ blue colour across the bead

Br- → blue colour across the bead and green on top of the flame

I- → green colour on entire flame

Metal	Oxidising Flame		Reducing Flame
Metal	Hot	Cold	Hot	Cold
Copper	Green	Blue	Colourless	Red
Iron	Brown	Yellow	Green	Green
Cobalt	Blue	Blue	Blue	Blue
Nickel	Violet	Brown	Grey	Grey

Sodium Carbonate Bead Test

A small amount of sodium carbonate (Na₂CO₃) is heated in a platinum wire loop in the bunsen flame. This results in the formation of a white, opaque bead. The bead is dipped in a small amount of KNO₃ and then in the salt sample and heated. The colour change of the bead is noted.

Sodium Carbonate Bead Test – Observations

The formation of a green bead confirms the presence of a manganese compound.

$M n O + N a_{2} C O_{3} + O_{2} \to N a_{2} M n O_{4} + C O_{2} ↑$

The formation of a yellow bead confirms the presence of a chromium compound.

$2 C r_{2} O_{3} + 4 N a_{2} C O_{3} + 3 O_{2} \to 4 N a_{2} C r O_{4} + 4 C O_{2} ↑$

Practice Problems

Find the total number of sp2hybridised boron atoms in borax.

Answer: B

Solution: Borax contains 2 sp2and 2 sp3hybridised boron atom per molecule.

So, option B is the correct answer.

The colour shown by cobalt in the borax bead test is:

Green
Purple
Blue
Olive

Answer: C

$C o S O_{4} (s) + B_{2} O_{3} (s) \to C o (B O_{2})_{2} + {S O}_{3}$

When a cobalt salt is subjected to borax bead test, a blue coloured bead Co(BO₂)₂ (cobalt metaborate) is formed.

So, option C is the correct answer.

Glassy bead appearance in microcosmic salt bead test is due to

Metal metaphosphate
Boric anhydride
Sodium metaborate
None of these

Answer: A

On heating, the microcosmic salt loses its water of crystallisation and forms metal metaphosphate which is transparent and liberates ammonia gas.

${N a (N H_{4}) H P O_{4}}_{.} 4 H_{2} O (s) \to N a P O_{3} (s) + N H_{3} (g) + 5 H_{2} O (a q)$

So, option A is the correct answer.

The formation of a yellow bead in the sodium carbonate bead test confirms the presence of

Iron
Copper
Manganese
Chromium

Answer: D

The formation of a yellow bead confirms the presence of a chromium compound.

$2 C r_{2} O_{3} + 4 N a_{2} C O_{3} + 3 O_{2} \to 4 N a_{2} C r O_{4} + 4 C O_{2} ↑$

So, option D is the correct answer.

Frequently Asked Questions – FAQ

1. What is the difference between oxidising and reducing flame?
Answer: A non-luminous flame is an oxidising flame and a luminous flame is a reducing flame. Oxidising flame is a flame produced with an excess amount of oxygen. With a good amount of oxygen, the flame height shortens, and the colour darkens and burns with sound.

2. What are meta acids?
Answer: When one water molecule is removed from 1 molecule of acid, meta acids are formed

$H_{3} B O_{3} (B o r i c a c i d) \to H B O_{2} (m e t a b o r i c a c i d)$

BO₂-- metaborate ion

So, the salt formed from metaboric acid is metaborate. NaBO₂ - sodium metaborate

H₃PO₄(Phosphoric acid)HPO₃ (metaphosphoric acid)

PO₃-- meta phosphate ion

NaPO₃-sodium metaphosphate

Note: When the name of acid ends with -ic acid, its anion name ends with - ate and the name of acid ends with -us acid its anion name ends with - ite.

$H_{2} S O_{4} (s u l p h u r i c a c i d) - {S O}_{4}^{2 -} (s u l p h a t e i o n)$

$H_{2} S O_{3} (s u l p h u r o u s a c i d) - {S O}_{3}^{2 -} (s u l f i t e i o n)$

What is the use of borax?

Borax is used to prevent fungal foot diseases in animals, used to treat wood products that are infected with woodworm. It is also used as a buffering agent.

3. What is the difference between hot and cold beads?
Answer: "Hot" describes a bead that has just been removed from the Bunsen burner's flame, while "cold" describes a bead that has reached room temperature.

4. Are the colours observed for cations the same in the borax bead test and microcosmic bead test?
Answer: Yes, generally colours produced by cations are the same in both the borax bead test and the microcosmic bead test.

Related Topics

Salt Analysis - Classification, Definition	Percentage Composition
Analysis of Group I Acidic Radicals	Analysis of Group II Acidic Radicals
Analysis of Group III Acidic Radicals	Brown Ring Test