BORIC ACID (H3BO3) and its structural features:

Orthoboric acid, H₃BO_3, commonly known as boric acid, and Metaboric acid, HBO₃ , are two common oxy acids of  boron . Orthoboric acid naturally found in volcanic steam vents called Suffioni. 

STRUCTURE OF BORIC ACID:

(1) In dilute Solution Boric acid exist as monomeric form;

(2) When concentration of the solution of acids  is very high then boric acid exist as polymeric metaboric and many more ions.

(3) In Solid State It exist sp3 hybridised BO3-  2D Sheet .

 GENERAL PROPERTIES:

(1) H₃BO₃ is soluble in water and behaves as weak monobasic acid. It does not donate protons but rather it accepts OH^- .Therefore it acts as a Lewis acid [B(OH)₃] .It is not a proton donor because it accept lone pair or hydroxyl ion from water.

(2) Since B(OH)₃ only partially reacts with water to form H₃O⁺ and [B(OH)₄]^- it behaves as a weak acid. Thus it cannot be titrated satisfactorily with NaOH as a sharp end point is not obtained.  

(3)  B(OH)₃ (Boric acid ) does not  titrated even strong alkali like NaOH but If certain polyhydroxy compounds such as glycerol, mannitol or sugar are added to the titration mixture then B(OH)₃ behaves as a strong monobasic acid and hence can be titrated with NaOH and end point is diluted using phenolphthalein as indicator.

(4) The added compound must be a cis-diol to enhance the acidic proprieties. In this way the cis-diol forms very stable complexes with [B(OH)₄]¯ formed in forward direction above, thus effectively removing it from solution. Hence reaction proceeds in forward direction (Le-Chatelier principle.)

PHYSICAL PROPERTIES:

CHEMICAL PROPERTIES:

USES:

Related Questions:

What is structure of solid Ortho Boric acid ?

Why Boric acid become strong acid in the presence of cis 1,2-diol or 1,3-diol ?

What is the structure of trimetaboric acid and trimetaborate ion?  

What is basicity of "Boric acid" ?

Why Boric acid exist in solid state ?

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BORAX BEAD TEST:

When Borax is heated on loop of platinum wire then loop of platinum wire swells up due to loss of water of crystal of borax, if it is further heated then a transparent glass of Sodium metaborates (NaBO₂) and boric anhydride (B₂O₃) is obtained which  is called borax Bead. 

When a solution of given salt is heated on bead , then volatile part of salt displaced by B₂O₃  and corresponding metaborates salt is formed which gives Characteristic colour on oxidizing and reducing flame and colour of the bead is noted in hot and cold for each type of flame .

Chemical reactions:

Illustrative example of copper:

In oxidising flame;

In Hot flame: green colour transparent glassy bead appear

In Cold: Blue colour transparent glassy bead appear

In reducing flame;

IMPORTANT NOTE:
In this bead test colour of bead of metaborates of given basic radicals are formed provided  the metal cation must contain at least one unpaired electron , Orthoborates are also formed  but they are not responsible for bead colour.

SUMMARY OF BORAX BEAD TEST:

ILLUSTRATIVE EXAMPLE (1): Which of the following do not respond borax bead test?

     (1) Nickel Salts               (2) Copper Salts             (3) Cobalt Salts      (d) Aluminium Salts

ILLUSTRATIVE EXAMPLE (2): the “Borax Bead “Contains:

       (1)  NaBO₃                    (2) NaBO₂                       (3) B₂O₃                 (d) Na₂ B₄O₇.10H₂O  

ILLUSTRATIVE EXAMPLE (3): This Cation gives a colourless bead in Borax Bead Test:

       (1) Mg²⁺                        (2) Ca²⁺                            (3) Cu⁺¹                  (4) Cu⁺² 

                                     Download as PDF click here..

Silicates (SiO4)-4: salts of silicic acids:

(1) Silicates is the general term applied for the solids with silicon – oxygen bonds.

(2) Silicates are regarded as the salts of silicic acid, H₄SiO₄. All the silicates are comprised of SiO₄ units.

(3) The Silicates units have a tetrahedral structure formed as a result of sp3 hybridization. Silicon atom has its complete octet but each oxygen atom is still short of one electron to complete its octet. They can complete their octet by taking up 4 electrons from a metal, getting converted to an anion [SiO4]^-4

(4) All the solids Silicates contain silicate ion (SiO4)^4- as the basic structural unit.

(5)The silicate ion is tetrahedral in structure and when the one or more oxygen atoms between such tetrahedrons, a complex structure arise.

(6) All the silicates units are non planer

(7) The Si atom in Silicates unit is covalently bonded to 4 oxygen atoms. Each oxygen atom possesses a formal negative charge. Hence each tetrahedral unit has a formal charge of –4. When linked together, the extended units are also negatively charged. Presence of other metallic ions such as Ca²⁺or Mg²⁺ are necessary for electrical neutrality.

(8) The covalent Si-O bond, having a bond enthalpy of 466 kJ mol^-1 is particularly strong compared with the C-C bond which has a bond enthalpy of 347 kJ mol^-1. The linkage -Si-O-Si-O- is very stable and instead of existing as discrete units of (SiO4)^4- ions, the silicates tend to form chains, sheets or networks.

 CLASSIFACATION OF SILICATES:

The silicates may be classified in to following groups chain silicates, ring silicates, cyclic silicates, sheet silicates, three – dimensional silicates depends on the way in which the (SiO4)4- tetrahedral units are linked together.

(1) Ortho silicates or Neso Silicates (SiO4)^4-: 

Number of oxygen shared by tetrahedron is Zero(0).Orthosilicates are salts of orthisilicic  acid

Example; Zircon ( ZrSiO₄ ), Phenacite (Be₂SiO₄) , Willemite (Zn₂SiO₄),  

(2) Pyro silicates or Sorosilicates or Disisilicates (Si₂O₇^-6) :    

(1) Number of oxygen atoms shared per tetrahedron is 1(one).

(2)Total number of shared oxygen towards one Si atom is ½

(3)Total contribution of all oxygen towards one Si atom is 3.5

(4) In pyrosilicates, one tetrahedron shares its one oxygen atom with other tetrahedron on they are salts of pyrosilicic acids .

Example: Thortveitite (Se₂Si₂O₇); lanthanoids disilicates (Ln₂[Si₂O₇] and Hemimorphite  Zn₄ (OH)₂[Si₂O₇].

(3) Meta silicates: 

 (A) Linear chain Meta Silicates [(SiO₃)_n ^-2n  ] :

In chain silicates each tetrahedron shares its  two  oxygen atoms with other tetrahedron atom such that a linear endless chain of silicates is formed , however in terminating chain silicates (n) tetrahedron  shares one oxygen.

Example: Spodumene (LiAlSiO₃)₂ ; Wollastonite [Ca₃(SiO₃)₃] Enstatite [Mg₂SiO₃)₃] and Diopsite [ CaMg(SiO₃)₂]

(B) Cyclic meta silicates [(Si_nO_3n) ^-2n  ] :

(1) Number of oxygen atoms shared per tetrahedron is 2(Two).

(2) Total number of shared oxygen towards one Si atom is one (1)

(3) In cyclic silicates each tetrahedron shares its two oxygen atoms with other tetrahedron atom such that a cyclo silicates is formed.

(4) There are several cyclo silicate is known like n= 3, 4, 5 ,6 but  3 and 6 is most common.

Example: Wollasponite [ Ca₃(Si₃O₉)] , Benitoite [BaTi(Si₃O₉)] , Beryl [Be₃Al₂(Si₃O₉)₂]

(4) Double chain Silicates or Amphiboles  (Si₄O₁₁)^-6n :

(1) Number of Silicon atoms in basic units is 2 .

(2) Total number of oxygen atoms in basic unit is 5.5.

(3) Number of oxygen atoms shared per tetrahedron is 3+2/2=2.5

(4) In this type silicates two strands of chain silicates are linked to each other by sharing oxygen atoms, Amphiboles are an asbestos mineral which contains magnesium.

(5)  General formula of double chain silicates (Si₄O₁₁)^-6n

Example: Tremotites  [Ca₂Mg₅(Si₄O₁₁)₂(OH)₂] Crocidolite [Na₂Fe₃Fe₂(Si₄O₁₁)₂(OH)₂]

(5) Sheet Silicates or phyllo silicates [(Si₂O₅)_n]^-2n:

In Sheet Silicates, one tetrahedron shares its three (3) oxygen atom such that  a two dimensional non planer layer is formed , such type of silicates have greasy touch or lubricating action.

Example:  Clay [Al₂(Si₂O₅)(OH)_{4 ,}Talc or Soap Stone  [Mg₃(Si₂O₅)₂(OH)_{2 ,} Kaolinite  [Al₂(Si₂O₅)(OH)₄  Chrysotite (white Asbestos)  [Mg(Si₂O₅)(OH)₄

(6) 3D-Silicates:

Examples: Silica, Quarts, feldspar, Zeolite, Ultramarine

Related Questions:

(1) What is "Zeolite's" ?

(2) What are most common cyclic meta silicates?

(3) What are the Silicones? Give one example one cyclic Silicone?