Experiment 4 - Preparation and Reactions of Boric Acid, H3BO3

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Experiment 4

Title: Preparation and Reactions of Boric Acid, H3BO3.

Objectives:

1) To synthesize the boric acid B(OH)3 from sodium tetraborate as starting material.

2) To use crystallization technique to obtain the final product.

3) To have a better understanding on physical properties predominantly and to study the  

structure of the boric acid.

Instruction:

Boron consist of 2 isotopes, which are 10B (19.6%) and 11B (80.4%). It is an element in p-block of the periodic table, with electron configuration of 1s22s22p1 in Group 13 (IUPAC classification). The important sources of boron are borax (sodium tetraborate) and kernite. The most common form of boron is amorphous boron, a dark brown to black amorphous powder, unreactive to oxygen, water, acids and alkalis. It reacts with metals to form borides.

All boron in nature is in oxygenated form. Boron oxides and their derivatives are technologically important and are relatively inexpensive to be produced. For Boron-oxygen compounds, it contains predominantly trigonal planar BO3, and to a lesser degree of tetrahedral BO4 units, as in the borate anions.

The principal oxide, B2O3 is very difficult to crystallize and normally exists in a glassy state (d=1.83 g cm-3) composed of randomly oriented B2O3 rings with bridging oxygen atoms. In the normal crystalline form, trigonal BO3 is linked through their oxygen atoms at high pressures and temperatures, and a high density (d=2.56 g cm-3) phase having BO4 tetrahedra is produced. The B-O bond is quite strong, with energy of 560-790 kJ mol-1. Boranes and related compounds are thermodynamically unstable relative to B2O3 and B(OH)3. The simple way to prepare boron of amorphous powder form is the reduction of boron trioxide by heating with magnesium.

Hydrolysis of B2O3 generates boric acid, B(OH)3, another major commercial product that is manufactured on a scale of hundreds of thousands of tonnes per year by acidification of aqueous solution of borax, a naturally occurring mineral.        Boric acid is produced mainly from borate ores containing sodium or calcium by the reaction with sulphuric acid in the presence of a hot aqueous boric acid liquor to recycle.

Boric acid, also known as boracic acid or orthoboric acid, is a weak acid usually used as an antiseptic, insecticide and also in nuclear power plants to control the fission rate of uranium. It exists in colourless crystals form or white powder which dissolves in water. It has chemical formula of H3BO3 or B(OH)3. Boric acid is poorly soluble in cold water but dissolves readily hot water, alcohol and glycerine. When it was heated above 170°C, it dehydrates to form metaboric acid (HBO2), which is a white, cubic crystalline solid and is only slightly soluble in water. It melts at about 236°C, and when heated above about 300°C, it dehydrates further, forming tetraboric acid or pyroboric acid (H2B4O7).

The chemical formula of Borax is generally described as Na2B4O7·10H2O, containing [B4O5(OH)4]2− ion. There are two four-coordinate boron atoms (two BO4 tetrahedra) and two three-coordinate boron atoms (two BO3 triangles). If Borax left exposed to dry air, it slowly loses its water of hydration and becomes the white and chalky mineral tincalconite, with formula of Na2B4O7·5H2O.

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Through crystallization process, the crystalline boric acid is formed which consists of layers of B(OH)3 molecules held together by hydrogen bonds. Boric acid is produced through heating and crystallization. The balance equation is shown as below:

Na2B4O7·10H2O(aq) + 2HCl(aq) → 4B(OH)3(aq) + 2NaCl(aq) + 5H2O(aq)

The yield of product can be measured through the reaction where the theoretical yield should be 44% over 100%. The amount of product should be more or less the same as the amount of reactant before heating. The lesser the amount of product, the lower the yield. The concentration of borax solutions was varied ...

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