Good Example Of The Chemistry Of Boron-Oxygen Compounds Report
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Experiment 1: Preparation of Sodium Peroxoborate
Sodium peroxoborate (Sodium perborate) finds a heavy usage as a bleaching agent in the detergent industry. This reagent comes with an advantage of low toxicity and a long shelf life. It is also a good substitute in organic synthesis for hydrogen peroxide solutions, which are highly concentrated, unstable and thus, pose great explosion hazards (Brotherton, 1994).
It is available as a crystalline reagent as a hydrate with a general formula: NaBO3 • n H2O. The chemical structure of the compound is as follows:
This experiment is designed to analyze the formation of a peroxy acid from a salt and hydrogen peroxide. In this case, the peroxy acid expected out of the experiment is sodium peroxoborate by using borax (or sodium borate) as the salt.
0.54 g NaOH
2.9 mL 30% hydrogen peroxide
0.7 g Potassium Iodide
10 mL of Hydrochloric Acid (2 mol L-1)
Prepared a solution of sodium metaborate by dissolving 2.38 g borax and 0.54 g solid sodium hydroxide in 10 mL warm water. It took a while for the borax to dissolve. Meanwhile, measured out 2.9 mL of hydrogen peroxide solution and chilled in an ice bath. Once borax has completely dissolved, the flask was put in the ice bath. The chilled hydrogen peroxide was diluted with 7.5 mL of distilled water. This diluted hydrogen peroxide (H2O2) solution was added to the sodium metaborate solution drop wise, while stirring. Stirring continued for another 15 minutes after all the H2O2 was added. Once the product started to crystallize, the crystals were filtered out by suction using a Buchner funnel. The filtered product was washed with 5 mL of alcohol and then dried by sucking air out of it for 30 minutes.
Weighing by Difference
Weighed out two 0.2 g sample of the product using an analytical balance and put into 2 test tubes. Re-weighed and noted the weight as weight A.
These contents were then transferred to 2 conical flasks and re-weighed. This weight was noted as weight B.
Added 30 mL of distilled water to each conical flask to dissolve the peroxoborate. This was followed by addition of approximately 0.7 g of solid potassium iodide (KI) and 10 mL of 2 mol L-1 of hydrochloric acid (HCl) drop wise to each flask. This was further titrated with 0.989 mol L-1 sodium thiosulfate.
Results, Observations and Discussion
The experiment was started with 2.38 g of borax. The weighing of the product by difference produced the below results:
The product, NaBO3.H2O, was a fine white colored powder. After the addition of HCl and KI, the HCl completely dissolved the solid and the test tube was covered in the bubbles. The formation of a brown colored precipitate was observed.
In the next step of titration, the liquid turned pale yellow in color after addition of about 10 drops.
Concentration of Hydrogen Peroxide (H2O2): 2.9/7.5 = 0.38
Volume of H2O2 used: 2.9 mL
Molar mass of H2O2: 34.016g/mol
Mass of H2O2: 0.38/100*2.9 = 0.01102
% Peroxide content of the perborate: Mass of H2O2 / Mass of Sodium Perborate
= 0.01102/0.2 * 100
Experiment 2: Boron Flame Test
0.2 g Boric Acid crystals
5 mL methanol
Sulphuric Acid drops
0.2 g of Boric Acid crystals was transferred to a test tube. 5 mL of methanol and some concentrated sulphuric acid (H2SO4) was added to the test tube. Sulphuric acid is added to play the role of catalyst to the reaction. This solution was then heated. The vapors produced on heating the solution were ignited using the flame splint and the color of the flame produced was recorded.
Results and Discussion
On the combustion of the solution, the flame produced was observed to be green in color.
Boric acid, in its crystalline form, is made up of layers of B(OH)3 molecules, which are held together by the hydrogen bonds. This hydrogen bonding allows boric acid molecules to form parallel layers in its solid state, giving it a sheet like structure (Greenwood & Earnshaw, 1997).
The chemical equation for the reaction of boric acid with methanol is (Brotherton, 1994):
H3BO3 (boric acid) + 3CH3OH (methanol) ↔ B(OCH3)3 (trimethyl borate) + 3H2O (water)
Boric acid reacts with methanol, with sulphuric acid acting as the catalyst, to produce trimethyl borate. Trimethyl borate is an inorganic ester, hence this reaction is an esterification reaction. During the reaction, an intense combustion is observed producing a green colored flame. This flame of green in color is specifically produced by the combustion of substances containing boron atoms. Below is a brief description of the process:
All electrons are in their lowest possible energy levels in their ground state. In the heat of a flame during combustion, electrons absorb its energy. When they absorb energy, they reach the excited state, but are short lived and tend to return back to the ground state. When they return back to the ground state, the excess energy absorbed during the process is released in the form of light. The color of the light is different for different elements. Green colored light is produced for copper, barium and boron (McKillop & Sanderson, 1995).
Brotherton, B.J. (1994). Boron: Inorganic Chemistry. Encyclopedia of Inorganic Chemistry. John Wiley & Sons
Greenwood, N., Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann.
McKillop, A., Sanderson, W. R. (1995). Tetrahedron, 51, 6145-6166
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