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 from 1120 to 1240 kg/m3 and 1180 kg/m3. Boric acid production yield was around 91% for a borax solution of 1180 kg/m3 and its purity is ranged from 94% to 97%.
Apparatus and Materials:
Sodium tetraborate, concentrated hydrochloric acid, concentrated sulphuric acid, methyl red indicator, mannitol, beaker, methanol, Butchner’s funnel, distilled water, electric heater, analytical balance, ice cubes and ice water, measuring cylinder, suction machine, Bunsen burner, wooden splinter, filter paper, test tube, test tube rack, glass rod.
Procedures:
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10g of sodium tetraborate (borax) was dissolved in 40 cm3 of distilled water in a 100 cm3 beaker.
- The solution in the beaker was heated using an electric heater to dissolve it completely. During heating process, the solution was stirred continuously using a glass rod.
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After the solution was dissolved completely, the heating process was stopped and 5 cm3 of concentrated hydrochloric acid was added.
- The beaker with the solution was left cooling by putting it in a steel bowl containing ice water to crystallize out the boric acid.
- The crystals formed were filtered off using filter paper above the butchner’s funnel through suction machine and washed well with ice-cold water.
- The crystals were transferred to an empty, dry and clean beaker from the filter paper carefully. The beaker with crystals was weighed and the yield was recorded.
- The following steps were carried out on the prepared sample of crystals.
- A little boric acid was dissolved in water in a test tube and a drop of methyl red indicator was added. This step was repeated in a second test tube using mannitol instead of the acid. Half of each solution was mixed in a third test tube and any colour changes were recorded. The results were explained in discussion part.
- A few crystals of boric acid were added in a test tube and followed by adding a little methanol and some concentrated sulphuric acid. The test tube was heated and the vapour evolved was ignited. The colour of the flame was recorded and explained.
Results:
Table 1: Mass of sodium tetraborate (borax) through heating process.
Table 2: Mass of boric acid in crystal form through crystallization method.
Table 3: Reaction of boric acid and observation of any changes.
Analysis and calculation:
- From Table 1:
mass of sodium tetraborate (borax) through heating process
= (mass of empty beaker + sodium tetraborate) – (mass of empty beaker)
= 60.3076 g – 50.3056 g
= 10.0020 g
- From Table 2:
mass of boric acid in crystal form through crystallization method
= (mass of empty beaker – crystallized boric acid) + (mass of empty beaker)
+ (mass of filter paper – crystallized boric acid) + (mass of filter paper)
= (45.6444 g – 42.5503 g) + (0.7520 g – 0.3359 g)
= 3.0941 g + 0.4161 g
= 3.5102 g
- The percentage yield of boric acid in the experiment was calculated:
Yield =
Borax has a formula of Na2B4O7.10H2O. It reacts with HCl and the equation is shown as follow:
Na2B4O7 + 5H2O + 2HCl 4H3BO3 + 2NaCl
According to the equation above, 1 mole of sodium tetraborate produces 4 mole of boric acid. Thus, mole of sodium tetraborate will produce 4 x mole of boric acid.
Theoretical weight of Boric Acid = (4 ) x [3(1.007) +10.811+3(15.9994)]
= 0.1049 x 61.8302
= 6.4900 g
In this experiment, the actual weight of boric acid obtained is 3.5102g referring to
Table 2. The yield of boric acid is calculated as shown as below:
Yield =
= 54.0863 %
The percentage yield of boric acid in this experiment is 54.0863 %.
Discussion:
In this experiment, preparation of boric acid from borax is done through crystallization. Crystallization is the (natural or artificial) process of formation of solid crystals precipitating from a solution, melts or more rarely deposited directly from a gas. Crystallization is also a chemical solid-liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid crystalline phase occurs.
Based on the result in Table 2, the mass of boric acid in crystal form through crystallization method is calculated by adding the mass of boric acid left on the filter paper. This is because when transfer the crystallized boric acid from filter paper to the beaker, there will be some boric acid left out on the filter paper. In order to get a more accurate result, the little amount of the mass boric acid was added.
In step 7, part (a) of the procedures in this experiment, a little boric acid was dissolved in distilled water in a test tube A and a drop of methyl red indicator was added. In test tube B, the boric acid was added with mannitol instead of adding with methyl red indicator. This two solution were mixed and the colour of solution turns purplish red, as shown in the observation in Table 3. This is due to the mannitol is a sugar alcohol that derived from sugar by reduction with the formula of C6H8(OH)6. The solution of dissolved mannitol with methyl red indicator turns purplish red because mannitol is mildly acidic.
In step 7, part (b) of the procedures in this experiment, a few crystals of boric acid were added in a test tube and followed by adding a little methanol and some concentrated sulphuric acid. The test tube was heated and the vapour evolved was ignited. The flame produced is a green flame, which shown in Table 3 in the result part. When boric acid is mixed with Methanol, Trimethyl Borate, a volatile (high vapour pressure) and flammable substance is formed. Trimethyl Borate burnt with a green flame without the need to add additional colorants. However, we should be caution as the colour produced by barium and copper ions is also green colour which is very close to our product. Thus, it would interfere experiment results if we mistaken the colour. This process was handled with caution as concentrated sulphuric acid is a strong acid and corrosive we should wear goggles and gloves when carrying out the test. Besides, the methanol is highly flammable when heating, hence, things that burn easily should be taken distance away from the flame.
Besides, the percentage yield of boric acid in the experiment was calculated. The way in calculating the percentage of yield is shown as below:
Yield =
One mole of borax (Na2B4O7.10H2O) reacts with hydrochloric acid to form 4 mole of boric acid.
Na2B4O7 + 5H2O + 2HCl 4H3BO3 + 2NaCl
The theoretical weight of boric acid was first calculated, which is 6.4900 g. Then, the actual weight of boric acid: 3.5102 g was divided by the theoretical weight. The result obtained was in percentage value, which is 54.0863%.
Questions
- Draw the structure of the borate anion.
- Find out the properties and structure of boric acid.
Boric acid, also known as boracic acid or orthoboric acid, is naturally occurring compound containing the elements boron, oxygen, and hydrogen (H3BO3). It is a weak acid usually used as an antiseptic, insecticide and also in nuclear power plants to control the fission rate of uranium. In nature, the element boron does not exist by itself. Boron is combined with other common elements, such as sodium to make salts like borax and with oxygen to make boric acid.
Boric acid is a crystalline solid at its physical state, which is white, odorless, and nearly tasteless. Boric acid is soluble in boiling water, but poorly soluble in cold water. Besides, it does not dissociate in aqueous solution, but is acidic due to its interaction with water molecules to form tetrahydroxyborate ion.
B(OH)3 + H2O ⇌ B(OH)−4 + H+ (Ka = 5.8x10−10 mol/l; pKa = 9.24)
Polyborate anions are formed at pH 7–10 if the boron concentration is higher than about 0.025 mol/L. The best known of these is the tetraborate ion, found in the mineral borax:
4B(OH)−4 + 2H+ ⇌ B4O2−7 + 9H2O
Crystalline boric acid consists of layers of B(OH)3 molecules held together by hydrogen bonds. Boric acid has the boiling point of 300 °C and a melting point of 170.9 °C. It dissolves 2.52 g/100 mL in water when the temperature is at 0 °C; dissolves 4.72 g/100 mL at 20 °C; dissolves 5.7 g/100 mL in water at 25°C; dissolves 19.10 g/100 mL in water at 80 °C; and lastly dissolves 27.53 g/100 mL in water at 100 °C. Boric acid also soluble in organic solvent like, in lower alcohols moderately soluble in pyridine and very slightly soluble in acetone. Boric acid has trigonal planar molecular shape.
The structure of boric acid is shown as below:
- How do you convert boric acid to boric oxide?
The boric acid is converted into boric oxide through heating. The boric acid will change into several different acids when bubbling the whole time. The bubbling action occurs when the boric acid is releasing the moisture from itself.
When boric acid is heated above 170 °C, it dehydrates and form metaboric acid, HBO2. Metaboric acid 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 further dehydrates and form tetraboric acid or pyroboric acid, H2B4O7.
Further heating leads to boron trioxide, also known as boric oxide. A trioxide of boron obtained as rhombic crystals melting at 460°C. It is used as an intermediate in the production of boron halides and metallic borides and as a thermal neutron absorber in nuclear engineering, also known as boron oxide.
Conclusion:
Through crystallization method, the mass of crystallized boric acid produced from 10.0020g of borax was 3.5102 g. Crystals of boric acid was formed when it left cooled in ice water.
The percentage yield of boric acid was calculated, which was 54.0863%. Two tests were carried out to determine the reaction of boric acid. The first one was mixing mannitol solutions with boric acid the second one was mixing methanol with boric acid. Mannitol showed pale yellow in methyl red indicator while boric acid in methyl red indicator showed brownish-orange colour. Both solution were mixed to the third test tube and the colour turned purplish-red due to mildly acidic property of mannitol solution. Green flame is ignited from the vapour evolved when boric acid was heated with methanol and concentrated sulphuric acid, due to the interfere of barium and copper in the test.
Reference:
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Boric acid. (2009). Accessed on July 6, 2009.
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Boric acid. (2008). Accessed on July 6, 2009.
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Boric acid preparation. (2002). Accessed on July 7, 2009.
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Borate: borate anion structure. (1998). Accessed on July 7, 2009.
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Chemical properties of boron. (2008). Accessed on July 7, 2009.