Some acids and bases are dibasic. The meaning of dibasic is that they either gain or release two protons per molecules rather than one.
- Containing two replaceable hydrogen atoms.
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Of or relating to salts or acids forming salts with two atoms of a univalent metal.
(Lexico Publishing Group, LLC)
An example of the above is Barium hydroxide which is a base in a form of solution;
Ba(OH)2 + H2O Ba2 + 2OH-
If one has been instructed to calculate the pH of a solution, one must therefore base it upon OH concentration, which is twice the size of base concentration. Therefore if 1 mole of barium hydroxide releases 2 moles of hydroxide ions then 0.1 molar solution of barium hydroxide will produce 0.1 moles of hydroxide ions.
There are dibasic acids too, although at times they are referred to as diprotic e.g. sulphuric acid.
H2SO4 2H+ + SO4- 2H3O++ SO4-
Method
For the experiment to begin, a sample of sodium hydroxide base with a known concentration of 0.1 mol dm-3 is provided, along side, hydrochloric acid, ethanoic acid and sulphuric acid, which all have a concentration of approximately 0.1mol dm-3.
The first experiment was the titration of a strong acid versus a strong base. Three conical flasks were rinsed out using distilled water. A beaker was rinsed out with hydrochloric acid solution and then 100cm3 of the acid was transferred to the beaker and some of the acid was used to rinse out the pipette. Then by using the pipette, 25cm3 of hydrochloric acid was transferred to each flask, from the beaker. About 2-3 drops of phenolphthalein indicator was added to each flask, which had to be gently swirled after the indicator was added.
The burette was then rinsed out with the sodium hydroxide (NaOH) solution; the burette was filled (using a beaker) with the NaOH solution to a point between 0 and 1 on the scale. The middle of the meniscus was used to take a reading. The conical flask was placed below the burette. Using one hand to hold the conical flask, the other hand was used to control the tap of the burette. Slowly the tap was opened to deliver the alkali to the acid. As the alkali met the indicator in the acid, there was a slight colour change. The conical flask had to be swirled as the alkali was let into the acid. This was done so that the solution became thoroughly mixed. It was important to keep an eye on the flask as the colour could have changed at any time. As the end-point of the titration grew closer the colour in the conical flask would remain in the flask for longer length of time before disappearing. As the colour become more permanent, the addition of the alkali had to be slowed down (preferably to a few drops after a swirl). Until a permanent colour change was present the alkali had to be added carefully. As soon as the coloration became permanent, it was important to end the addition of alkali. This meant that it was the end point of the reaction. This was done at least three times to get an average mean volume was obtained or till concordant results were obtained. Concordant results are results that are + or – 0.1cm3 of another, for example if someone had got results that looked like, 0.5cm3, 0.6cm3 then they could stop the experiment.
The titration techniques used above were the same for both experiments 2 & 3. Experiment 2 was when a weak acid (in this scenario ethanoic acid) was titrated against a strong base, and experiment 3 was when a dibasic acid (in this scenario sulphuric acid) was titrated against a strong base.
For each titration the initial and final burette readings were noted as well as the volume of NaOH necessary to neutralise the acid.
Results
Strong Acid
Mean Volume Added = 24.1 cm3
Weak Acid
Mean Volume Added = 25.05 cm3
Dibasic Acid
Mean Volume Added = 55.20 cm3
Discussion
Most, if not all experiments undergone are subject so some degree of uncertainty or error. For example the experiment which we had carried out was done in a group, which meant that we were sharing some chemicals with other people. It was apparent that a few people were unaware of the protocol which should have been followed to make sure that contamination was not an issue; unfortunately some chemicals had become contaminated. Another example of a possible error could be the meniscus, as people range in heights it is impossible to say if the meniscus is accurately on zero or not. Also in some of the titrations the end point came around really quickly, and if you were not cautious it would be really easy to over shoot the mark, thus going over the end point (this is apparent when you see a real dark colour in the conical flask)
Conclusion
In conclusion it must be said that it is apparent from the set of results provided that the concentration of the dibasic acid was far greater than those of either the strong acid or weak acid, nearly twice as much base had to be titrated against the dibasic acid in comparison to both the weak and strong acid, for it to reach the end point. For example it took around 24.1cm3 for the strong acid reach its end point, and around 25.10 cm3 for the weak acid to reach its end point, whilst it took around 55.20cm3 to reach the end point of the dibasic acid.
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HCl +NaOH NaCl + H2O
(ratio = 1:1)
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0.1 x 24.1 = 2.41 x 10-3 x 1000 = 0.0964 mol dm-3
1000 25
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CH3COOH + NaOH CH3COONa + H2O
(ratio = 1:1)
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Mol of NaOH =
0.1 x 25.05 = 2.50 x 10-3 = 0.1002 mol dm-3
1000 2
5.
6. H2SO4 + 2NaOH Na2SO4 + 2H2O
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x 55.20 = 5.52 x 10-3)
- Ratio = 1:2
•
• • 2.76 x 10-3 x 100 = 0.1104 mol dm-3
25
Please note that as the ratio was 1:2, 5.52 had to be divided by 2.
- Contamination was one error source that had occurred; this was due to some people sticking the pipette directly into the acids, which meant that there could have been a chance that the acid had been contaminated. In the future this can be avoided by the proper use of a conical flask to transfer to the pipette.
- Weak acid – substances capable of donating hydrogen but do not completely ionize in solution.
Strong acid - an acid that completely dissociates to produce an H+ ion and the conjugate base