Background theory
Equipment used in the experiment: Burette, pipette, pipette filler, clamp and stand, a small and a large beaker, three conical flasks, white paper, phenolphthalein indictor, dilute solutions of HCl and H2SO4 of unknown molarities, and 0.10 dm–3 NaOH.
Equations, diagrams, and calculations:
Concentration of the sulphuric acid solution
Molarity of the alkaline solution of NaOH 0.10 mol. dm–3
As found in the experiment,
Average volume of H2SO4 is 11.15 cm3 = 0.01115 dm3
H2SO4 → H2+ + SO4–
NaOH →Na++ OH–
H2+ + SO4–+ 2Na + 2OH → Na2+ + SO4–+ 2H2 O
Thus, the hydrogen ions in H2SO4 reacted with hydroxide ions in NaOH in a one-to-two ratio to produce water as an end product:
H2+SO4–+ 2NaOH → Na2+SO4–+ 2H2O
As one mole of sulphuric acid reacts with two moles of sodium hydroxide solution and
As Molarity = Moles/Volume dm3
0.10 = Moles/0.01115
0.10 × 0.01115 = Moles
Therefore, Moles = 1.115 × 10–3 moles of alkali
The number of moles of acid reacting will be half that number of moles = 0.56 × 10–3 moles of alkali
Since Molarity = Moles/Volume dm3, and the volume of the acid is 0.01115 dm3,
Molarity of H2SO4 is 0.56 × 10-3/1.115 × 10–2 = 0.050 mol. dm–3
Concentration of the hydrochloric acid solution
Molarity of the alkaline solution of NaOH 0.10 mol. dm–3
As found in the experiment, Average volume of HCl is 22.35 cm3 = 0.02235 dm3
HCl → H++ Cl–
NaOH →Na++ OH–
H+ + Cl– + Na+ + OH– → Na+ + Cl– + H2O
Thus, the hydrogen ions in HCl reacted with hydroxide ions in NaOH in a one-to-one ratio to produce water as an end product:
H+Cl– + Na+OH– → Na+Cl– + H2O
As one mole of hydrochloric acid reacts with one moles of sodium hydroxide solution and
As Molarity = Moles/Volume dm3
0.10 = Moles/0.02235
0.10 × 0.02235 = Moles
Therefore, Moles = 2.235 × 10–3 moles of alkali
The number of moles of acid reacting will be half that number of moles = 1.16 × 10–3 moles of alkali
Since Molarity = Moles/Volume dm3, and the volume of the acid is 0.02235 dm3,
Molarity of HCl is 1.16 × 10–3 /2.235 × 10–2 = 0.052 mol. dm–3
Hypothesis
If the concentration of one a chemical reagent and the reactions of both the reagents are known in an acid-base titration, then the unknown concentration of the reagent can be determined.
Method
This experiment was conducted using titration. The base was NaOH, and its concentration was known. The unknown concentrations of acids HCl and H2SO4, were to be found. Phenolphthalein was the indicator used to determine the results.
Results
The concentrations of HCl and H2SO4 were determined as 0.052 mol. dm–3 and 0.050 mol. dm–3, respectively.
Discussion
An acid is a chemical substance, and has properties of neutralizing alkalis, dissolving certain metals, and making litmus red. It is generally found to have a corrosive action and in the form of a sour-tasting liquid (Bettelheim, 2010). To determine the concentration of an acid, a titration process is used (Bettelheim, 2010).
The pH indicates the hydrogen ion concentration in a solution, which is used to determine the acidity or alkalinity of a solution (Bettelheim, 2010). Values below 7 indicate acidity, whereas those above it indicate alkalinity, and 7 indicates that the solution is neutral (Bettelheim, 2010).
The hydrogen ions in water are connected the oxygen in a “V” shape (Bettelheim, 2010). This is because the electrons in a water molecule mostly orbit the nucleus of the oxygen atom and not that of the hydrogen nuclei (Bettelheim, 2010). Thus, the oxygen atom gains a negative charge and the hydrogen atoms become positively charged. Hence, the water molecule is polar in nature, that is, there is a positive and negative charge at both its ends (Bettelheim, 2010). In a reaction, water dissociates into hydronium ion (H3O+) and a hydroxide ion (OH−) (Smith et al., 2005).
The reactions in this paper have low activation energy as ions are simple and the positive and negative charges instantaneously attract each other without the need to dissociate any bonds (Bettelheim, 2010). In this experiment the slight pinkish hue indicated that that the solution had become alkaline, that is pH had become more than 7.
In an acid-base reaction an indicator, which is a large organic molecule, is used to function somewhat like a color dye (Acid-base indicators). Most indicators are weak acids that respond to the change in hydrogen ion concentration in a solution with a change in color (Acid-base indicators).
Strengths and weaknesses of the experiment: While titration is indeed a very useful means of determining the concentration of a substance, it can be used only under the above-mentioned conditions (Skoog, Holler, and Nieman, 1998). Moreover, factors like the mass of a substance cannot be determined with titration (Skoog, Holler, and Nieman, 1998). Thus, in analytical chemistry, the elemental and molecular concentrations of a substance must be sometimes determined by measuring the amount of light absorption and emission (Skoog, Holler, and Nieman, 1998). The presence of different ions in a solution can also be found by methods such as electrode separation and chromatography (Skoog, Holler, and Nieman, 1998).
Conclusion: Conducting this experiment has shown that the concentration of a dilute chemical reagent can be calculated by titration provided the concentration of the reagent it is being titrated with is known. The reactions of the both the reagents should be known for this purpose as well. This paper also found that if in a chemical reaction, the reaction of either one of the reagents is unknown, but the concentrations of both the reagents is known, a titration enables the calculation of the unknown reaction (Bettelheim, 2010).
Future perspectives: As listed below, knowing the concentration of a substance is useful for a number of purposes (de Levie, 2001).
In the food processing industry in order to determine the fat content in a food product
The pH of a substance determined by titration is useful for determining the acidic content of substances like vegetable oil
Pharmacists and nurses to mix various dosages of medicine and to administer intravenous medication
Doctors such as diabetologists, gynecologists, and urologists often use titration to measure blood glucose amount and to measure the levels of various elements in body fluids
In metal analysis for knowing the exact concentration of metal ions, such as iron, magnesium, and calcium, in samples, such as hard water
An innovative if unusual method of using titration techniques could be used to manufacture strips—such as the pregnancy strips that are available at a pharmacy to determine the hormonal levels in pregnant women’s urine— to determine thee glucose level in people. As far as automating the procedure is concerned, conducting the titration with a pH meter rather than an indicator would enable the endpoint detection with more ease and accuracy, as the result would not depend on visual analysis but color changes, but a change in the pH.
References
Acid-base indicators. Retrieved on January 9, 2012 http://www.elmhurst.edu/~chm/vchembook/186indicator.html
Bettelheim, F. A. (2010). Introduction to general, organic, and biochemistry. Belmont: Thomson Brooks/Cole, p. 259.
de Levie, R. (2001). Aqueous Acid-base Equilibria and Titrations, 1st edition. California: Oxford University Press, p. 54.
Skoog, D. A., Holler, F. J., and Nieman, T. A., (1998). Principles of Instrumental Analysis, 5th edition. Orlando: Harcourt Brace College Publishers.
Smith, J., et al. (October 4, 2005). Unified description of temperature-dependent hydrogen-bond rearrangements in liquid water. The National Academy of Sciences of the USA, 102(40), 14171–14174.
