One important area of chemistry is the analysis of substances to determine their composition, either qualitatively or quantitatively.In qualitative analysis, we identify the elements and compounds that are present in a sample of a given substance. In quantitative analysis, we calculate the amount or quantity of an element or compound present in a given sample using chemical processes called titrations. There are two approaches to quantitative analysis, namely volumetric analysis and gravimetric analysis.
Volumetric analysis is based on volume measurements of solutions while gravimetric analysis is based on direct mass measurements of the substances. Volumetric analysis is commonly used as it is faster and more convenient, although less accurate, than the gravimetric method.
What is Titration?
Titration is the method employed in volumetric analysis. In this method, a solution from a graduated vessel is added to a known volume of a second solution until the chemical reaction between the two is just completed. This is shown by a colour change in the resulting solution or in an added indicator.
In any titration, a standard solution, i.e. one with a concentration which is accurately known, must be used to react with a solution of unknown concentration. The reacting volumes of solutions are then used to calculate the unknown concentration of the solution.
Important Notes on Titrations and Volumetric Analysis
1. There is no suitable indicator for titration of weak acid and weak base. Any indicator could be used for strong acid and base.
2. Volumetric analysis usually includes titrations of acid against base or trioxocarbonate(IV); oxidizing oxidizing agent against reducing agent; one substance against another substance, giving a precipitate.
3. Acid-base or acid-trioxocarbonate(IV) titration are actually neutralization reactions. We use indicators to determine the end-point of this type of titrations.
4. In titrations, the concentrations of the reacting solutions give an accurate picture of the quantitative behaviour of the reacting particles.
The following equations are useful in calculations involving concentration.
A Standard Solution is a solution of known concentration. the amount of solute in a given volume of solvent must be known. For example, 1.06g of Na2Co3* dissolved in 1dm3 of water gives a standard solution.
Acid-Base Titrations
Some materials used during acid-base titrations and other precautions employed while using these materials:
- Weighing bottle
- Chemical balance
- Pipette
- Burette
- Retort stand
- Filter paper
- Funnel
- White title
- Standard volumetric flask
- Conical flask
Precautions Employed while using the Pipette, Burette and Conical Flasks
(a) Pipette
- Rinse the pipette with the solution it should be used to measure, i.e base.
- Avoid air bubbles in the pipette.
- Make sure that the mark to be read is at the level with your eye.
- Do not blow the last drop on the pipette.
(b) Burette
- Rinse the burette with acid or allow it to drain after rinsing with distilled water.
- make sure that the burette jet is filled.
- Avoid air bubbles in the burette.
- Make sure that the burette is not leaking.
- Take your burette readings with your eyes at the same level as the meniscus to avoid error due to parallax during titration
- Remove the funnel before taking your readings.
- Avoid inconsistent burette reading.
Conical Flasks
- Do not rinse it with any of the solutions used in the titration but with distilled water.
- Wash down with distilled water any drop of the solution that stick by the sides of the conical flask during titration.
Molar Solution
The molar masses of sodium and potassium hydroxides are 40g and 56g respectively. Therefore, a molar solution of sodium hydroxide contains 1 mole or 40g of the hydroxide in 1 dm3 of the solution, while a molar solution of potassium hydroxide contains 1 mole or 56g of the hydroxide in 1 dm3 of the solution.
From table 10.2, we get following relationship which are helpful in calculations involving molarity.
- By definition, molar concentration C = mol dm-3
- By definition, number of specified entitles per dm3 = mo1 dm-3 x 6.02 1023 x 6.02 x1023
- Concentration (g dm3)
= mo1 dm3 x molar mass
= molarity (M) molar mass.
Acid-Base Indicators
What are Acid -base indicators? Acid-base indicators are dyes that can change color according to the pH of the medium they are found. The most common indicator is litmus which is red in an acidic medium and blue in an alkaline medium. This indicator changes from a color gradation of red through purple to blue over a pH range of 5.0 to 8.0 confirming the fact that each pH has its own specific range of pH over which it changes color.
The pH of a solution can be measured using the following methods:
(i) Universal indicator
(ii) pH meters.
A universal indicator is made up of a mixture of various indicators that work at different pH ranges. Through a series of successive color changes, the universal indicator can indicate pH values of 3 to 11 and these pH changes can be obtained by comparing the color seen with that of the standards given. However, this method of measuring pH is not very accurate for scientific precision during titrations.
On the other hand, the pH of a solution can be measured accurately with the aid of a pH meter. A p meter can accurately measure the pH of even very dilute solutions as well as that of opaque and colored liquids.
A Universal Indicator can be used to compare the pH values of 1.0M solutions of the following
(i) Strong alkalis such as sodium hydroxide solution
(ii) Strong acids such as hydrochloric acid
(iii) Weak alkalis such as aqueous ammonia
(iv) Weak acids such as ethanoic acid
(v) Pure distilled water
It can be observed that strong acids and strong alkalis have pH values that are at the extremes of the pH scale, whereas weak acids and weak alkalis have pH values close to 7. For pure distilled water, its pH value is 7. Indicators are used in acid-base titrations and the best indicator that can be used for this is the universal indicator.
Buffer Solutions
A Buffer solution is a solution that resists changes in pH on dilution or on addition of small amounts of acids or alkalis. Buffer solutions usually consist of a weak base or a weak acid in the presence of one of its salts. Some examples of buffers used in titrations are:-- Ethanoic acid and sodium ethanoate, trixocarbonate(IV) acid and sodium hydrogen trioxocarbonate(IV) and aqueous ammonia and ammonium chloride.
Buffer solutions are of huge significance in fields such as biochemistry and medicine. This is because pH values are often very critical and germane and therefore have to be maintained at a steady value for biological systems to function optimally. For instance, blood has a pH value of 7.4 and even a slight change of +0.5 may prove to be very fatal. Therefore, injections into the blood stream needs to be buffered so as not to alter this delicate balance. Also, many fermentation processes and enzyme facilitated reactions depend largely on pH, which can only possible vary within narrow limits. Buffer solutions are widely used in processed drinks and foods to prevent acidity.
The completion of many chemical reactions during titrations depends largely on the pH of the solutions in which they take place. The pH of aqueous solutions is extremely sensitive to the addition of small amounts of alkalis or acids. For instance, when 0.1cm3 of 1.0M hydrochloric acid is added to 1dm3 of distilled water or sodium chloride solution, the pH changes from 7.0 to 4.0 and this sort of sharp pH change would prove to be very fatal to living organisms. However, animals are protected from this sort of sharp pH changes by the presence of buffers.
