The Use of Volumetric Flask, Burette and Pipette in Determining the Concentration of NaOH Solution

Authors Avatar

The Use of Volumetric Flask, Burette and Pipette in Determining the Concentration of NaOH Solution


* To determine the number of ionizable hydrogen in an unknown acid.

* To determine the equivalent weight of an unknown acid.

* To determine the enthalpy change for the ionization of an unknown acid.

* To use the technique of volumetric analysis or titration to determine the concentration of a given NaOH solution.

Theory And Background

In 1855, the German chemist, Friedrich Mohrn defined titration as the "weighing without scale" method because this process allows determination of the concentration of a sample without using complex instrumentation. A manual titration requires high accuracy and precision, both in the preparation of the material, and the use of different precisely dosed reagents. The operation must be repeated at least 3 times to obtain a reliable measured value. This procedure makes the manual analytical technique very long and fastidious.

Titration is the quantitative measurement of an analyte in solution by reacting it completely with a standardized reagent. For example, a given volume of a solution of unknown acidity may be titrated with a base of known concentration until complete Neutralization has occurred. Acids and bases react until one of the reactants is consumed completely. A solution of base of known concentration can therefore be used to titrate an acid solution of unknown concentration. Likewise, an acid solution of known concentration can be used to titrate a base solution of unknown concentration. The point at which all of the analyte is consumed is the equivalence point and is generally determined by observing a color change in an added indicator such as phenolphthalein.

The term "end point" is where the indicator changes colour. That isn't necessarily exactly the same as the equivalence point. This means that at the equivalence point (where you had mixed the solutions in the correct proportions according to the equation), the solution wouldn't actually be neutral. To use the term "neutral point" in this context would be misleading. The equivalence point is often determined by visual indicators are available for titration based on acid-base neutralization, complexation, redox reactions and is determined by some type of indicator that is also present in the solution. For acid-base titration, indicators are available that change color when the pH changes. When all of the analyte is neutralized, further addition of the titrant causes the pH of the solution to change causing the color of the indicator to change.

The G.N. Lewis (1923) idea of acids and bases is broader than the Brønsted-Lowry model. The Lewis definitions are: acids are electron pair acceptors, and bases are electron pair donors. Each ionizable pair has a proton donor and a proton acceptor. Acids are paired with bases. One can accept a proton and the other can donate a proton. In the Lewis theory of acid-base reactions, bases donate pairs of electrons and acids accept pairs of electrons. A Lewis acid is therefore any substance, such as the H+ ion, that can accept a pair of nonbonding electrons. In other words, a Lewis acid is an electron-pair acceptor. A Lewis base is any substance, such as the OH- ion, that can donate a pair of nonbonding electrons. A Lewis base is therefore an electron-pair donor. The Lewis theory suggests that acids react with bases to share a pair of electrons, with no change in the oxidation numbers of any atoms. Many chemical reactions can be sorted into one or the other of these classes. Either electron is transferred from one atom to another, or the atoms come together to share a pair of electrons.

The true point of neutralization in any titration occurs when the amounts of acid and alkali added together are chemically equivalent to each other.

Using a calibrated burette to add the titrant, it is possible to determine the exact amount that has been consumed when the endpoint is reached. The endpoint of a titration is when the pH of the reactant is just about equal to 7, and when the reactant stops reverting back to its original color.

Acid-base indicators are substances, which change colour according to the hydrogen ion concentration of the liquid in which they are placed. They either weak acids or weak bases, and are therefore slightly dissociated when dissolved in water. The color of the indicator depends on the colour of the ions produced.
Join now!

As the titration is close to its end, indicator molecules near a drop of added base change colour due to the temporary excess of OH- ions there. As soon alence point in the titration occurs when all the moles of H+ ions present in the original volume of acid solution have reacted with an equivalent number of moles of OH- ions added from the burette.

Moles of H+ (originally in flask) = moles of OH- (added from the burette)

The end point of the titration occurs when a tiny excess of OH- ions changes the indicator ...

This is a preview of the whole essay