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

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UESB 1113

General Laboratory I 

Practical Component (II) : -Atomic Structures & Periodicity

Group 6 / 2 

 

Name:                (1) Yeo Shi Wei (05UEB01783)

Bachelor of Science (Hons) Biotechnology           

(2)Timothy Lee Tek Wang (05UEB02658)

Bachelor of Science (Hons) Biotechnology  

Experiment No:         TWO (2) 

Title:         The Use Of Volumetric Flask, Burette and Pipette in Determining the Concentration of NaOH Solution.

Date:                         10 / 6 / 2005 

Laboratory:         SD017

Lecturer:                 Mr. Lee Chong Yong

Experiment 2

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

Objectives:  - To carry out acid-base titration and determine the end point with the use  of indicators such as phenolphthalein.

- To get the concentration of base when the concentration of acid is known.

- To do calculations related to titration.

Theory & Background:

Titrations are used to determine the concentration of acids or bases in solution. 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. This point is called the equivalence point and is generally determined by observing a color change in an added indicator such as phenolphthalein. From the volume and concentration of added base and the volume of acid solution, the unknown concentration of the solution before titration can be determined.

 Titrations can also be used to determine the number of acidic or basic groups in an unknown compound. A specific weight of the compound is titrated with a known concentration of acid or base until the equivalence point has been reached. From the volume and concentration of added acid or base and the initial weight of the compound, the equivalent weight, and thus the number of acidic or basic groups, can be computed. Instead of adding an indicator to observe the equivalence point, one can construct a graph on which the pH (see separate article) at regular intervals is plotted along one axis and the number of moles of added acid or base at these intervals along the other axis; such a plot is called a titration curve and is usually sigmoid (S-shaped), with the inflection point, where the curve changes direction, corresponding to the equivalence point.

 From the pH at the equivalence point, the dissociation constant of the acidic or basic group can be determined (see chemical equilibrium). If a compound contains several different acidic or basic groups, the titration curve will show several sigmoid-shaped curves like steps and the dissociation constant of each group can be obtained from the pH at its corresponding equivalence point. In pure water, there are always small amounts of hydrogen ions and hydroxyl anions present.

A definition of acids and bases, provided by the Swedish chemist Svante Arrhenius. According to this definition, an acid is any substance which, when dissolved in water, tends to increase the amount of H . An example is hydrochloric acid:

or perchloric acid:

A base is defined to be any substance which, when dissolved in water, tends to increase the amount of hydroxide. Examples include sodium hydroxide:

potassium hydroxide:

ammonia:

When an acid solution is mixed with a basic solution, a neutralization reaction occurs:

which is the opposite of the water ionization reaction. It can also be written in the form of a reaction including spectator ions. For example, when an HCl solution is mixed with an NaOH solution, the reaction, with spectator ions, would be

i.e., acid + base water + salt.

The titration is carried out by placing the solution of unknown concentration in a flask. To titrate it with a solution of known concentration, the latter is placed in a buret. Small amounts of this solution are allowed to run into the flask until a change in color of the solution or of an indicator occurs. It is often difficult to know when the end point of the titration will occur, so that overshooting the endpoint is common. In this case, one can back titrate by adding a little more of the unknown solution to the flask until the color disappears and then titrating in tiny amounts until the color change just begins to appear. If done carefully, this can lead to relatively accurate measurements of the unknown concentration.

Apparatus & Materials:

  • NaOH solution
  • HCL solution (1.00 x 10‾²M) or H2SO4 solution (1.00 x 10‾²M)
  • Phenolphthalein solution
  • Beaker (500cm³)
  • Burette(50cm³)
  • Pipette(20cm³ or 25cm³)
  • Volumetric flask (250cm³)
  • Funnel
  • Watch Glass
  • Erlenmeyer flask(250cm³)

Procedures:

  1. The volumetric flask is cleaned and rinsed with distilled water.
  2. All NaOH solution is transferred into the volumetric flask using funnel and the remaining NaOH solution is washed into the flask several times using distilled water.
  3. The NaOH solution is topped up to 250cm³ with distilled water, the cap is closed and the flask is rotated several times to get a homogenous solution.
  4. This solution is poured into a clean and dry beaker, labeled, covered with with a watch glass and putted aside. The volumetric flask is cleaned.
  5. The burette is cleaned with distilled water and rinsed with 5cm³ NaOH solution a few times. The burette is filled with NaOH solution using a funnel.
  6. The pipette is cleaned and rinsed a few times with the acid to be used in the titration.
  7. 20.0cm³ (or 25.0cm³) acid solution is pipetted into three clean Erlenmeyer flasks.
  8. Two drops of phenolphthalein is added into the acid solution. (the solution should remain colorless and do not use in excess.)
  9. The initial burette reading is recorded and the acid solution with NaOH solution is titrated in the burette until end-point is reached (when the solution turns pink). The end reading is recorded to obtain the volume of NaOH solution used.
  10. The titration is repeated for several times until the NaOH solution volumes remain constant within 3/1000 for three titrations, ie:
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Volume reading – Average volume reading

---------------------------------------------------       X    1000   ≤   3

Average volume reading

Data / Results:

Titration of NaOH with HCl

Titration of NaOH with H2SO4

Analysis & Calculations:

  1.  Titration of NaOH with HCL

(a)        Mean Titre

Average        =         X1 + X2 + X3

                        ---------------

                                3

                =        11.80 + 12.00 + 11.80

                        ---------------------------

                                        3

                =        11.87cm³

        (b)        Determination of accuracy

        X1        =        11.80 – 11.87

                        -----------------        X        1000                ≤        3

                                11.87

         

                =        | -5.89 |

                =        5.89cm³

        X2        =        12.00 – 11.87

                        -----------------        X        1000                ≤        3        

                                11.87

                

                =        2.53cm³

        

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