Quantitative Estimation [ Theory and Procedure ]

Objectives

• To prepare a standard solution of sodium carbonate.
• To determine the strength of a given solution of hydrochloric acid by titrating it against standard sodium carbonate solution.

The Theory

What is Titration?

One of the important methods in Quantitative Analysis is Volumetric Analysis, a commonly used laboratory technique. It is used to determine the unknown concentration of a sample by measuring its volume. This process is also called titration. In a titration, a solution of unknown concentration is reacted with a solution of known concentration. The solution taken in the burette is called the titrant and the solution taken in the conical flask is called the analyte.

What does the end point of a titration mean?

The endpoint of a titration is the point at which the reaction between the titrant and the analyte becomes complete. Generally the endpoint of a titration is determined using indicators.

What is a standard solution?

A solution of known concentration is called the standard solution. A standard solution can be prepared by dissolving a known quantity of the substance in a definite volume of the solvent. The substance used to prepare the standard solution can be classified into two types.

1. Primary standard

A primary standard has the following features.

• It is highly pure and cheaply available.
• It is highly soluble in water.
• It is neither deliquescent nor hygroscopic.
• It is highly stable.

Oxalic acid, Mohr's salt, potassium dichromate are some examples of primary standards.

2. Secondary standard

Substances whose standard solutions cannot be prepared directly are called secondary standards.

Some examples are sodium hydroxide and potassium permanganate.

How do we express the concentration of a solution?

The concentration of a solution can be expressed in the following ways.

Normality: It is defined as the number of gram equivalent of solute dissolved in one litre of the solution. It is denoted by the letter 'N'.

Molarity: It is defined as the number of gram moles of solute dissolved in one litre of the solution. It is denoted by the letter 'M'.

Molality: It is defined as the number of moles of the solute dissolved in 1Kg of the solvent. It is denoted by the letter 'm'.

What does Acid-Base titration mean?

Titration can be classified into various types depending upon the chemical reactions occurring during titration. One of the commonly known titrations is the Acid-Base titration. It is a method used to determine the strength of an acid or alkali and this type of titration is based on the neutralisation reaction. In this reaction, acids and bases react to form salt and water.

           

What is an indicator?

An indicator is a chemical substance that undergoes a colour change at the endpoint. The endpoint of an acid-base titration can be determined using acid-base indicators. Acid Base indicators are either weak organic acids or weak organic bases. The colour change of an indicator depends on the pH of the medium. The un-ionized form of an indicator has one colour, but its ionized form has a different colour.

For example, consider the indicator phenolphthalein, whose ionization can be written as,

Some common examples of acid-base indicators

Indicators	pH Range	Acid	Base
Phenolphthalein	8.0 - 10.0	Colourless	Pink
Methyl Orange	3.1 - 4.4	Red	Orange
Methyl Red	4.4 - 6.2	Red	Yellow
Phenol Red	6.4 - 8.0	Yellow	Red
Thymol Blue	1.2 - 2.8	Red	Yellow
Thymol Blue	8.0 - 9.6	Yellow	Blue
Methyl Yellow	2.9 - 4.0	Red	Yellow

What are the different types of Acid-Base titrations?

Acid-base titration can be classified into the following types.

1. Strong acid-Strong base titration:In this type, a strong acid is titrated against a strong base. Both the acid and base are of equal strength, so at the endpoint, the pH will be neutral. The indicators used are phenolphthalein and methyl orange.
   • Example: Titration of HCl Vs NaOH
2. Strong acid-weak base titration: In this type, strong acid reacts with a weak base to form an acidic solution. So the pH of the solution is <7. Methyl orange is the indicator used to determine the endpoint.
   • Example: Titration of HCl Vs NH4OH
3. Strong base-weak acid titration: Here a strong base reacts with a weak acid to form a basic solution. So the pH of the solution is >7. In this type phenolphthalein is a suitable indicator for the determination of the end point.
   • Example: Titration of CH3COOH Vs NaOH
4. Weak acid-weak base titration: This type of titration is not very practical. Here both the acid and base are very weak so they do not ionize completely. So, it is difficult to determine the pH range around the end point and it is difficult to choose a suitable indicator for this type of titration.
   • Example: Titration of CH3COOH Vs NH4OH

How do we determine the strength of a given acid or base?

Determination of the strength is based on the Law of Equivalents. According to this law, the number of equivalence of the substance to be titrated is equal to the number of equivalence of the titrant used.

Consider an acid-alkali titration. V1 cm3 is that of an acid solution of normality N1 required to neutralize V2 cm3 of a base of normality N2.

We know that 1000 cm3 of 1N acid solution contains acid = 1 gram equivalent

V1 cm3of 1N acid solution contains acid= V1/1000 gram equivalents

Thus number of gram equivalents of acid in V1 cm3 of N1 acid solution is; 

Similarly, number of gram equivalents of base in V2 cm3 of N2 basic solution is; 

By the law of equivalents, at the end point, 

i.e, 

This is called the Normality Equation.

Similarly, Molarity equation can be written as,

Here we determine the strength of HCl by titrating it against a standard solution of sodium carbonate and they react to form NaCl, CO­2 and water. The chemical reaction can be represented as;

 

Here 1 mole of sodium carbonate reacts with 2 moles of HCl. So according to Molarity equation,

Learning outcomes

1. Students understand the terms: quantitative estimation, acid-base titrations, end point, standard solutions, molarity, molality, normality and indicators..
2. Students can calculate the strength of a given acid or base using molarity or normality equations.
3. Students acquire the skill to prepare the standard solution and to determine the end point.
4. Students acquire the skill to select the indicators based on the nature of the solution.
5. Students are familiar with the apparatus used for titration.
6. Students acquire the skill to perform the titration using sodium carbonate and hydrochloric acid in the real lab once they visualize the different steps.

Procedure

 Materials required

Real Lab Procedure

Preparation of 250ml M/10 (0.1 M) sodium carbonate solution

•  Using an electronic balance. we’ll first weigh about 2.65g of sodium carbonate crystals in a weighing bottle and transfer these into a 250ml beaker.
• Wash the weighing bottle 2 or 3 times using distilled water and transfer all the washings into the beaker. Dissolve the sodium carbonate crystals in the beaker by gentle stirring with a clean glass rod.
• When the sodium carbonate crystals in the beaker dissolve completely, transfer the entire solution from the beaker into a 250ml standard flask through a funnel, using a glass rod.
• Wash the beaker 2 to 3 times with distilled water and transfer all the washings into the standard flask.
• Finally wash the funnel thoroughly with distilled water.  This transfers the solution completely into the standard flask.
• Using a wash bottle, add enough distilled water to the standard flask so that the level is just below the calibration mark on it. Add the last few drops of distilled water with a pipette until the lower level of the meniscus just touches the mark on the standard flask.
• Stopper the measuring flask and shake gently to make the solution uniform throughout.  

Determination of strength of the given hydrochloric acid

• Take a burette and wash it with distilled water.
• Rinse and fill the burette with the given hydrochloric acid and set the initial burette reading as zero.
• Clamp it vertically to the burette stand.
• Rinse the pipette first with water and then with the given sodium carbonate solution.
• Pipette out 20ml of the given sodium hydroxide solution into a conical flask and add 1-2 drops of methyl orange into it.
• Titrate it against the hydrochloric acid taken in the burette till the colour of the solution in the conical flask changes from  yellow to light red.
• Now note down the final burette reading.
• Repeat the same procedure until concordant values are obtained.

Simulator Procedure (as performed through the Online Labs)

• You can select the type of titration from the 'Titration type' drop down list.
• Select the titrant from the 'Titrant' drop dowm list.
• Adjust the speed of the drops using the slider.
• Select the titrate from the 'Titrate' drop down list.
• Use the slider to select the molarity of the titrate.
• Use the slider to select the volume of the titrate.
• You can choose the indicator corresponding to each titration from the 'Indicators' drop down list.
• You can see the zoomed view of the burette reading on the right side.
• Click on the 'Start' button or on the 'Nozzle' of the burette to start the titration.
• You can see the volume of the titrant used for titration by clicking on the 'Show the volume of titrant' check box.
• To stop the titration either click on the 'Stop' button or click on the 'Nozzle' of the burette.
• You can see the chemical equation of the reaction on the side menu.
• Find out the number of moles of the titrant (n1) and that of the titrate (n2) from the chemical equation and enter the values in the respective text boxes and verify the values.
• Calculate the molarity of the titrant using the given equation and enter the value in the corresponding text box and verify the value.
• The molar mass of the titrant is shown on the side menu.
• Calculate the strength of the given titrant (in g/lit) and enter the value in the corresponding text box and verify the result.
• To redo the experiment click the 'Reset' button.

Observations

Observations can be recorded in a tabular form as shown and the molarity of the given HCl can be calculated using the molarity equation.

Sl. No.	Initial Reading of Burette	Final Reading of Burette	Volume of HCl used (ml)
1
2

Result

The strength of given Hydrochloric acid solution is  _________________g/litre.

Precautions

• Handle the apparatus and chemicals with care.
• Never close the pipette with the thumb. Close it with the index finger.
• Always keep the lower end of the pipette in the liquid while sucking it.
• Never pipette out hot or corrosive solutions.
• Do not blow out the last drop of the solution from the jet end of the pipette.
• Carefully fill the burette with the solution and see that the stopcock does not leak.
• Remove the funnel immediately after filling the burette.
• Do not allow any air bubbles to remain inside the burette.
• To take the reading, place the eye exactly at the level of the meniscus of the solution.
• There shoule not be any drops of solution at the tip of the burette at the end point.
• Place the titration flask containing the solution on a white tile to see the colour change correctly.
• Give rotary motion to the titration flask throughout the titration.
• Use one or two drops of the indicator that should be added using a glass dropper.
• The same number of drops of the indicator should be used for each titration.