Introduction to Titration: Standard Solution, Washing, Set-up

This is part of the HSC Chemistry course under the topic Quantitative Analysis

HSC Chemistry Syllabus

  • Conduct practical investigations to analyse the concentration of an unknown acid or base by titration

 

What is Titration?

Titration is an analytical technique used to determine the concentration of a solution. It is a quantitative technique since it involves numerical measurement. 
It is used to calculate the concentration of unknown solutions including acid and base solutions. 

There are two two types of titration involving acids and bases:
  • Indicator-based titration: uses colour of indicator to measure equivalence point
  • Conductometric titration: uses electrical conductivity to measure equivalence point

 

What is a Standard Solution? 

Titration requires a solution of accurately known concentration called a standard solution. If the unknown solution is basic then the standard solution will be acidic and vice versa.

Titration is considered to be a volumetric analysis as it measures the volume of standard solution required to exactly neutralise the unknown solution This required volume is called the titre or titre volume.

 

Properties of a primary standard include:

  • High purity
  • Accurately known chemical composition
  • Free of moisture (does not absorb water, which would reduce purity)
  • Chemically stable
  • Readily soluble in pure water
  • High molar weight (variations in mass have reduced effect on moles)

 

Examples of substances unsuitable for making primary standard solutions

  • Hydrochloric acid (HCl) of very high concentration because it becomes volatile, resulting in losses as fumes
  • Sodium hydroxide (NaOH) because it is hygroscopic (absorbs water)

 

Examples of good standard solutions

  • Oxalic acid
  • Benzoic acid
  • Anhydrous sodium carbonate
  • Anhydrous sodium hydrogen carbonate

Note that anhydrous compounds are prepared by drying in an oven until all water content is evaporated (mass remains constant).

 

What is a secondary standard?

A secondary standard is one whose concentration is accurately determined via titration with a primary standard solution prior to its use. For example, NaOH is often used as a secondary standard as its concentration can be determined by titrating with a primary acid standard solution e.g. oxalic acid. 

Secondary standards should be avoided if possible as the process of standardisation introduces more sources of error, therefore affecting the accuracy of results. 

 

Preparing a Standard Solution for Titration

How to prepare a standard solution for titration

 

Step 1: calculate and weigh the correct quantity of the substance

The first step of making a standard solution is to calculate the quantity of the substance we want to make the solution out of. 

For example, if we want to make 250.0 mL of a 0.0500 mol/L solution of sodium carbonate:

$$n = c \times V$$

$$n(Na_2CO_3) = 0.0500 \times 0.250 = 0.0125 \hspace{0.1cm} mol$$

 

$$m = n \times MM$$

$$m(Na_2CO_3) = 0.0125 \times (2 \times 22.99 + 12.01 + 3 \times 16.00)$$

$$m(Na_2CO_3) = 1.33 \hspace{0.1cm} g \hspace{0.1cm} (3 s.f.)$$

 

Therefore, 1.33 g of sodium carbonate is required to make this solution.

 

Step 2: dissolve the solid

From step 1, 1.33 g of sodium carbonate is dissolved in a small amount of distilled water in a beaker of appropriate size. Use a stirring rod to help completely dissolve the solid.

 

Step 3: transfer the standard solution to a volumetric flask

Transfer the solution to a 250.0 mL volumetric flask using a funnel. Ensure that the volumetric flask is rinsed with distilled water prior to use. It is appropriate to clean the volumetric flask with water because distilled water will be added to the flask eventually to make the standard solution. 

Ensure complete transfer of sodium carbonate by rinsing the beaker, stirring rod and funnel with distilled water and discarding the rinsing into the volumetric flask.

 

Step 4: add distilled water until required volume

Once the solution is transferred to the volumetric flask, add distilled water until the level reaches 1 cm below the graduation mark of the flask.

Use a plastic pipette to add distilled water dropwise until the bottom of the meniscus is level with the graduation mark.

 

Step 5: homogenise the solution

Stopper the flask and invert it up to 10 times.

 

Preparing the Titrant & Burette

A burette is a piece of volumetric equipment used in all types of titrations. it usually can contain up to 50.0 mL of a particular substance. The volume measurements are marked such that 0.0 mL is at the top of the burette while 50 mL is at the bottom of the burette.

The titrant is the solution to be added to the burette. The burette is washed with distilled water, followed by a small amount of titrant prior to use. The distilled water and titrant should coat all the entire interior surface of the burette and be discarded via the tip (ensure the stopcock is open). 

It is important to finish the washing process with the titrant as residual amounts of water would dilute the concentration of the titrant. Residual amounts of titrant will not affect the concentration of the titrant. 

Burette

 

The titrant of a titration can either be the standard solution or the unknown solution to be analysed. If the standard solution is to be added to the burette, then the unknown solution will be added to the conical flask.

After the burette is clamped to a retort stand and filled with the titrant, record the initial volume - this is the reading that is level with the bottom of the meniscus. After each titration trial, record the final volume.

The titre volume equals to the difference between the initial and final volumes recorded. 

 

Preparing the Conical Flask

A pipette is used to transfer a specific volume of either the standard solution (from the volumetric flask) or the unknown solution to the conical flask. This specific volume is known as the aliquot

The pipette is rinsed with the solution that is to be transferred. For example, if the standard solution is to be transferred using the pipette, it must be rinsed with the standard solution prior to use. 

conical flask

The conical flask is to be rinsed with water prior to use. It is appropriate to use distilled water for rinsing because any dilution of the solution in the conical flask will not affect the number of moles of the substance. As long as the pipette is rinsed appropriately and the correct quantity of the solution is transferred into the conical flask, its concentration in the flask will not affect the titration itself. 

After the solution is transferred, add a few drops of an appropriate acid/base indicator and place the conical flask on a white tile. The white tile allows for a better visualisation of the indicator's endpoint.

  

Example Titration

An experiment is conducted to investigate the concentration of a solution of HCl. Exactly 50.00 mL of HCl is required to neutralise 20.00 mL of a 0.0500 mol/L solution of sodium carbonate.

In this example, the titrant is the HCl solution and the titre volume is 50.00 mL.

Therefore,

  • Volumetric flask rinsed with distilled water
  • Pipette rinsed with sodium carbonate solution (standard)
  • Conical flask rinsed with distilled water
  • Burette rinsed with HCl solution (titrant)