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Study Notes: Laboratory Solutions

Laboratory solutions are important laboratory tools. Improper management of laboratory solutions can result in inefficiencies and incorrect results.

A solution is a mixture of two or more substances uniformly dispersed through a single phase, so that the mixture has the same composition throughout.

A solution may be a solid dissolved in a liquid, such as sodium chloride (salt) dissolved in water to make a salt solution.

A solution is comprised of a solvent and a solute, where:

  • the solvent is the liquid in which the solid dissolves. Water is the most common liquid solvent
  • the solute is the solid that dissolves in the solvent. In the example of the salt solution given above, sodium chloride is the solute.

Stock (or standard) solutions
A stock, or standard, solution is a concentrated solution with an accurately known concentration. Stock solutions can be diluted to prepare a range of working solutions, of lower concentration, for use in the laboratory.

Stock, or standard, solutions are useful because:

  • their concentration is accurate, as they contain large amounts of solute (weighing errors are reduced)
  • many are commercially available (eg stock solutions of concentrated acids and bases)
  • some concentrated solutions are more stable, and less likely to support microbial growth than working solutions used routinely (eg 9% NaCl is stable and won't allow bacterial growth, whereas 0.9% NaCl will allow microbial growth at room temperature)
  • one stock solution can be used to prepare many different concentrations of working solutions - this saves time and space in the laboratory.

A stock solution may be either a primary or a secondary standard solution.

Primary standard solutions
A primary standard solution has the following characteristics.

  • The solute is a pure chemical with a known formula.
  • The solute is stable in storage.
  • The solute dissolves well in water.
  • The solute's reactions are known.
  • The solution made from the solute is stable.

The solid (solute) used to make the primary standard must be properly dried before being dissolved to form the primary standard solution with a known concentration. The high purity of the solute and its accurate weighing and dilution enables the concentration of the solution to be calculated when the solution is made up. No further procedures have to be undertaken.

Titration
Titration is a procedure where a burette is used to add a known amount of solution to a measured amount of a standard solution until a given reaction, which is often indicated by a colour change, is completed.

To view an animation of the titration process click on the 'Start' button below.

Note that distilled water is not always added during the process, depending on the type of titration being carried out.Skip flash movie

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The purpose of a titration is to determine the concentration of a solution by comparing it with a solution with an accurately known concentration.

Secondary standard solutions
The concentration of a secondary standard solution is obtained by titrating against a primary standard solution. This process is called standardisation. The concentration of the secondary standard is determined by stoichiometric calculation.

Some chemicals, such as hydrochloric acid and sodium hydroxide, are not suitable for use as a primary standard and must be made up as a secondary standard solution.

  • Hydrochloric acid is stable and will keep almost indefinitely in the bottle. However, the composition of concentrated hydrochloric acid is variable so there is no easy way to make up solutions of this acid with a known concentration. To make a standard solution of hydrochloric acid, the solution is made up to the approximate concentration and then titrated with a suitable primary standard (such as sodium carbonate) so that its exact concentration may be obtained.
  • Sodium hydroxide absorbs water from the atmosphere, and reacts with atmospheric carbon dioxide to form carbonates, so its composition at any one time is unknown. Even a solution of sodium hydroxide will continue to react with atmospheric carbon dioxide in the stock bottle. A standardised solution of sodium hydroxide can be obtained by titrating a portion of the aqueous sodium hydroxide against a primary standard such as KHP or previously standardised hydrochloric acid. A standardised sodium hydroxide solution can only be used for a limited time before being restandardised, because of its instability.

Working solutions
A working solution is obtained by the accurate dilution of a primary or secondary standard solution. The concentration of the working solution is determined by a ratio (dilution factor) calculation.

Working solutions are made up in small quantities for use in the short term. They are often consumed on the day of preparation.

Advantages of making up fresh working solutions on a regular basis include:

  • the technician is assured that the solution is the correct concentration as it will not have had time to deteriorate
  • contamination is minimised.

When making up a working solution it is important to minimise waste, so working solutions should be made up in the minimum amount necessary for immediate use.

Stock solutions are prepared at relatively high concentrations and diluted to make working solutions. In laboratories, the concentration of solutions is commonly expressed in molar units (M).

You must be able to determine the amount of stock solution you need to make the required volume and molarity of the working solution. The easiest way to calculate the amount is to first determine a dilution factor by the following formula:

Dilution factor equals concentration of stock solution divided by concentration of working solution

Then divide the required volume of working solution desired by the dilution factor to determine the amount of stock solution to add, ie:

Volume of stock solution equals volume of working solution divided by dilution factor

For example, if you are making 250 mL of a working solution of 0.05M NaCl from a stock solution of 0.5M NaCl:

Dilution factor equals 0.5M divided by 0.05M equals 10

Volume of 5M NaCI to add equals 250mL divided by 10 equals 25mL

The 25 mL volume of stock solution is then accurately dispensed (with a pipette) and made up to 250 mL to prepare the working solution.

To see how 250 mL of 0.05M NaCl solution is made from a stock solution of 0.5M NaCl, click below.Skip flash movie

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