Quality control of mixtures
Before you start your investigation, you should carry out a risk assessment and have it checked by your
teacher. For help with this, read through our health and safety information and look out for health and
safety warnings in the text.
Goldilocks was rather fussy – too hot, too cold, too big, too small! But some things were just right. She knew
what she liked. Goldilocks was a typical consumer, just like you and me. We get used to particular brands of
product, and complain if something is not quite the same as usual. Manufacturers use quality control
procedures to check that their products are consistent and up to standard. This poses some interesting
challenges. In this project you must rise to the challenge. Try to make contact with a business that performs
these sorts of analyses. This might be a manufacturer’s quality control department, or an independent
organisation, such as a public analyst who tests samples for the Trading Standards Department.
Getting started
Quality control includes checking samples, to make sure that:
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They contain the correct substances
A solution has the correct concentration
A mixture has the correct composition
Think about sampling single substances, solutions, homogeneous mixtures and heterogeneous mixtures.
Consider how many samples must be taken from a batch of product to obtain reliable results. If more than
one, could a single sample be split into several portions, or should multiple samples be taken? Find out about
‘representative sampling’. Draw up guidelines to show how to sample different categories of products.
Remember, some of the chemicals used in your tests may be hazardous, make
sure you complete a risk assessment.
Checking a single substance
Before packaging a batch of tablets, a pharmaceutical company must check to make sure they contain the right
compound, and the correct dose. Your first task is to research and carry out procedures for both qualitative
and quantitative analysis of tablets with a single active ingredient, such as aspirin, paracetamol or ferrous
sulphate. How closely do your results match the label? Suggest reasons for any significant discrepancies. In
case of customer complaint, you must keep a concise record of your analyses.
Checking a solution
Concentration is critical. Vinegar or lemon juice on fish tastes good, but concentrated ethanoic acid or citric
acid crystals wouldn’t! Devise a procedure for sampling and checking the concentration of one ingredient in a
consumer product that is sold as a solution. Compare your results with quantities shown on the label, if any.
Possibilities include:
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Citric acid in lemonade - most lemonade has never been anywhere near a lemon!
Hydrogen peroxide in hair bleach
Sugar in a soft drink
Salt in soy sauce.
You may have done something similar earlier in your school career to compare brands of household bleach or
vinegar. Don’t repeat it - try a new challenge!
Checking a heterogeneous mixture
Analysing a mixture composed of separate parts is more of a challenge. Labels don’t usually show the amount
of each component, but they do list them in decreasing order, so you can check relative amounts. You need to
consider the properties of the main components and devise a way to separate them, so you can measure
them.
Devise a procedure for determining the amounts of main components in a heterogeneous mixture.
Possibilities include:
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Oil, vinegar and herbs (as dry mass) in a French dressing
Spaghetti and sauce in a tin
Fruit pieces and liquid/gel in jam
Cereal, fruit, nuts, and milk powder in a muesli.
You must ensure that you check representative samples of your mixture.
Remember, food and drink must not be consumed in the laboratory. Food and
drink that’s been handled in the lab shouldn’t be consumed either.