A formulation is a mixture with a purpose
Definition
Formulation
A mixture designed for a particular use.
Definition
Component
One of the substances present in a mixture.
- Each component has a specific purpose, such as giving colour, protecting a surface or making the product easier to use.
- The components are mixed in measured quantities, because changing the proportions changes the properties.
- The components are mixed rather than reacted, so each keeps its own properties within the product.
- A formulation is therefore a mixture, and it is not pure in the chemical sense.
- Manufacturers follow a precise recipe, so every batch has the same properties.
Key Idea
A formulation is useful because both its components and their amounts are chosen deliberately for one purpose.
Paints, medicines, cleaning products and fertilisers are everyday formulations
- Paint contains a pigment for colour, a binder that holds the pigment on the surface and a solvent that keeps the paint runny until it dries.
- A medicine tablet contains the active drug with fillers that bulk it out, binders that hold it together and a coating that makes it easier to swallow.
- Washing-up liquid combines detergents that lift grease with water, fragrance and colouring.
- Fertilisers are blends of nitrogen, phosphorus and potassium compounds in proportions suited to particular crops.
- Fuels, cosmetics, sun creams and many foods are also formulations.
Practical
Making Prussian blue paint
- Aim: make a pure, dry sample of the pigment Prussian blue and find out which binder turns it into the best paint.
- Apparatus: two 100 cm3100\,\text{cm}^3100cm3 conical flasks, a 25 cm325\,\text{cm}^325cm3 measuring cylinder, a glass rod, a filter funnel and filter paper, a wash bottle of distilled water, a dropping pipette, propanone, a watch glass, a pestle and mortar, a spatula, a digital balance, iron(II) sulfate, potassium hexacyanoferrate, binders such as linseed oil, egg yolk and dilute PVA glue, and white paper.
Part 1: making the pigment
- Dissolve a spatula of iron(II) sulfate in about 20 cm320\,\text{cm}^320cm3 of distilled water in one conical flask.
- Dissolve a spatula of potassium hexacyanoferrate in about 20 cm320\,\text{cm}^320cm3 of distilled water in the second flask.
- Pour one solution slowly into the other while stirring, so that a dark blue precipitate forms.
- Filter the mixture, then wash the solid on the filter paper with distilled water to remove the soluble by-products.
- Rinse the solid with a few drops of propanone, which mixes with the water left on it and evaporates quickly.
- Scrape the solid onto a watch glass and leave it to dry.
Part 2: testing the binders
- Grind the dry pigment to a fine powder with a pestle and mortar.
- Weigh equal masses of the pigment and mix each one with the same volume of a different binder.
- Paint each mixture onto the same kind of paper, using the same number of brush strokes.
- Compare how easily each paint spreads, how even the colour is and how long it takes to dry.
Accuracy and safety
- Use the same mass of pigment and the same volume of binder each time, so that the paints are compared fairly.
- Keep acids well away from potassium hexacyanoferrate, because they react to release a very toxic gas.
- Keep propanone away from flames, because it is highly flammable.
- Wear eye protection throughout.
Changing one component changes the whole formulation
- Adding more pigment gives a paint a stronger colour, but too much can make it crack as it dries.
- Adding more solvent makes a paint thinner and easier to spread, but it covers the surface less well.
- In a medicine, the dose of active ingredient must be exact, because too little does not work and too much can be harmful.
- Formulations are tested so that the product is safe, effective and the same from batch to batch.
Common Mistake
Do not describe a formulation as a compound, because its components are mixed and are not chemically bonded together.
A formulation melts over a range of temperatures
- Like any mixture, a formulation has no single melting point.
- A heating curve for a formulation shows the solid melting over a range, with no flat section at one temperature.
- A pure substance gives a flat section at its melting point, because the temperature stays constant while it melts.
- This lets a chemist tell a formulation apart from a pure substance using melting data.
- Chocolate is a formulation of cocoa solids, cocoa butter, sugar and milk, which is why it softens gradually as it warms.
Example
Sample A stays at 80 ∘C80\ ^{\circ}\text{C}80 ∘C while it melts, but sample B melts between 55 ∘C55\ ^{\circ}\text{C}55 ∘C and 62 ∘C62\ ^{\circ}\text{C}62 ∘C, so sample B is the formulation.
Self review
- What is a formulation?
- Why are the components of a formulation measured carefully?
- Name three components of paint and give the job of each.
- Why is a formulation not pure in the chemical sense?
- How would a heating curve show that a substance is a formulation?
