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pH curves, Titrations and Indicators

A2

Acids & Bases

Physical

AQA Content

Perform calculations for these titrations based on experimental results.
Sketch and explain the shapes of typical pH curves
Use pH curves to select an appropriate indicator.

Specification Notes

Titrations of acids with bases.
Typical pH curves for acid–base titrations in all combinations of weak and strong monoprotic acids and bases.

Notes


 





Titration

Volumetric analysis used to accurately determine the concentration, molar mass or formula of a compound.

Main Terms
Titrant: standard solution of known concentration
Titrate/Analyte: solution of an unknown concentration
Indicator: reagent used to determine the endpoint of the titration
End/equivalent point: colour changes. moles of titrant = moles of analyte.
Burette: dispenses accurate and known volumes of solutions
Volumetric flask: graduated flask used to prepare solutions.
Solute: substance dissolved in the solvent.
Solvent: substance in which the solute is dissolved.
Solution: the solute + solvent

Indicators
Indicators are either weak acids or bases – they dissociate slightly, forming ions in solution. They tend to have a number of alternating (conjugated) carbon-carbon double bonds and single bonds that can absorb wavelengths from visible light (appear coloured).

In acidic solutions, the molecule will not dissociate much so its colour is that of the original molecule.

In basic solutions (less H+ in solution), the molecule loses a hydrogen ion changing the arrangement of electrons so that different wavelengths of light are absorbed (ie the colour changes)

Common indicators:

Phenolphthalein:
Strong acid (indicator is colourless) & strong base (pink)
pH range of 8.3 - 10.0: strong acids with strong alkalis or weak acids with strong alkalis.
Methyl orange:
Strong acid (red) and weak base (yellow).
pH range of 3.1 - 4.4: strong acid & strong alkali or strong acid & weak alkali.
Phenol red:
Yellow to bright reddish pink, turning pink in alkaline solutions.
pH range of 6.8 - 8.2
Bromothymol blue:
Yellow to blue
pH range of 6 - 7.5

The equivalence point and the neutral point are not the same – the equivalence point doesn’t indicate that the solution is neutral. Use an indicator that has a colour change at a pH close to the equivalence point.

The pH scale measures how acidic or alkaline a substance is. The scale is logarithmic – at a pH of 3 the hydrogen ion concentration is ten-times greater than for pH 4.






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