1. ACCESS TO SCIENCE: CHEMISTRY Learning aims for this session: The pH scale and the use of indicators in acid-base titrations Show a practical where you have determined the strengths of acids and alkalis by use of Universal Indicator paper and using a standardised pH meter (1.1) In this section relate the colour changes of common indicators to the pH at the equivalence points (1.2) Interpret pH titration curves (1.3) numeracy Literacy

2. WHAT IS AN ACID AND A BASE / ALKALI?  An acid is something that tastes sour.  An acid is something that donates hydrogen ions / protons, so increases the acidity of a solution  We can describe hydrogen ions as a proton, because once hydrogen has lost an electron, to become H+, all that is left is a proton in its electronic structure of the atom.  A base / alkali is something that tastes bitter, think of soap, this contains alkali – sodium hydroxide.  A base is a proton / hydrogen ion acceptor, so it ‘mops up’ hydrogen ions leaving the solution less acidic by the formation of hydroxide (OH-) ions.

3. USING A PH METER FOR TITRATIONS  When an acid is titrated by a base, the hydrogen ion concentration changes.  The reaction can then be followed by monitoring the change in pH value.  pH is the potential or the power of hydrogen  A pH meter provides a convenient and accurate measurement of pH as long as it is calibrated first.  We plot the pH of the solution when acids and bases are added together, and so find the equivalence point, where acid = base.  The resulting graph that is plotted representative of the type of acid and base used.

4.  Indicators show colours that indicate the pH of a solution  Some show only two colours, others like universal indicator consist of a few different indicators and give a better indication of the pH of the solution Pros  It’s a quick way of telling what pH the solution is  It’s a cheap method Cons  It doesn’t give the exact pH of the solution just an estimated guess from the colour chart  Human error- may be hard to interpret the pH from the colour Different indicators are used depending on the pH of the equivalence point of the acid and base used. Methyl orange = 3.7, litmus = 6.5 phenolphthalein = 9.3

5. HOW DO WE MEASURE P H :  We measure pH by the strength of the hydrogen ion concentration.  pH 0.1 has the most hydrogen ions and is therefore the most acidic  pH 14 has the least hydrogen ion concentration and is therefore most basic. Its official measure is –log10 [H+] This is seen as a logarithmic scale So pH 1 = 0.1 H+ mol/l pH 2 = 0.01 H+ mol/l pH 3 = 0.001 H+ mol/1 pH14 = 0.00000000000001 H+ mol/l

6. PH GRAPHS  The graphs that result from different acids and base types look different.  For strong acids and strong bases where they fully dissociate into ions H + and OH -

7. FOR STRONG ACIDS AND WEAK BASES WEAK BASES LIKE AMMONIA GAIN THEIR HYDROXIDE IONS FROM A REVERSIBLE REACTION WITH WATER SO LITTLE HYDROXIDE IONS IN SOLUTION. Alkaline buffer solutions are commonly made from a weak base and one of its salts. A For a weak base the pH is less than that of a strong base pH still falls rapidly but then becomes less steep The gradient is less due to buffer formation A buffer solution is one which resists changes in pH when small quantities of an acid or an alkali are added to it.

8. WEAK ACID AND STRONG BASE WEAK ACIDS DO NOT FULLY DISSOCIATE INTO IONS Past the equivalence point the solution resists changes in pH due to buffer conditions within the solution. Acid buffers: weak acid and one of its salts - often a sodium salt.

9. WEAK ACID AND WEAK BASE – NEITHER HAVE THE A LOT OF HYDROGEN IONS OR HYDROXIDE IONS IN SOLUTION.

10. CHOICE OF INDICATORS

11. PRACTICAL  1. test common substances with universal indicator paper and record your findings  2. carry out pH titrations using a pH meter for various acids and bases.

12. PORTFOLIO PRACTICAL