2(f) Acids, alkalis and titrations

what do the numbers on the pH scale indicate?

0-3 strongly acidic

4-6 weakly acidic

7 neutral

8-10 weakly alklaine

11-14 strongly alkaline

what is the pH scale a measure of?

the amount of H⁺ ions present in a solution

litmus

acid → alkali

red → blue

phenolphthalein

acid → alkali

colourless → pink

methyl orange

acid → alkali

red → yellow

define an acid

an acid is a source of H⁺ ions in an aqueous solution

define an alkali

an alkali is a source of OH^- in an aqueous solution

what is a neutralisation reaction?

when an acid reacts with an alkali to form water

the H⁺ ions react with the OH^- ions to produce water

not all reactions of acids are neutralisations

e.g. when a metal reacts with an acid a salt is produced, but no water, so it is not a neutralisation

what is neutralisation important for?

the treatment of soils

some plants cannot pH levels below 7

explain how to do an acid-alkali titration

method:

1. use the pipette and pipette filler to put 25cm³ of sodium hydroxide solution into the conical flask

2. place the conical flask on a white tile so the tip of the burette is inside the flask

3. add a few drops of suitable indicator to the solution

4. perform a rough titration by taking the burette reading and running in the solution in 1-3cm³ portions while swirling the flask vigorously. quickly close the tap when a sharp colour change occurs and read the burette to calculate the volume of solution used

5. repeat the titration, but when approaching the final volume, close the tap so that the solution drips one drop at a time

6. close the tap when a sharp colour change occurs and record the volume to the nearest 0.05cm³

7. repeat this until you have concordant results (within 0.1cm³ of each other) and take the mean