9 - Investigating pH changes

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Last updated 4:27 PM on 8/24/26
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7 Terms

1
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What is an acid & what is a base?

  • Acid: proton donor

  • Base: proton acceptor


2
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What is an alkali?

A base that dissolves in water

3
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Why are acid-base indicators used?

To detect when a reaction reaches completion/becomes neutral, usually by the presence of a colour change 

4
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Why does a pH probe need to be calibrated?

So that the pH values for each pH reading are accurate 

5
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What is the method to calibrating a pH probe?

  1. Rinse the pH probe thoroughly with distilled water & shake gently to remove excess water

  2. Place the probe in standard pH 7.00 buffer solution & record the pH reading

  3. Repeat this process using standard pH 4.00 & pH 9.00 buffer solutions & record the pH readings

  4. Plot a graph of the recorded pH readings (x-axis) against the pH of the buffer solution (y-axis) → should be a curve


6
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What is the method to measuring pH using an acid-alkali mixture?

  1. Rinse a burette with 0.100moldm-3 ethanoic acid solution & then fill the burette with this solution (label this burette)

  2. Use the burette to transfer 20cm3 of ethanoic acid to a clean 100cm3 beaker

  3. Rinse a second burette with 0.1moldm-3 NaOH solution & then fill this second burette with this solution (label this burette)

  4. Rinse the pH probe with distilled water & clamp it so that its bulb is immersed in the ethanoic acid solution in the beaker

  5. Use a rod to stir the solution & record the pH reading in a table

  6. Add 2cm3 of NaOH solution from the burette at a time. Stir the solution & record the pH alongside the volume of NaOH added

  7. When the end-point is nearly reached, add 0.2cm3 of NaOH each time. Stir the solution & record the pH alongside the volume of NaOH added

  8. After this, continue adding 2cm3 NaOH until it’s in excess. Stir the solution & record the pH alongside the volume of NaOH added


<ol><li><p>Rinse a burette with 0.100moldm<sup>-3</sup> ethanoic acid solution &amp; then fill the burette with this solution (label this burette)</p></li><li><p>Use the burette to transfer 20cm<sup>3</sup> of ethanoic acid to a clean 100cm<sup>3</sup> beaker</p></li><li><p>Rinse a second burette with 0.1moldm<sup>-3 </sup>NaOH solution &amp; then fill this second burette with this solution (label this burette)</p></li><li><p>Rinse the pH probe with distilled water &amp; clamp it so that its bulb is immersed in the ethanoic acid solution in the beaker</p></li><li><p>Use a rod to stir the solution &amp; record the pH reading in a table</p></li><li><p>Add 2cm<sup>3</sup> of NaOH solution from the burette at a time. Stir the solution &amp; record the pH alongside the volume of NaOH added</p></li><li><p>When the end-point is nearly reached, add 0.2cm<sup>3</sup> of NaOH each time. Stir the solution &amp; record the pH alongside the volume of NaOH added</p></li><li><p>After this, continue adding 2cm<sup>3</sup> NaOH until it’s in excess. Stir the solution &amp; record the pH alongside the volume of NaOH added</p></li></ol><p></p>
7
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Why is it important to stir the resulting solution?

Ensures that all of the acid & base is thoroughly mixed, so the pH doesn’t significantly vary throughout the solution, as this would give an inaccurate reading