0620 · 12.2Acid–base titrations

Titration purpose and apparatus

0:00Two colourless solutions can react without producing an obvious visible change. That makes it difficult to judge when enough acid has been added to neutralise an alkali. A titration solves this problem using accurate volume measurements and an indicator. By the end, you will understand what each piece of apparatus does and how to recognise the stopping point. An alkali is a base that dissolves in water. When an acid reacts with an alkali, neutralisation produces a salt and water.

0:25For example, hydrochloric acid plus sodium hydroxide gives sodium chloride plus water. We will use these solutions throughout this video, with the alkali in the flask and acid above it. An acid–base titration measures the volume of one solution needed to react with a measured volume of another. The important idea is controlled addition, rather than simply pouring the solutions together. We add acid carefully until an indicator shows that the end-point has been reached.

0:50Accurate volumes make the result useful for investigating the solutions. The volumetric pipette measures one fixed volume accurately. Its narrow tube has a calibration mark showing where the liquid level must reach. A burette has a graduated scale and a tap, so it can deliver a variable volume accurately. These instruments have different jobs because the alkali is measured out in advance, but the amount of acid needed must be found.

1:14Wear eye protection, and use a pipette filler rather than drawing liquid into the pipette by mouth.

Preparing solutions and recognising end-points

1:18First, rinse the volumetric pipette with a little sodium hydroxide solution. This prevents water left inside from diluting the sample. Draw in the alkali and adjust the bottom of its curved surface to the calibration mark at eye level. Transfer twenty-five cubic centimetres of sodium hydroxide solution into a conical flask using the pipette. Allow it to drain as designed, rather than blowing out the small amount remaining in the tip.

1:41The flask gives us space to swirl the mixture without easily spilling it. Its markings are not used to measure the starting volume accurately. Add a few drops of a suitable indicator, such as methyl orange or thymolphthalein. An indicator changes colour over a particular pH range, making an otherwise invisible reaction easy to follow. Only a few drops are needed to show that change. A large amount is unnecessary and can affect the result because indicators themselves have acid–base properties.

2:06Methyl orange is yellow in the sodium hydroxide solution. As hydrochloric acid is added, the alkali is gradually neutralised. At the end-point, the indicator changes from yellow to orange. Adding excess acid makes it red, so aiming for a strong red colour would take us beyond the intended stopping point. With thymolphthalein, the starting alkaline solution is blue. The end-point is reached when that blue colour just disappears, leaving the mixture colourless.

2:31These indicators signal the stopping point through different colour changes. Always connect the observation to the indicator being used, rather than learning one colour change for every titration. The end-point is the stage at which the indicator just changes colour. A suitable indicator changes in the region where neutralisation is complete, giving a useful practical stopping signal. This does not mean every titration must finish at exactly pH seven. Our observation is the indicator change, not a direct measurement of pH.

Carrying out accurate titrations

2:59Rinse the burette with a little hydrochloric acid, then fill it with that solution. Run some acid through the tap so the tip is filled and contains no air bubble. Remove the filling funnel before taking a reading. Otherwise, liquid dripping from the funnel could change the amount in the burette during the experiment. Read the burette at eye level from the bottom of the meniscus. The meniscus is the curved surface of the liquid inside the tube.

3:22Looking from above or below can make its position appear different, giving an inaccurate reading. Record the initial reading before any acid enters the conical flask. Place the flask on a white tile beneath the burette. The pale background makes the indicator change easier to see. Open the tap and add acid while swirling the flask so the solutions mix thoroughly. Without mixing, acid near the point of entry could change the indicator before the whole sample has reacted.

3:46First, carry out a rough titration to find approximately where the end-point occurs. Then repeat with a fresh measured sample of alkali and a few drops of indicator. Near the expected end-point, add acid drop by drop while swirling. Stop when the indicator just changes colour and the new colour stays after mixing. A brief colour change where a drop lands does not necessarily mean the end-point has been reached.

4:08If alkali remains, swirling can make that colour disappear. Near the stopping point, even a small addition can take the mixture too far. Slowing down lets you identify the first lasting change instead of overshooting it. Burette readings are recorded to two decimal places. Suppose our initial reading is one point one five cubic centimetres. At the end-point, the final reading is twenty-four point seven zero cubic centimetres. To find the volume delivered, subtract the initial value from the final one.

4:35Twenty-four point seven zero minus one point one five gives twenty-three point five five cubic centimetres. This delivered volume is called the titre. The final reading alone is not the titre unless the burette started at zero. The burette scale is numbered from the top downwards, so the reading rises as liquid leaves. Subtracting the readings tells us how much acid entered the flask, regardless of the starting position. After the rough trial, repeat accurate titrations until two titres agree closely.

5:03These are called concordant titres. Use a fresh measured portion of alkali each time, so every trial starts with the same sample volume. Agreement helps show that the end-point has been identified consistently, rather than reached by chance. Several common mistakes come from confusing the jobs of the apparatus. A measuring cylinder is less suitable than a volumetric pipette when an accurate fixed volume is needed. Universal indicator gives a broad sequence of colours rather than the sharp change wanted here.

5:30Adding acid rapidly near the end-point risks recording too large a titre. In the Practical Test or Alternative to Practical, you may be asked to describe this method or identify the end-point. Begin by stating how the fixed alkali volume is measured, then explain the controlled acid addition. Name the indicator and give its specific colour change. A clear method links each instruction to an observable or measurable result. Use a volumetric pipette to transfer a fixed volume of sodium hydroxide solution into a conical flask.

5:59Add a few drops of methyl orange. Run in hydrochloric acid from a burette while swirling, then add it drop by drop near the end-point. Stop when the indicator just changes from yellow to orange. Record the initial and final burette readings at eye level from the bottom of the meniscus. Calculate the titre by subtracting the initial reading from the final one. Carry out a rough titration, then repeat accurate trials until two titres agree closely.

6:23These statements describe both how the volume is measured and how the result is made reliable. The central idea is to combine accurate measurement with a visible stopping signal. A volumetric pipette fixes the starting alkali volume, while the burette measures the acid delivered. The indicator identifies the end-point through a lasting colour change. Careful mixing, dropwise addition and concordant titres turn that observation into a reliable measurement.