Conductometric Titrations

Transcript

0:00Introduction

Hello everyone! Okay, a small puzzle to start. You are doing an acid base titration, but the solution in your flask is dark coloured, or cloudy. You add the indicator, but the colour change? You simply cannot see it! So how will you find the end point? Hmm. Here is the clever answer: forget the colour, and watch the conductance instead. That is a conductometric titration. Questions on the shapes of these curves do come up in JEE, so let's begin.

0:32Watch the conductance, not the colour

So, here is the idea. Look at the three graphs on screen. We keep adding base, and plot the conductance going up, against the volume added going across. That volume is V base: a capital V with the small word base written below it. Each graph is made of straight lines. So, where is the end point? Yes, exactly! At the point where the two straight lines meet. See the dashed line dropping down to the axis? That marks the end point. Right? And why do we need this at all? Back to our puzzle. It works for coloured or turbid solutions; turbid simply means cloudy. No colour to watch, no problem. Clear? It also works for very dilute, or weak, acids and bases, where the colour change of an indicator is not clear. So the conductance does the job of the indicator. Got it?

1:26The one question: which ion replaces which?

Now, before we read any curve, one key idea. The conductance of a solution depends on its ions: how many there are, and how fast each one moves. So suppose, during the titration, a fast ion is removed, and a slow ion takes its place. What happens to the conductance? It falls, correct! The same number of ions, but slower ones, so less current flows. Okay? Now, mobility simply means how fast an ion moves through the solution. So if the new ion has a similar mobility to the old one, the conductance hardly changes: nearly flat. With me? And when extra ions are just added, with nothing removed, the conductance rises. So here is the topper's trick: for every curve, ask which ion leaves, and which ion comes in. Got it?

2:17Strong acid, strong base: HCl with NaOH

Let's read graph a, the dark blue one. Hydrochloric acid, H C l, is in the flask, and we add sodium hydroxide, N a O H, a strong base. Before the end point, the conductance falls! Strange, right? We are adding ions, and still it falls. Hold on to that question, because it is exactly our Example one. Now, after the end point, all the acid is used up. So what is extra in the flask now? Excess sodium hydroxide, yes! Its hydroxide ions, O H minus, have nothing left to react with. The small minus written up top means a negative ion. So the hydroxide ions just pile up, and the conductance rises. Okay? Falls, then rises: that is a V shape. And the end point sits right at the bottom of the V, where the two lines meet.

3:12Weak acid with strong base (graph b)

Clear? Now graph b, the orange line. Acetic acid, with sodium hydroxide. Acetic acid is a weak acid. So, does the conductance start high, like hydrochloric acid, or low? Low, correct! Acetic acid, C H three COOH, is only weakly ionised, so there are few ions to begin with. The small numbers written below, like this three, are subscripts: they count the atoms. Okay? Then, a small dip right at the start; just note it on the graph. After that, a slow rise. Why? As the base is added, the salt, sodium acetate, forms. So the weak acid becomes ions. More ions, more conductance. Right? Now look after the end point. The line gets steeper. Why? Because now the excess hydroxide ions just pile up, and the hydroxide ion is a fast ion. So the conductance rises more steeply than before. Got it?

4:14Precipitation: KCl with AgNO₃

Last one: graph c, the green line. Potassium chloride, K C l, is in the flask, and we add silver nitrate, written A g N O three. This time, the volume going across is the volume of silver nitrate added. The silver ions take the chloride ions, C l minus, out of the solution, as solid silver chloride, the precipitate. Okay? So chloride ions leave, and nitrate ions come in to take their place. What happens to the conductance? Rise, fall, or nearly flat? Nearly flat, yes! The nitrate ion, N O three minus, and the chloride ion have similar mobility; they move at about the same speed. One ion out, a similar one in, so the green line stays nearly flat, right up to the end point. Clear? And after the end point? No chloride is left, so the extra silver nitrate just adds ions, and the conductance rises. Our rule again: ions added, nothing removed. With me?

5:15Example 1: why conductance falls when NaOH is added to HCl

Now, Example one: our question from graph a. Why does the conductance fall at first when sodium hydroxide is added to hydrochloric acid? Pause the video, and try it first. Done? Okay, let's check. Sodium hydroxide is a strong base, so it brings plenty of ions. We are adding ions, so the conductance should go up, right? No, no, careful! That is the trap. Write the full ionic equation. H plus, plus C l minus, plus the sodium ion, N a plus, plus O H minus, gives N a plus, plus C l minus, plus H two O, water. The small plus up top means a positive ion, and the arrow means gives. Okay? Look what happens. H plus and O H minus join to make water molecules, not ions. And the sodium ion takes the place of the hydrogen ion. So in the flask, H plus and C l minus become N a plus and C l minus: the same number of ions! Right? So the difference must be speed. Small lambda naught, the Greek letter lambda with a little circle on top, is the limiting molar conductivity of each ion: its molar conductivity at infinite dilution. For H plus it is about three hundred and fifty, and for the sodium ion only about fifty, both in siemens centimetre squared per mole. The wavy equals sign means approximately. Three hundred and fifty against just fifty: H plus is much, much faster. Clear? So each fast H plus is replaced by a slow sodium ion. Same number of ions, but much slower ones, so the conductance falls. That is why the V starts by falling. Puzzle solved!

7:05Example 2: curve for NH4OH added to acetic acid

Now a harder one, because this time both are weak. Sketch the conductance as ammonium hydroxide, N H four O H, is added to acetic acid. Use our one question: which ions leave, and which come in? Pause and try. Done? Let's check. Before the end point, the base reacts with the acid. The weak acid, which was only weakly ionised, becomes the salt ammonium acetate, C H three COONH four. And that salt is fully ionised. Okay? So, more and more ions appear as we add the base, and the conductance rises, right up to the end point. With me? Now the trap. After the end point, we keep adding base. With sodium hydroxide, the line shot up steeply. So here also, a steep rise, right? Oh no, careful! Ammonium hydroxide is a weak base. So the excess adds very few ions. Got it? So after the end point, the conductance stays nearly flat. Our sketch: a rise up to the end point, then nearly flat. One simple question built the whole curve. Very good!

8:22Quick revision: end point, ion swap rule, curve shapes

So, quick revision. One: the end point is where the two straight lines meet. Two: for every curve, ask which ion leaves, which ion comes in, and how fast it moves. Three: strong acid with strong base gives a V; weak acid with weak base gives a rise, then nearly flat. Now go and try the practice problems on this page. All the best!