Acid–Base Indicators

Transcript

0:00Introduction

Hello everyone! Okay, one small puzzle before we start. You titrate hydrochloric acid with sodium hydroxide. At the equivalence point the solution is neutral: p H seven, at twenty five degrees Celsius, right? But phenolphthalein changes colour only between p H eight point three and ten. So it should turn pink too late and give the wrong answer, isn't it? Yet for this titration, it works perfectly well. How? Today we find out. Choosing the right indicator comes up often in JEE, so let's begin.

0:36What is an indicator?

So, what is an indicator? It is a weak acid, or sometimes a weak base, whose two forms have different colours. Okay? We write the acid form as HIN; the I N stands for indicator. HIN gives H plus and I N minus. The small plus and minus up top are charges, and the two half arrows mean equilibrium. Each form has its own colour. Right? Now, this is the Henderson equation from buffers. p H equals p KIN, plus log of, I N minus over HIN, as you see on screen. The square brackets mean concentration, the small I N below the K is for indicator, and the log is to the base ten. With me? So tell me. If the p H is below p KIN, is the log term positive or negative? And which form is more? Negative, yes! So there is less I N minus than HIN, and you see the colour of the acid form. Above p KIN, the log is positive, so I N minus is more, and you see its colour. Clear?

1:41The colour-change range

Now, when does your eye actually see the colour change? Only while the ratio, I N minus over HIN, goes from about one tenth to ten. Got it? So, put these two ratios into our equation. What is log of one tenth, and log of ten? Minus one, and plus one. Correct! So across that window, the log term moves from minus one to plus one. Right? And that gives the colour-change range: p H equals p KIN, plus or minus one; the plus-minus sign means both ends. So an indicator changes colour over about one p H unit on each side of its p KIN value. Remember this line.

2:22Three indicators you must know

Now, three indicators you must know. Methyl orange changes between p H three point one and four point four, from red to yellow as the p H rises. Clear? Methyl red: four point four to six point two, also red to yellow. Got it? And phenolphthalein: eight point three to ten, colourless to pink. Right? Now look at the three ranges together. Two of them change colour in acid, below seven. Which one changes on the basic side? Phenolphthalein, exactly! Its whole range is above seven. Methyl orange and methyl red change on the acid side. Keep this in mind for the next part. With me? One small note. These acid base indicators respond to the p H of the solution. Redox indicators are different: they respond to the electrode potential.

3:14Choosing the right indicator

Okay? Now, the big question: which indicator for which titration? Look at this graph from the course: p H against the volume of zero point one molar N a O H added, in millilitres. The solid curve is H C l, a strong acid; the dashed one is acetic acid, C H three COOH, a weak acid. The lower shaded band is the methyl orange range, the upper one is phenolphthalein. The tall dotted line marks equivalence at twenty five millilitres, and the black dot, with its short dotted line, is the half-way point for acetic acid, where p H equals p K a. Right? Here is the rule. The indicator's range must lie in the steep part of the titration curve. There, a tiny amount of base changes the p H a lot, so the colour flips right at equivalence. Clear? Now, our puzzle. Look at the solid H C l curve at twenty five millilitres: it shoots almost straight up, through both bands. So phenolphthalein turns pink practically at the equivalence volume, even though eight point three is not seven. That is why, for a strong acid with a strong base, most indicators work. Puzzle solved! Now the trap. Methyl orange worked for H C l. So it works for acetic acid too, right? No, no, careful! Look at the dashed curve. It crosses the methyl orange band near the start, in the slow, gentle part, long before equivalence. So methyl orange would change colour far too early. The dashed curve jumps mainly on the basic side, through the phenolphthalein band. So, weak acid with strong base: phenolphthalein. Got it? Flip it around: strong acid with weak base, not on this graph. Now the equivalence point is below seven, so we need an indicator that changes in acid: methyl orange, or methyl red. Okay? And finally, weak acid with weak base. There is no sharp jump at all, so no simple indicator works. Clear?

5:20Example 1: indicator ratio at pH 4 and pH 6

Now, Example one. An indicator has p KIN equal to five. Find the ratio, I N minus over HIN, at p H four and at p H six. Pause the video, and try it first. Done? Okay, let's check. Rearrange our equation: log of the ratio, I N minus over HIN, equals p H minus p KIN, right? At p H four: four minus five is minus one. So the ratio is minus one, isn't it? Oh no, careful! Minus one is the log of the ratio, not the ratio. So the ratio is ten to the power minus one: zero point one. Okay? At p H six: six minus five is plus one, so the ratio is ten to the power one, which is ten. With me? And see, p H four and six are exactly p KIN minus one and plus one: the two ends of the colour-change range. That's it!

6:20Example 2: fraction in basic form at pH = pKIn + 1

Now a slightly harder one, JEE style. What fraction of an indicator is in its basic form, I N minus, when the p H equals p KIN plus one? Pause and try. Hint: find the ratio first. Done? Okay, let's check. p H minus p KIN is one, so log of the ratio is one. The double arrow means implies: the ratio, I N minus over HIN, is ten. Right? So the fraction in the basic form is ten, correct? No, no, wait! The fraction can never be more than one. The fraction compares I N minus with the total: HIN plus I N minus, both together. So, ten parts of I N minus for every one part of HIN: ten, over one plus ten. Got it? That is ten over eleven, about ninety one percent; the wavy equals sign means approximately. So at the top of the colour-change range, about ninety one percent is already in the basic form. Very good!

7:27Quick revision: pKIn plus minus 1 and choosing an indicator

So, quick revision. p H equals p KIN, plus log of, I N minus over HIN, and the colour changes over p KIN plus or minus one. The range must lie in the steep part of the titration curve. Weak acid with strong base: phenolphthalein; strong acid with weak base: methyl orange, or methyl red. Now go and try the practice problems on this page. You will find them easy, I promise. All the best!