Permanganate and Dichromate Titrations

Transcript

0:00Introduction

Hello everyone! Let's start with a small puzzle. Take two titrations of the same iron solution. In the first, with potassium permanganate, we add no indicator at all, and hydrochloric acid would spoil the answer. In the second, with potassium dichromate, we must add an indicator, and hydrochloric acid is perfectly fine. Why such a difference? Strange, right? By the end of this video, you will explain both. These titrations come up often in JEE problems. So, let's begin.

0:36Permanganometry: KMnO₄ is its own indicator

So, first, potassium permanganate, K M n O four. The small four below is a subscript: four oxygen atoms. Its solution is deep purple. We add it from the burette into a pale, nearly colourless solution of F e two plus, iron two plus; the two plus up top is the charge. Now tell me: do we need an indicator to see the end point? No, we don't! While iron two plus is left, each drop of permanganate reacts, and its purple colour vanishes at once. But once all the iron has reacted, the first excess drop has nothing to react with. So the whole solution turns pale pink. That pink is our end point. Okay? So permanganate is its own indicator. Titrations with permanganate are called permanganometry. Right?

1:23Which acid? Only dilute H₂SO₄

Now, this reaction needs acid. Which acid? Dilute sulphuric acid, H two S O four. Not hydrochloric acid, H C l, and not nitric acid, HNO three. But H C l is so common in the lab. Why can't we use it? The answer is in the potentials. E naught, the E with a small circle up top, is the standard reduction potential, measured at two hundred and ninety eight kelvin, with ions at one molar and gases at one bar. Clear? For permanganate, M n O four minus, going to M n two plus, E naught is one point five one volts; the minus up top is its charge. The slash separates the oxidised form from the reduced form. For chlorine going to chloride, C l minus, it is one point three six volts. And one point five one is greater than one point three six. With me? So permanganate is the stronger oxidant, and it oxidises the chloride ions to chlorine as well. Some permanganate is wasted on the acid, so the titre, the volume we read from the burette, comes out too high. Wrong answer! Got it? And nitric acid? It is itself an oxidant, so we avoid it too.

2:32Warm or cold, and standardising KMnO₄

Okay? Next, temperature. With oxalic acid in the flask, we titrate warm, at sixty to seventy degrees Celsius. The small circle means degrees. Why do we heat it? Because the reaction is slow at first. And here is the nice part: the M n two plus ions it forms act as a catalyst and speed it up. So the reaction is catalysed by its own product. This is called autocatalysis. Right? But iron two salts, like Mohr's salt, are titrated cold. The Roman two in brackets means iron in the plus two state. So remember the pair: oxalic acid warm, iron two cold. Clear? Now one more thing. Permanganate is not a primary standard. The solid contains some M n O two, manganese dioxide, and permanganate decomposes in light. So how do we find its exact molarity? We standardise it, exactly! Titrate it first against oxalic acid or Mohr's salt. Got it?

3:33Dichrometry: K₂Cr₂O₇

Now the second oxidant, potassium dichromate, K two C r two O seven. Titrations with it are called dichrometry. Dichromate is a primary standard: pure, stable, and non-hygroscopic, which means it does not pick up moisture from the air. So we can weigh it and trust the molarity. Okay? Dichromate solution is orange, and as it reacts, the solution turns green. So, is it its own indicator, like permanganate? Not really! The arrow means changes to: orange changes to green, but the change is not sharp enough to catch the exact end point. Right? So we add an internal indicator: diphenylamine, or barium diphenylaminesulphonate. Got it? Now, remember the hydrochloric acid problem? Can dichromate titrations be done in H C l? Compare the potentials. Yes, they can! For dichromate going to chromium three plus, E naught is one point three three volts, less than one point three six volts for chlorine. So dichromate cannot oxidise chloride, and H C l is safe. And that solves our puzzle from the start! Clear?

4:42Example 1: percentage of iron in an ore using KMnO4

Now, example one. We have a zero point five zero gram sample of an iron ore. It is dissolved in acid, and all the iron is reduced to F e two plus. The solution needs twenty point zero millilitres of zero point zero two zero molar K M n O four; capital M means molar, moles per litre. Find the percentage of iron in the ore. Pause the video and try it first. Done? Okay, let's check. First, the ratio. One permanganate ion oxidises how many F e two plus ions? Five, correct! In acid, manganese goes from plus seven to plus two, so each M n O four minus takes five electrons; e minus is an electron. Each F e two plus gives just one. So one permanganate is equivalent to five iron ions. The sign with three lines means is equivalent to. Okay? Now the moles. Twenty point zero millilitres is zero point zero two zero litres. So the moles of permanganate are zero point zero two zero molar times zero point zero two zero litres, and the moles of F e two plus are five times that. Two point zero times ten to the power minus three. Oh no, careful: forget the five, and your answer is five times too small! Right? All the iron was F e two plus, so that is also the moles of iron. Times fifty five point eight five grams per mole, the molar mass of iron, gives zero point one one one seven grams. With me? Percentage: zero point one one one seven over zero point five zero, times a hundred. Twenty two point three percent. That's it!

6:20Example 2: purity of an iron wire using K2Cr2O7

Now example two, with dichromate. We have a one point zero zero gram piece of iron wire, dissolved in acid as F e two plus, and titrated with zero point one zero zero molar K two C r two O seven. Twenty eight point zero millilitres is needed. Find the purity of the wire. Pause and try it. Hint: count the electrons one dichromate takes. Done? Okay, let's check. Many students simply reuse the permanganate ratio, one to five. Is that right here? No, no, careful! In dichromate, each chromium goes from plus six to plus three, so it takes three electrons. And there are two chromiums in one dichromate ion; the two minus up top is its charge. So one dichromate takes six electrons. Okay? Each F e two plus still gives one electron. So one dichromate is equivalent to six iron ions. Right? Moles of F e two plus: six, times zero point one zero zero molar, times zero point zero two eight zero litres. That is zero point zero one six eight moles. Clear? Mass of iron: zero point zero one six eight times fifty five point eight five. Zero point nine three eight grams. Got it? Purity: zero point nine three eight over one point zero zero, times a hundred. Ninety three point eight percent. Perfect!

7:50Quick revision: indicators, acids, Fe2+ equivalents

So, quick revision. Permanganate is its own indicator; use only dilute sulphuric acid, and standardise it first. Oxalic acid is titrated warm, because of autocatalysis by M n two plus; iron two salts are titrated cold. Dichromate is a primary standard, uses diphenylamine as the indicator, and works fine in H C l. And the ratios: one permanganate for five iron two plus ions, one dichromate for six. Now go and try the practice problems on this page. All the best!