Tests for Nitrogen and Sulphur

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Hydrazine is a compound made only of nitrogen and hydrogen. So it is full of nitrogen, right? Yet when you fuse it with sodium and do the nitrogen test perfectly, no blue colour appears at all. The test says there is no nitrogen! How can that happen? Keep this question in mind, because we will answer it in this lesson. And look, these colour tests for nitrogen and sulphur come up often in JEE questions. So, let's begin.

0:33One extract, many tests

So, here is a map of this whole chapter. At the top is the sodium fusion extract: the compound is fused with sodium, plunged into water, boiled and filtered. In it, nitrogen is now sodium cyanide, N a C N, and sulphur is sodium sulphide, N a two S, where the small two written low is a subscript. When both are present, we may also get N a SCN, sodium thiocyanate. And a halogen gives N a X, where X stands for chlorine, bromine or iodine. With me? Below the extract are four arrows, and the boxes at the bottom show what you see. Today we follow the first three: nitrogen, which ends in Prussian blue; sulphur, which ends in a black precipitate or a violet colour; and both together, which end in a blood red colour. The halogen arrow on the right is for the next lesson. Okay?

1:27Nitrogen: the Prussian blue test

Let us start with nitrogen. Take the extract and boil it with freshly prepared ferrous sulphate solution, F e S O four. Then acidify it with dilute sulphuric acid, H two S O four. Two simple steps: boil, then acidify. Clear? Now, what happens inside the tube? Six cyanide ions, C N minus, where the small minus sign written high is the charge, join one ferrous ion, F e two plus. The arrow means gives. We get one complex ion, held in square brackets: iron, with C N in round brackets taken six times, and a charge of four minus. Okay? Next, three of these complex ions combine with four ferric ions, F e three plus. The product is F e four, then the complex in square brackets, taken three times. This deep blue solid is Prussian blue, and it tells us that nitrogen is present. Got it?

2:22What happens in the test tube

Now remember, the extract is alkaline. So the ferrous ions first give a dirty green precipitate of ferrous hydroxide, F e, O H in round brackets, two. But wait, Prussian blue needs ferric ions, F e three plus, and we added only ferrous sulphate. So where does the F e three plus come from? Yes, exactly, it comes from the air! On boiling, the air oxidises part of it to F e three plus. And to make sure, a drop of ferric chloride, F e C l three, may be added. With me? Then the dilute acid dissolves the hydroxides, and the blue colour appears. That is the job of the acid. Right? And here is one practical point. If you see a green or bluish green solution instead of a clear blue, it means the compound has only a little nitrogen. Clear?

3:15When the nitrogen test fails

So, back to our puzzle. Hydrazine is full of nitrogen, yet it gives no Prussian blue. Think about what the blue compound is built from. Why does the test fail? It is built from cyanide ions, yes! And in the fusion, sodium, carbon and nitrogen together give N a C N, sodium cyanide. See, the carbon is needed too. Okay? Now, hydrazine and hydroxylamine have nitrogen but no carbon. No carbon, so no cyanide, and so no Prussian blue. That solves our puzzle: the test fails even though nitrogen is present. Got it? One more case to remember is diazonium salts. On heating, they lose their nitrogen as nitrogen gas, N two, so they fail the test too. Clear?

4:03Sulphur: two tests

Now let us move to sulphur. For the first test, we add sodium nitroprusside to the extract, and a violet colour appears, which is also described as purple. Okay? Here is the reaction. The sulphide ion, S two minus, joins the nitroprusside ion: iron with five cyanide groups and one N O group, with a charge of two minus. The product is the violet ion, the same group with an extra sulphur, and a charge of four minus. With me? For the second test, we acidify the extract with acetic acid, and then add lead acetate. If you get a black precipitate, sulphur is present. Right? The black solid is lead sulphide: S two minus plus P b two plus gives P b S, where P b is lead. So for sulphur, remember two colours, violet and black. Got it?

4:54Nitrogen and sulphur together

Finally, let us test for nitrogen and sulphur together. The extract is acidified, and ferric chloride is added. Then a blood red colour appears. Okay? The colour comes from the thiocyanate ion, SCN minus, which holds sulphur, carbon and nitrogen together. With F e three plus, it gives the blood red ion: F e, then SCN in round brackets, all inside square brackets, with a charge of two plus. Clear?

5:23Example 1: blood red with FeCl3 but no Prussian blue

Alright, Example one. The extract of a compound gives a blood red colour with ferric chloride, but no Prussian blue. What does this show? Pause the video and try it first. Done? Okay, let's check. There is no Prussian blue, so there is no nitrogen in the compound, right? No, no, wait, this is exactly the trap! The blood red colour with ferric chloride tells us that thiocyanate ions, SCN minus, are present. Okay? And thiocyanate contains both sulphur and nitrogen. So the nitrogen is there, but it is combined with the sulphur as thiocyanate, and that is why no Prussian blue appeared. Right? Why did they stay together? Because too little sodium was used in the fusion. So the answer is that nitrogen and sulphur are both present, combined as thiocyanate. Got it?

6:17Example 2: moles of NaCN for one mole of Prussian blue

Now let us try a harder one. How many moles of sodium cyanide are needed to form one mole of Prussian blue? Pause the video and try it first. Done? Okay, let's check. Here is Prussian blue again: F e four, then the complex in square brackets, taken three times. We can see C N six in the formula, so six moles of sodium cyanide, right? Oh no, careful, look at the small three outside the square bracket! One mole of Prussian blue contains three moles of the complex ion, with its charge of four minus. With me? Each of these ions needs six cyanide ions, just as in our first equation. So three times six equals eighteen cyanide ions. Okay? And each sodium cyanide gives one cyanide ion. So the answer is eighteen moles of sodium cyanide; the m o l on screen is short for moles. Clear?

7:17Quick revision: colour tests for nitrogen and sulphur

So, quick revision. For nitrogen, boil with ferrous sulphate, acidify with dilute sulphuric acid, and look for Prussian blue; the compound must contain carbon too. For sulphur, sodium nitroprusside gives violet, and lead acetate gives black lead sulphide. And for nitrogen and sulphur together as thiocyanate, ferric chloride gives a blood red colour. Now go and try the practice problems on this page. You have got this. All the best!