Volumetric Analysis (9) Yet another back titration

Measuring the amount of Nitrogen in a Fertiliser

Introduction

Ammonium salts are used as fertlisers e.g. Nitram or ammonium nitrate NH₄NO₃

Ammonium salts release ammonia gas on treatment with sodium hydroxide solution.  If the ammonia released is caught in a solution of a strong acid then it is possible to measure the proportion of nitrogen in the fertiliser.

The residual unreacted acid can be determined using a titration and you can then use the results of the titration to work back to the proportion of nitrogen in the fertiliser.  

This approach can be used with fertilisers that might contain trace elements to be added to the soil such as zinc, as these trace elements will not affect the release of ammonia.  

Procedure

1. Place 50ml of Mhydrochloric acid (HCl) in a 250ml conical flask.  This is the acid that will be used to trap the ammonia.
2. Weigh accurately about 1.70g of ammonium sulphate ((NH₄)₂SO₄ ) into a 50ml distillation flask.  Add an excess of 25ml 2M sodium hydroxide solution (NaOH) to the distillation flask and a few anti-bumping granules. 
3. Quickly put the distillation apparatus together and gently warm the distillation flask so that it boils VERY slowly.
4. Watch out for rapid sucking back of the solution from the conical flask.  This can happen because the ammoina gas is so soluble in the acid. 
5. Opening the tap of the funnel (see diagram) equalises the pressures in and out of the apparatus and prevents suck back.  (This is a typical apparatus to use but you would need to change the receiving vessel from the 50ml round bottomed flask in the diagram a 250ml conical flask with the same fitting.)

6. You will need to ensure you distill over 10-15ml of the solution to ensure all the ammonia has been removed.
7. Then wash the apparatus and make sure all the washings enter the conical flask.
8. Now add the contents of the receiving flask to a 250ml volumetric falsk and make up to the mark with distilled water.  
9. Titrate 25ml samples of the distillate with 0.1M sodium hydroxide solution using methyl orange indicator to determine the residual acid in the distillate.

Typical analysis and calculation

In this analysis, we begin from the final titration results and work back to the percentage of nitrogen in the original fertiliser.  

The titration results show that 25.0ml of 0.1M sodium hydroxide neutralised the excess acid in the distillate.  This is

25.0  ×  0.1    =    0.0025 moles NaOH     

  1000

This number of moles of sodium hydroxide was contained in 25ml of the distillate from the volumetric flask.  This is a tenth of the volumetric flask so the actual number of moles of excess acid is 

10  ×   0.0025    =    0.025moles  HCl          

This now allows us to determine the number of moles of hydrochloric acid that reacted with ammonia from the fertiliser.

First, let’s calculate the total number of moles of hydrochloric acid in the receiver.

50  ×   1    =    0.050moles     

  1000 

Therefore, the number of moles of hydrochloric acid reacting with the ammonia from the fertiliser was    

0.050   —   0.025    =    0.025moles            

And as 1 mole of ammonia reacts with one mole of hydrochloric acid this is also the number of moles of ammonia from the fertiliser.

And as 1 mole of ammonia contains 1 mole of nitrogen atoms this is the number of moles of nitrogen atoms in the sample of fertiliser.

Therefore the mass of nitrogen in the fertiliser is 

0.025    ×    14    =   0.35g            

And therefore the percentage of nitrogen in the fertiliser is

0.35   ×   100    =    21%      

1.70

This is the percentage of available nitrogen in ammonium sulphate.