Determining amount of Nitrogen in Fertiliser Question relating to aliquots

[VCE247]

Why must 20mL Aliquots be used rather than simply boiling the whole mixture? Just asking because in the textbook (heinemann) they have just boiled and titred the whole thing rather than seperating into aliquots?

Is it so that you can perform 3 titrations and achieve more accurate results?

[Mao]

I'm assuming you are taking aliquots of the ammonium(fertilizer)/NaOH solution, then boiling, then titrating against an acid.

That's because it's a lot easier (faster) to boil a small amount (20mL aliquot) than to boil a whole beaker full of stuff.

[VCE247]

yeah thanks

one more thing, before boiling the the aliquot of ammonium/fertilizer and naoh solution we had to add some water to it. would you know why? :/

my teacher said something about cooling it/maintaining a constant level...but i still dont really get it :S

[Greatness]

I think putting water into the solution cools it to room temperature so it doesnt affect the results of the titration...

[VCE247]

but we did it before heating? so how would that help?

[VCE247]

would it help to speed up the reaction?

[Greatness]

Ohhh right which prac is this? THe back titration one? I forgot..

[Mao]

yeah thanks

one more thing, before boiling the the aliquot of ammonium/fertilizer and naoh solution we had to add some water to it. would you know why? :/

my teacher said something about cooling it/maintaining a constant level...but i still dont really get it :S

Since you're boiling it, some water will evaporate as steam. If you want to keep a constant liquid level, you need to add some water to it beforehand. (no accurate way to tell exactly how much water will be evaporated though, so it won't be an 'accurate' addition of water)

[VCE247]

oh but why is the constant liquid level required? does it affect the experiment if it isnt?

[Mao]

oh but why is the constant liquid level required? does it affect the experiment if it isnt?

The only problem I can think of is if the solution becomes supersaturated (too much water boil off, the thing becomes more like a 'toffee'). But that won't happen, since everything is way below their solubility limit.

So, no idea.

[VCE247]

alright thanks ^^

oh and one more thing, if it wasnt boiled long enough and the NH4+ didnt fully react with the NaOH, it would result in an underestimation of the amount of N yeah?

[Readinya]

It's to make sure the solution doesn't get too concentrated. There is still excess NaOH in the flask, and NaOH can be dangerous if exposed to air.

[VCEMan94]

doesnt NaOH absorb water though
wouldnt that affect the experiment?

[Mao]

doesnt NaOH absorb water though
wouldnt that affect the experiment?

Not really, since it is in a solution of water. We're more worried about absorption of CO2.