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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: leena on June 17, 2009, 07:22:48 AM

Title: Inorganic question:finding the concentration of ions
Post by: leena on June 17, 2009, 07:22:48 AM
The question
An aqueous solution containing sulphate and phosphate ions provided.Using this solution,dil. nitric acid and dil.NaOH only,outline a method to determine the concentration of the sulphate and phosphate ions

I had posted this question on another forum and I was told to consider using titration to find the molar concentration of each ion present.
After adding dil. nitric acid to the above solution,we get 2 acids,namely phosphuric and sulphuric acid.
So.. to do the titration,this is what I thought,
Take a known volume of the acidic solution into a titraion flask and add phenolphalein as indicator.
In the burette we have a known concentration of the NaOH sol' provided to us,
Now... at the end point...this is where my problem starts,
what happens ? I know that titrating  Na2CO3 with an acid gives 2 endpoints with methylorange as indicator,but is it relevant to this as well.
Does phosphuric acid act similar to Na2CO3 cause it'a weak acid just like Na2CO3 is a weak base?If so...I don't know how ???
On the other hand my whole method of titrating maybe completely wrong.
I'm looking for someone to guide me here.

A thorougly confused student seeking urgent help.
Title: Re: Inorganic question:finding the concentration of ions
Post by: Borek on June 17, 2009, 08:02:11 AM
No solution to the problem as worded.

Quote from: leena on June 17, 2009, 07:22:48 AM
After adding dil. nitric acid to the above solution,we get 2 acids,namely phosphuric and sulphuric acid.

No - you have three acids. Phosphoric, sulfuric and nitric.

At this stage if you start titration with NaOH you will be only able to determine amount of HNO3 added. Well, with correct selection of indicators you may be also able to determine concentration of PO43- - but to do so you can as well titrate the original solution with nitric acid (treating PO43- as a strong base). You won't be able to determine SO42-.

Unless what you wrote is wrong and you have not

Quote from: leena on June 17, 2009, 07:22:48 AM
An aqueous solution containing sulphate and phosphate ions

but solution containing mixture of phosphoric and sulfuric acid. Then you will be able to determine their sum - and kowing sum and phosphoric, you can calculate sulfuric.

http://www.titrations.info/acid-base-titration-phosphoric-acid
Title: Re: Inorganic question:finding the concentration of ions
Post by: leena on June 18, 2009, 05:05:53 AM
Quote
No - you have three acids. Phosphoric, sulfuric and nitric.

At this stage if you start titration with NaOH you will be only able to determine amount of HNO3 added. Well, with correct selection of indicators you may be also able to determine concentration of PO43- - but to do so you can as well titrate the original solution with nitric acid (treating PO43- as a strong base). You won't be able to determine SO42-.

Unless what you wrote is wrong and you have not


Quote from: leena on Today at 01:22:48 AM
An aqueous solution containing sulphate and phosphate ions

Oh,I see what you mean,but unfortunately this is exactly how it appears in my question paper.I wonder if there's another way of doing this ,cause we're told to use only nitric and NaOH only,so wouldn't an indicator also count as a reagent ?
I'll check this with my teacher and see if it was mistake after all.
Thanks.