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Topic: Analysis of Cobalt by Gravimetric  (Read 8558 times)

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  • Guest
Analysis of Cobalt by Gravimetric
« on: October 18, 2005, 10:03:35 PM »
Hi, need some help here. (But not asking you guys to spoon feed me thou)

I am doing this prac,  to prepare CIS and TRANS [Co (en)2 Cl2 ]Cl. I complete that and answer most of the question but i am stuck at the analysising part. This salt is then use for the anaylsising part.

Title: Analysisation of Cobalt By Gravimetric Method.

Step 1 : Wet Ashing with Sulphuric Acid and Nitric Acid.

Meaning of Wet Ashing :    A method for the decomposition of organic material, such as resins or fibers, into an ash by treatment with nitric or sulfuric acids.

Reason for this step : It is to break down the Co(III) complex to Co(II).

Step 2 : Dilute the solution with Conc Ammonia.

Reason for this step : I believe it's to adjust the solution neutrailty.

Step 3 : Add Ammonium Thiocyanate (NH4CNS) and prydine

Reason for this step : [Co (C5H5N)](CNS)2 is precipitated. Use for analysis.

Step 4 : Wash the precipitates with NH4CNS, Pyridine (C5H5N) in water.

Reason for this step : To ensure the reaction is complete? Not really sure thou.

Step 5 : Wash the precipitates AGAIN with Lesser NH4CNS , More Prydine ,95% Alcohol and water.

Reason for this step : To be really really sure that it is complete? No sure thou.

Step 6 : Wash again !!!!! (Damn!) with More Prydine,and more alcohol (Repeat twice)

Reason for this step : Wanna torture the student? No idea thou.

Step 7 : Wash AGAIN !!!!! (&*$%^) with 4 drop of prydine in 30ml sodium dried ether. (Repeat 8 times!)

Reason for this step : To be so very very sure that the reaction is complete?..No idea too. But whats dry ether for afterall?

Step 8 : Dry and weight . Calculate the percentage of cobalt from the weight.

Reason for this step : TO see the % yield and to determine the previous compound?

Question 1,

Why it is important to destroy the Co(III) to Co(II)?

My ans: Believe that Co(III) cannot form the [Co(C5H5n)4](CNS)2 precipitates. Thats why.

Why is the precipitate washed with NH4CNS and prydine instead of just water alone?

My ans: Because water will form ligands with the complex thus changing the precipitates structure.(Water is higher in spectrochemical series compare to CNS and prydine?

Why wash with prydine in dry ether in the final part?

My ans: Dry ether will disspciate pyridine. So it will... hmmm do something?

Senario Question:
3 X 0.10g GREEN colour all of the same sample( u got no idea what it is thou). Were prepare and as the above method, form the
[Co(Prydine)](CNS)2 just like the above experiment.
Weight of each yield is 0.1700, 0.1710g, 0.12610g

Whats the % of cobalt of each green complex?

my ans: I can calculates this thanks!

Is the green complex is likely to be [Co(en)2Cl2]Cl ?

My ans: Yes... because ..hmmm.... think i can calculate this too..

This is the tough lab i am doing kinda know alittle about the experiment except the washing part....any help or reference is greatly appreciated


  • Guest
Re:Analysis of Cobalt by Gravimetric
« Reply #1 on: November 04, 2005, 04:32:53 AM »
Hmmm.... just wanna check here that is that nobody knows the answer or i didn't make myself clear? .  ::) :1eye:


  • Guest
Re:Analysis of Cobalt by Gravimetric
« Reply #2 on: November 04, 2005, 03:50:51 PM »
one at a time my friend, one at a time....

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