[shalikadm]

what is the use of sodium carbonate in charcoal briquette test?
Does only aluminum compounds need sodium carbonate for the test or it is compulsory with any compound in the test?

[9-92-6-19]

I believe you have misconstrued the Na₂CO₃ + C_(s) test.

I believe it is a way of testing for the presence of a metal, for example:

AgNO₃ + Na₂CO₃  NaNO₃ + Ag₂CO₃

Ag₂CO₃  Ag₂O + CO₂

Ag₂O + C  Ag + CO₂

I have never tried this experiment myself, though; however, that's what I'd logically presume.

[shalikadm]

see what's stated in my tute..this is where i find it complicated.

Zn²⁺
it becomes green when zink salts are heated with cobalt nitrate in a charcoal block

Al³⁺
when aluminium compounds mixed with sodium carbonate are heated in carbon block it gains a white Al₂O₃.when  cobalt nitrate is added to this and heated again.it becomes blue in color

why it goes to an intermediate step when Al is for the test.why can't those aluminium compounds go alone with cobalt nitrate for the test.?

[shalikadm]

any one who knows the reason for this ?

[Arkcon]

The carbon block test, like the borax bead test, are ancient methods of element qualitative analysis.  I read about them as a grammar school student on my own, I never did them in high school or university -- and that was more than 20 years ago.  You're not likely to find someone who does them much these days, and you're looking instead for specific knowledge on the chemistry behind the technique.  That's asking a lot.  Here's something you can try -- hit the university library for Paulings' Chemistry and for Feigel's Laboratory Manual of Spot Tests.  They may have explanations, or references to the procedures that describe them better.  Be prepared to search for century-old reference documents.

[shalikadm]

The carbon block test, like the borax bead test, are ancient methods of element qualitative analysis.  I read about them as a grammar school student on my own, I never did them in high school or university -- and that was more than 20 years ago.  You're not likely to find someone who does them much these days, and you're looking instead for specific knowledge on the chemistry behind the technique.  That's asking a lot.  Here's something you can try -- hit the university library for Paulings' Chemistry and for Feigel's Laboratory Manual of Spot Tests.  They may have explanations, or references to the procedures that describe them better.  Be prepared to search for century-old reference documents.

thanks Arkcon for the support...anyway i wonder why this phenomena cannot be explained by normal chemistry knowledge...I have ultimately decided to learn it by heart to save my limited time for studies and not to go searching those century old documents... but i prefer understanding for memorizing..
anyway thanks a lot !

[Borek]

i wonder why this phenomena cannot be explained by normal chemistry knowledge

I don't think it is a correct way of assessing the situation. Trick is, these old methods were sometimes based on rather unexpected things - like they were in fact detecting not the element itself, but impurity that was usually accompanying the element and had an easy to detect properties. In such cases the chemistry behind is perfectly sound, it just has nothing to do with what we expect. I am not saying that's the case here, I am just signaling possible problems.

Actually that's one of the reasons why these methods are no longer used nor taught - we have better ones, easier to understood.

[curiouscat]

Trick is, these old methods were sometimes based on rather unexpected things - like they were in fact detecting not the element itself, but impurity that was usually accompanying the element and had an easy to detect properties.

Interesting. Which test for example? I never knew the associated impurity trick.

[Borek]

Sadly, I can remember the exact details, I read about it long ago.

But I remember similar (even if slightly different) case. Thiophene is a common impurity in benzene, and for many years some thiophene reaction giving an easy to observe dye was though to be a benzene reaction and was used for benzene identification.