Sciencemadness Discussion Board

need explanation pls

aeacfm - 7-12-2010 at 12:59

some times we measure sulfide by oxidation (conversion to sulfate and then determine sulfate) but during the expirement there are some observations which i cant explain !!

the steps like that :
- sample containing sulfide + Na2CO3 +Na2O2 .
- heating 30 min in a crusible after that the contents coverted to dark red solid in the crusible when it is left to cool.
- dissolve the solid material in the crusible with water , here come what i cant explain, during the dissolution a strong effervescence occure and the crusible's temperature increases ( it seemsit is exothermic reaction ) and the solid dissolved in amazing way

the completeness of steps is addition of Whatman accelerator and boiing + neutralization with HCl and filter and measure sulfide as sulfate .
for your knowledge the sample contain (FeS , Fe2O3 , FeCO3 , CaCO3 , NaCl )

thanks in advance

ScienceSquirrel - 7-12-2010 at 13:12

Almost certainly there is excess sodium peroxide present in the fused mixture.
This hydrolyses to hydrogen peroxide that then decomposes to oxygen and water.
Try using a glowing splint to test the gas I bet it will catch fire showing that the gas is oxygen.

aeacfm - 7-12-2010 at 13:27

i thought that for the first time but i didnt thought it is exothermic reaction
to be honest i even didnt try the fire test you noted

ScienceSquirrel - 7-12-2010 at 13:46

The reaction of sodium peroxide with water is pretty exothermic.
A lump of it in contact with tissue paper or similar flammable material and a few drops of water will catch fire.
The strong hydrogen peroxide formed by the hydrolysis of the salt plus the heat will cause the mixture to catch fire and it will then burn vigorously.
It has been used as an improvised fire bomb!

matei - 8-12-2010 at 08:14

I think the iron compounds in the crucible will catalyze the decomposition of the hydrogen peroxide.

aeacfm - 8-12-2010 at 10:23

Quote: Originally posted by matei  
I think the iron compounds in the crucible will catalyze the decomposition of the hydrogen peroxide.

aha , good catch .