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While testing for anions using dilute $\ce{H2SO4}$, we prefer using the soda extract for tests rather than the actual salt. But we don’t use it for tests of carbonates, bicarbonates and sulphites. Why is this so?

I get that soda extract itself is a carbonate so no point to use it for carbonates but why not bicarbonates and sulphites?

Also I’m told that we use soda extract because sodium salts are mostly soluble. Then why specifically sodium? Isn’t there any other metal we can use?

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If a solution of $\ce{H2SO4}$ is added to a some carbonate, bicarbonate or sulfite, it will produce some $\ce{CO2}$ or $\ce{SO2}$ effervescence with gas bubbles filling the container where this test is done, and maybe cause it to overflow. With sodium bicarbonate the equation is : $$\ce{H2SO4 + 2 NaHCO3 -> Na2SO4 + 2 H2O + 2 CO2}$$ With sodium sulfite, it is : $$\ce{Na2SO3 + H2SO4 -> Na2SO4 + H2O + SO2}$$

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  • $\begingroup$ I don’t understand why “Cause it to overflow” is a problem. Why can’t we just use a bigger container and prevent overflowing? $\endgroup$
    – WhySee
    May 21, 2021 at 13:52
  • $\begingroup$ Also, is there any chemical aspect to this problem? $\endgroup$
    – WhySee
    May 21, 2021 at 13:54
  • $\begingroup$ @Apoorv Sharma. Before doing the experiment, you don't have any idea how far and high the foam may go up. You may have to give up your first experiment, and start again with a second sample. Apart from this aspect, there is no chemical difficulty or secondary reaction. $\endgroup$
    – Maurice
    May 21, 2021 at 16:17

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