qualitative analysis

what is qualitative analysis?

a technique which relies on observations to detect which substances are present in a sample

what are the three tests for anions?

carbonate, sulfate, halide test

what is the carbonate test?

tests for the carbonate ion CO32-
add dilute nitric acid to the solution
if effervescence occurs, could indicate presence of carbonate
to prove gas is CO2, bubble gas through lime water - CO2 should react to form a white precipitate of calcium carbonate, which turns lime water cloudy

ionic equation for carbonate test

CO3^2-(aq) + 2H+(aq) → H2O(l) + CO2(g)

what is the sulfate test?

tests for SO4^2- ions
acidify solution with dilute hydrochloric acid then add a few drops of barium chloride
if sulfate is present, a white precipitate of barium sulfate is formed (as BaSO4 is very insoluble)

what should i change to the sulfate test if i plan on doing a halide test afterwards?

use barium nitrate - with barium chloride, chloride ions are introduced to the solution

ionic equation for sulfate test

Ba2+(aq) + SO4²-(aq) → BaSO4(s)

what is the halide test?

add aqueous silver nitrate to an aqueous solution of a halide
silver chloride - white, silver bromide - cream, silver iodide - yellow
add aqueous ammonia to test solubility of precipitate
silver chloride - soluble in dilute NH3, silver bromide - soluble in conc NH3, silver iodide - insoluble in conc NH3

general equation of halide test

Ag+(aq) + X-(aq) → AgX(s)

what is the sequence for anion tests?

carbonate, sulfate, halides

why is carbonate test carried out first?

neither sulfate nor halide ions produce bubbles with dilute acid
carbonate test can be carried out without possibility of incorrect conclusion

why is sulfate test carried out second?

barium carbonate is also white and insoluble in water, the same as barium sulfate
important to carry out carbonate test first then proceed to sulfate once known that no carbonate is present

why is halide test carried out last?

silver carbonate and silver sulphate are both insoluble in water and form precipitates, similar to the silver halides
important to carry out halide test last once known that no carbonate and sulfate is present

what is the process for the anions test on a mixture of ions?

1) carbonate test
effervescence seen, continue adding dilute HNO3 until it stops (not use H2SO4 or HNO3, as contain sulfate or chloride ions)
all CO3^2- ions removed, none left to react in next tests
2) sulfate test
add excess of Ba(NO3)2, any SO4^2- ions will precipitate out as BaSO4
filter solution to remove BaSO4
3) halide test
add AgNO3, any CO3^2- or SO4^2- ions must have been removed, so any precipitate formed involves halides
add NH3 to confirm which halide is present

what is the test for cations?

test for NH4+ ion
NaOH added to solution of an ammonium ion
NH3 gas produced (unlikely to see effervescences as NH3 very soluble in water)
mixture is warmed and NH3 gas released
test gas with moist pH indicator, turns paper blue as NH3 is alkaline