Anion tests Flashcards
What term is used to describe this type of analysis?
Qualitative analysis
Define the term anion
Negatively charged ion
Why is deionised water used in all of the tests?
Tap water contains too many dissolved ions which could interfere with the tests, however deionised water which contains no dissolved ions is perfect
What is a precipitate?
Material that settles out of solution, insoluble in water
In the test for the chloride ion, how can the presence of silver chloride be confirmed?
By adding dilute ammonia solution to the silver chloride, since silver chloride is soluble in dilute ammonia the cloudiness dissapears
What is the difference between a sulfate ion and a sulfite ion and how can these two ions be distinguished from eachother in a lab
The sulfate ion has the formula SO4^2- and the formula of the sulfite ion is SO3^2-
Distinguished : adding barium chloride - white percipitate in each case
add dilute hydrochloric acid to the precipitate and the precipitate dissapears in the case of the sulfite ion but not in the case of the sulfate ion
Why is it best to make up a fresh solution of limewater before using it to confirm the presence of CO2
Limewater reacts with carbon dioxide in the air which makes the solution milky
In testing for the presence of the nitrate ion, why must extreme care be taken when using concentrated sulfuric acid
Conc sulfuric acid is very corrosive and causes burns to skin
KCl, KNO3, Na2HPO4.12H2O,Na2SO3.7H2O, NaHCO3 and Na2SO4.10H2O.
Which of the substances listed above was identified by the addition of silver nitrate, AgNO3, solution
to a solution of each sample in turn? What observation indicated a positive test result?
KCl white precipitate (ppt)
KCl, KNO3, Na2HPO4.12H2O,Na2SO3.7H2O, NaHCO3 and Na2SO4.10H2O
One of the samples gave a brown ring when a little concentrated sulfuric acid was carefully poured
down the inside of a slanting test tube which contained a solution of the salt, together with another
reagent. What was the other reagent? Which salt was identifiable by the appearance of a brown
ring?
reagent - iron(II) sulfate
KNO3
Describe how you would test the samples for the presence of the phosphate anion.
Add ammonium molybdate
few drops of concentrated nitric acid and warm gently
yellow precipitate (ppt) formed
Balanced equation when hydrochloric acid is added to hydrogencarbonate
HCO3- + H+ -> CO2 + H20
Balanced equation when hydrochloric acid is added to carbonate
CO3^2- + 2H+ -> CO2 + H20
Balanced equation when magnesium sulfate is added to hydrogen carbonate
Mg2+ + 2HCO3- -> Mg(HCO3)2 (soluble)
Balanced equation when mangensium sulfate is added to carbonate
Mg2+ + CO32- -> MgCO3 (arrow down)
Balanced equation when barium chloride is added to sulfite
Ba2+ + So3^2- -> BaSO3 (arrow down)
Balanced equation when barium chloride is added to sulfate
Ba2+ + SO4^2- -> BaSO4 (arrow down)
Balanced equation when hydrochloric acid is added to sulfite
BaSO3 + 2HCl -> BaCl + SO2 + H20
Balanced equation for when hydrogen carbonate is heated
Na2CO3 + MgSO4 -> Na2SO4 + MgCO3
Describe the procedure for the test of carbonate and hydrogencarbonate ions
Add sodium carbonate and sodium hydrogen carbonate to two separate clean test tubes
Using a dropping pipette add HCl to both test tubes
Check for the release of CO2 using limewater = milky
Add sodium carbonate and hydrogen carbonate solutions to two other clean get tubes and using a dropping pipette add magnesium sulfate
Carefully heat the contents and note white precipitate of carbonate
What is observed upon addition of HCl to
I) carbonate
ii) hydrogencarbonate
both -> effervescence and co2 gas given off
What is the observation from the limewater test on the carbonate and hydrogencarbonate ion
both turn limewater milky
What is the observation of the addition of magnesium sulfate to carbonate and hydrogen carbonate
carbonate -> white ppt
hydrogencarbonate -> no white ppt
Observation upon hearing of carbonate and hydrogen carbonate
Carbonate - white ppt remains
Hydrogencarbonate - white ppt formed
What is the observation of the addition of 1) BaCl2 and 2) HCl to sulfate and sulfate
1) both form white ppt
2) sulfate - ppt does not dissolve
sulfite - does dissolve
Give a safety precaution
Wear safety glasses
How do you identify unknown anions
Add unknown solutions to clean test tubes
Using dropping pipette add dilute hcl to test tube and record observations
If gas evolved establish using limewater weather it is CO2 or not. If it is, proceed with hydrogen carbonate and carbonate identification test (magnesium sulfate)
If no gas evolved, take further samples in clean test tubes and test in turn with barium chloride followed by dilute hcl, silver nitrate , cold saturated iron II sulfate followed by concentrated sulfuric acid and ammonium molybdate reagent until positive results obstained
