What are the 3 anion tests?
Carbonate
Sulfate
Halide
Describe the carbonate test.
Carbonate + acid > CO2
- Add dilute HNO3 to solid/solution
- Bubbles form > could be a carbonate
- To prove the bubbles are CO2, bubble the gas through limewater, if it goes cloudy it is CO2
CO2(g) + Ca(OH)2(aq) > CaCO3(s) + H2O(l)
Calcium carbonate precipitate shows presence of CO2
Describe the sulfate tests.
Sulfates are soluble in water, BaSO4 is insoluble white precipitate
1. Add Ba2+(aq) to unknown compound
2. White precipitate forms
Ba(2+)(aq) + SO4(2-)(aq) > BaSO4(s)
Describe the halide test.
- Ass AgNO3(aq) to X(aq)
- AgCl(s) - white, AgBr(s) - cream, AgI(s) - yellow
AgCl - soluble in dilute NH3(aq)
AgBr - soluble in conc. NH3(aq)
AgI - INSOLUBLE in conc. NH3(aq0
What is the sequence of anion tests and why?
Carbonate, Sulfate, Halide
- Eliminate possiblity of CO3(2-), neither sulfate, nor halide with HNO3 acid gives bubbles
- Why eliminate? If there are carbonate ions, a white BaCO3 forms, how do you know which is which.
- With halides, you are looking for a precipitate, Ag2CO3 and Ag2SO4 are both insoluble precipitates (hence halide test is last)
Describe the process of testing a mixture of ions.
Same sequence: Carbonate, Sulfate, Halide
1. CO3(2-) - keep adding HNO3 until bubbling stops - ensures no more carbonate ions (otherwise precip. forms in subsequent tests)
Use HNO3, not H2SO4 or HCl, they will show up in sulfate and halide tests
- SO4(2-) test - add excess Ba(NO3)2(aq)
- Filter precip of BaSo4(s), if you want to test for halide ions, don’t use BaCl - Proceed with halide test as normal
Describe the test for NH4(+) (cations)
- NH4 + NaOH(aq)
- NH3(g) is produced - no gas bubbles as NH3 gas is soluble in water
- Warm the mixtiure, NH3(g) is released
- You can smell the NH3(g) or moist pH indicator paper turns red to blue.
