Qualitative Analysis of Cations Flashcards
qualitative vs quantitative analysis
qualitative
- what is present
quantitative
- how much is present
group 1 is known as
the silver group
group 2 is known as
the copper group
group 3 is known as
the zinc-aluminum group
ions in group 1
Ag+
Hg2 +2
Tl+
Pb2+
ions in group 2
Hg2+
Bi3+
Cu2+
Sn2+
Sn4+
what group is Hg2+
apart of
group 2
what group is Bi3+
apart of
group 2
what group is Cu2+
apart of
group 2
what group is Sn2+
apart of
group 2
what group is Sn4+
apart of
group 2
what group is Ag+
apart of
group 1
what group is Hg2 +2
apart of
group 1
what group is Tl+
apart of
group 1
what group isPb2+
apart of
group 1
what ions are in group 3
Zn2+
Fe2+
Fe3+
Ni2+
Cr3+
Al3+
what group is Zn2+
apart of
group 3
what group is Fe3+
apart of
group 3
what group is Fe2+
apart of
group 3
what group is Ni2+
apart of
group 3
what group is Cr3+
apart of
group 3
what group is Al3+
apart of
group 3
what solution precipitates group 1
0.3 M HCl
what group does HCl precipitate
group 1
why does group 2 precipitate in HCl
these soft acids react strongly enough with a borderline base to precipitate in acid solution
what solution precipitates group 2
0.3 HCl containing H2S
what group precipitates with HCl containing H2S
group 2w
why does group 2 precipitate with H2S
because soft acids react with a very soft base
what solution precipitates group 3
alkaline solution of H2S
how can group 3 be distinguished into two different groups
- Fe+3, Cr+3 and Al+3 will redissolve if precipitate layered with weak acid
- Ni+2, Fe2+ and Zn+2 will not
why does Fe+3, Cr+3 and Al+3 redissolve if the precipitate is layered with a weak acid
these are hard acids that reacting with hard base (OH-) and as cations they precipitate when the Ph equals the Pka
what are the hard acids
Fe+3, Cr+3 and Al+3
why don’t Ni+2 and Zn+2 and Fe2+ dissolve if the precipitate is layered with a weak acid
they are borderline acids and bind to very soft base when Ph is adjusted to weaken the hydroxide hydrated cation
what are confirmatory tests
these that determine CONCLUSIVELY that a specific ion is present
are interfering ions left in a sample before a confirmatory test is done
NO, they are removed prior to running the test
what are separations
procedures that separate ions from other groups
what follows separations
confirmatory tests
what was important to due throughout testing cations
label ALL the test tubes
where were all solutions discarded after the lab
into the HEAVY WASTE bucket
is qualitative analysis precise and what does that mean
no it’s not = measurements don’t have to be exact
how are liquid reagents measured
in drops
how many drops equals 1 ml
20 drops
what techniques were used to mix solutions
- flicking the test tube when the volumes were small
- inversion when the volume was larger
what type of sample is centrifugation done
for a solution containing a solid and a liquid
why is centrifugation done
to separate liquid from a solid
what is the solid called after centrifugation andwhere is it found
precipitate - found at the bottom of the test tube
what is the liquid called after centrifugation and were is it found
supernatant - found above the solid
how is the supernatant and precipitate separated after centrifugation
supernatant poured carefully from the solid into a new test tube, LEAVING the solid in the original test tube
decanting
technique for separating solid from liquid
how is decanting done
let the solid settle at the bottom of the test tube and CAREFULLY pour off the liquid WITHOUT disturbing the solid
precipitation is most often done to
remove ions
how to test for completeness of precipitation
add a drop or two of the precipitating reagent to the supernatant
if more precipitate forms than there are still ions in the solution and need to centrifuge again and combine precipitates
what is desirable while doing precipitation
to make sure all ions are removed from the solution OR precipitation is complete
how were test tubes heated
in a water bath with the cork REMOVED and the opening of the tube pointing AWAY from you
when is evaporation done
when the volume of a solution needs to be concentrated or reduced
how can evaporation be done
transfer the desired solution to an evaporating dish and use tongs to GENTLY move the container back and forth over the hot plate
most common cause of false observations leading to incorrect conclusions
cross contamination
how are ions in groups related
based on similar chemical and analytical properties
how are ions in groups NOT related
based on the periodic table
why are some ions omitted from experiments
because of their toxicity, expense or rarity
what group 1 cation was not tested and why
Tl - toxicity
how are cations differentiable among and between groups
based on differential precipitation properties between groups and among members of a group
how can group 1 cations be separated from other groups
they form slightly soluble chlorides with HCl
are there other groups besides group 1 that precipitate with HCl
NO - others will remain in solution
precipitate reaction of silver
precipitation reaction of mercury +2
precipitation reaction of lead 2+
what are the colours of the precipitates of group 1 with HCl
all are white precipitates
what was the first step of group 1 analysis
adding 0.5 ml (10 drops) of 6M HCl and stirring well
what is done after adding HCl to group 1 sample
centrifuge and decant the supernatant into another test tube and test for completeness
what might appear if HCl is added to the unknown sample and what does that mean
a white precipitate forms which indicates at least ONE group 1 cation
what is done to the precipitate in group 1 after decanting the supernatant
add 5 ml of RO water and heat the contents for 5 mins WHILE STIRRING
after heating centrifuge any precipitate and decant the supernatant into another tube
what can the heated sample of group 1 cations reveal
- the saved precipitate can contain Ag+ or Hg2 + 2
- Pb2+ ONLY cation soluble in water (must test supernatant for lead)
what is the ONLY cation of group 1 that is soluble in water
LEAD
steps for the confirmatory test for lead in the supernatant of group 1
- add 2 to 3 drops of 6 M acetic acid (Ch3COOH)
- add 3 to 4 drops of 1.5 M K2CrO4
- a yellow precipitate will form in presence of lead
what is the yellow precipitate that confirms presence of lead in sample
PbCrO4
what is added to group 1 to test for Ag+ and Hg2 +2
NH4OH
what cation is a part of a disproportionation reaction
Hg2 2+
reaction for confirmation of Ag+
reaction for confirmation of Hg2 + 2
what is a disproportionation reaction
The reaction in which one reactant gets oxidized and the same reactant gets reduced
why is the reaction of Hg2Cl2 disproportionate
because the Hg2Cl2 oxidizes and reduces itself to produce Hg and Hg(NH2)Cl
what product of Hg2Cl2 reaction is black
Hg
what product of Hg2Cl2 reaction is white
Hg(NH2)Cl
what is the precipitate if Hg2 +2 is present in the sample
grey/black precipitate
what is present if the chloride precipitate dissolves with Nh4OH
Ag+
how can we be sure there is silver in the sample if the precipitate turns from white to grey (indicating presence of mercury)
test with HNO3 to the supernatant after decanting from precipitate
what indicates presence of silver in the group 1 sample
if the supernatant has a white precipitate form AFTER adding HNO3
what is the reaction causing the white precipitate of Ag to form
how did we ensure the sample in group 1 was free of lead
wash the precipitate that came after heating the sample with 2ml of HOT RO water and filter through Whatman paper
when testing for confirmation of silver, is the solution acidified or basic
acidified with HNO3
if red litmus turns blue the solution is
basic
if blue litmus turns red the solution is
acidic
are group 1 soluble or insoluble chlorides
slightly INSOLUBLE chlorides
are group 2 soluble or insoluble chlorides
soluble
what cations in group 2 precipitate with H2S in 0.3 H+
HgS
Bi2S3
CdS
CuS
SnS
SnS2
will other sulfides in other groups precipitate out with H2S at 0.3 H+
no
what are the colors of precipitates of groups 2 cations with H2S
HgS - black
Bi2S3 - brown
CdS - Yellow
CuS - Black
SnS - Brown
SnS2 - Yellow
what is the only group 2 sulfide that is SOLUBLE in ammonium sulfide
Tin
what is the only group 2 cation NOT soluble in nitric acid (HNO3)
HgS (mercury)
what is IMPORTANT for steps containing thioacetamide
all steps are done in a fume hood because it is highly toxic
how is it known if group 2 cations are present in the sample
if there is a precipitate formed after adjusted to 0.3 M in respect to H+
what group 3 cations are soluble in H2S when more basic than 0.3 M
- Zn2+
- Fe2+/Fe3+
- Ni2+
- Cr3+
- Al3+
what colour sulfide precipitate does Zn2+ form when H2S is more basic
white
what colour sulfide precipitate does Ni2+ form when H2S is more basic
Black
what colour sulfide precipitate does Fe2+ form when H2S is more basic
Black
what colour sulfide precipitate does Fe3+ form when H2S is more basic
reddish-brown
what colour sulfide precipitate does Cr3+ form when H2S is more basic
blue-green
what colour sulfide precipitate does Al3+ form when H2S is more basic
white
what cations are dissolved when precipitate is treated with SO4-2 / HSO4 0 buffer
Fe2+ /3+
Cr3+
Al3+
what cations are NOT dissolved when precipitate is treated with SO4-2 / HSO4 0 buffer
Ni2+
Zn2+
where will Ni2+ and Zn2+ be found in group 3
in precipitate (after treated with buffer)
where will Fe2+/3+, Cr3+, Al3+ be found in group 3
in supernatant (after treated with buffer)
what happens to the Ni2+ and Zn2+ in the precipitate after it is treated with NaOH
Ni2+ will form a green precipitate
Zn2+ is in solution (dissolved)
what is the equation for Zn2+ reacting with OH
what is the equation for Ni2+ reacting with OH
how is the supernatant (after treated with NaOH) tested for presence of Zn
testing with H2S will form a white precipitate
how is the precipitate (after treated with NaOH) tested for presence of Ni
tested with dimethylglyoxime should form a red precipitate
the supernatant with Al3+, Fe2+/3+ and Cr3+ is treated with NaOH and H2O2, what happens to each ion
Iron
- oxidization of Fe2+
- forms reddish-brown precipitate
Cr3+
- oxidized
- forms blue-green precipitate
Al3+
- remains in solution
what happens when Fe reacts with Potassium thiocyanate
dark red solution
what indicates the presence of chromium
if the solution containing the aluminum, chromium OR iron is adjusted to proper H+ and a green solution occurs (Cr6+) indicating chromium ions might be present
confirmatory test for iron
- in a separate test tube add 1 ml 6 M HCl, 2 ml H20 and a few drops of 1 M KSCN
- add the solution from step 1 to the precipitate that formed after adding H2O2 and heating
- appearance of dark red colour confirms presence of iron
confirmatory test for aluminum
- collect the filtrate through a filter paper after the solution was treated with H2O2 and heated (centrifuge to separate supernatant from precipitate)
- add 6M HCl until just acidic to BLUE litmus paper
- add 3 M NH4OH till the solution is JUST basic to red litmus paper
- warm in water bath before centrifuging to collect the precipitate AND the supernatant (test for chronium)
- a white precipitate indicates Aluminum
confirmatory test for chromium
- solution tested for aluminum should be YELLOW/ORANGE
- acidify the supernatant from testing Aluminum with 6 M HCl
- add a few drops of H2O2
- appearance of dark blue complex confirms the presence of Cr
confirmatory test for nickel
- a pale green precipitate that forms after adding 6 M NaOH till basic to red litmus paper INDICATES nickel is present
- dissolve the precipitate in 6 M HCl and make solution alkaline to red litmus paper with 6 M NH4OH
- add few drops of dimethylglyoxime
- red solution will form IF NICKEL PRESENT
confirmatory test(s) for zinc
- after treating the solution with 6 M NaOH till basic to red litmus paper and centrifuge the supernatant might hold zinc
- divide the supernatant into two parts
- to one part, add 15 drops thioacetamide and heat in water bath for 5 mins. white precipitate will form if Zinc present
- acidify other portion of supernatant using 3 M HCl till solution JUST acidic to blue litmus paper. white precipitate will form and redissolve if zinc present
