Chern 12:

3.5 NOTES: Separating Ions

We can find out which ions are in a solution and separate ions out of solutionusing 2 methods:

- Quantitative analysis (experimental procedures)Precipitation

Quantitative analysis

Example: we have a solution which we know contains either Ag+ or Ba2+, butwe're not sure which. A -1- ba ~t·Solution: use the solubility table! 3

Find a aD" 0 (\ 8 ion that will form a _p( f2 (i P \'-\-a{Gone but not the other! This takes some searching!+ IJ ').~J .w G\(\~- ~ ~ f <- one. ,'Dn (A-g or Dd ,f\

withil ' )\.-Du) SO I.

e:l, " An otbers Soluble

Sulphate, 8042-

Low Solubility

I - - a ,-'1~ c "t, - --

: AU others ~ UQ \ ("\ a\\ 0.\ ~~ Soluble

- ~J>b-+, Cu+ Low SoJubility

- I tC,\ br or JAWe could add ) to precipitate

A ~ - c,- ~ A3C\-e,l( " 9 + c s), ta;~ (c).j\)

'- Pp"T

UV\ o~/~er j 00 d cho\"c<- ~ o 1).-( ppt- A3 +)

Example 2: A solution is known to contain either Ag+ or Ba2+ ions. (One but notthe other). S2- ions are added to this solution and a precipitate DOES NOTFO~ Which ion, (A~+ or !3a2+) is present in the ~ol~tion? ~_

\he... \0(\ \ S ;SO \ \.A'o \ e. \,v \ 1-tl S

Q ~-\-Answer: _Q-,Q~ is present in the solution

Example 3: You have three beakers each containing a different ion.

Solution:

One has Pb 2-; one has

~ eel + and one has Mg 2+

Which is which?

Start by selecting an anion to add that will only form a ppt with ONE cation.\CO" ~ t .o« unc\{( \1 '0..0 So\\..t~)\;-\j;·-

, ---- --..../~ Chlmide, .T i Al! otbers Soluble

or

or ::~/;. --~:9~~~-----------------PP;~--PbCi-~-----~~-~::~;;,:~__ ~~

Set aside this beaker (label it U1.k;L")

Now, add an anion that will form a ppt with only ONE of the remaining

cations. CG\ ?--\ ) 09 'J--t

~-_______ AU athel.'S Soluble

Aj\·(" '1' ..(]\.O· Sulphate, so4~ l::'.--------------------------------------------------------------------------------

~ . Ag'" Ca2+ Sr:!+, 9:11+, 'Pb2~ p-pl: Co.. SO~ LowSolubitity

~i- Set aside this beaker and label it II CIA ", You can also label the last beakerl~

"~"!~ p\flO(~\) o{ ~\; m " l\Q h on ~

Now in reality, we cannot just ADD ANIONS. We can add them in the formof a salt (pair it with a soluble cation).Recall: Net +

.~ C\clo\ ~ C\ c\.0 --

A9 f ~ C\tlo~ ~ND3

- When you want to introduce an a . n, pair it with-,-,

{Y-: C\- When you want to introduce a ca '. n, pair it with

-+

Example: A solution is known to contain either CI- (chloride) ions or S042-(sulphate) ions. Name a compound which you could add to tell you which ion waspresent.

or <; :J--Vo\{

Solution: - look up these two anions on the solubility table:

\.. Chloride.Cl ")or-~

Bromide, Dr-or

Iodide, 1- 1AUothers Soluble

, -------~~- --;- - - - -------------- - - ---- - -- ---------------- -- - - - -- - - - ---------------. -- -- - --

Ai w"+ Cu+ Low So!ubl1it'l'/""""'7-- .,.-.- " - .r

AUothers Soluble

l Sui , SOil ;= }---------- --- ---;.::----:------ ---- --- ---- -------------- --- ---- -------------------- -----A ./ / 2+ IS 2+' 2+. P"£ 'S 1 Ie '1',/ £ ,<1 \:r . <1 Y U LOW 0 Uul Ity .0\-

Bad choices of cations to add would be: A..a -r ?b 'd-;- (y'\ok -. C01 ,S. /"10+d.O .) L ~+ab\L ._. wd \

(\ 'J.1 ~ d-.-+ 0... ~i +0 (u. L",Good choices of cations to add would be: LlA ) 0( > uCt C'V\Cl1-t I D )

,'{\. vJ {,\-+<-- r -.

So, the compound we add would be:(~Cl (ND-;JL'\ ~h ~~ )5, (\\)°1)>-

( t)o.ll\lO~) L

Example. A solution is known to contain either Ba2+ or Mg2+ ions.

Suggest a method by which these solutions could be analyzed to find outwhich ion is present. Be specific about any compounds that are added.

Clo..&~~ 0IJL\L.- -7 PPt ·BQl.-\- 0("\ \j .

\ Ii DD N0. ~ 'SO cf l

Example: A solution is known to contain!!.!1:£of these .ions; Mg2+, Ca2+, Sr2+, orBe2+: Mixing samples of the solution with various reagents gives thefollowing data: <' d-- SO'~ - - I I -

/"- -0 ~ '-\ ~OM,( " ,(

Reagent i Na S i Na2S04) i N~H··..·· ·· ·..···;··············t ······..·· ,..··········,·"'-j- ~ ..=·",··,··t · ,··,..· ' .Result i no_ppt. i ppt. i no ppt.

From these data, which one of the four ions is present?

f\\ D \<. e. o so\V\b ~,\'tj c\t a (-t ~-~- :SOl{

d-.- OH.5tv\s

2-.,.no ppt- no pr-\- no ppt

-CC\ ~-t y\ v PPT- VPT

Sf ~~' -1--.. - ..__ . ~-

B-e'J.-t

PP'T hO ppt- rvo ppl

Precipitation Methods of Separating Ions ( sa me. as \ as r mett16a)b\.{ r .

This method involves forming a precipitate and removing it by filtration.

The precipitate remains as a residuer--------t on the filter paper.

The filtrate containsany solublesubstances. These are able to penetratethrough the filter paper.

If there are more ions in the "filtrate", we can repeat the process.

Example:You are given a solution containing silver ions (Ag'"), strontium ions (Sr2+),and magnesium ions (Mg2+). You must separate them, one at a time from thesolution using precipitation reactions. Show how you would do this. Write theNet-Ionic Equation for each precipitate formed.

Solution:A -r. 3

choose an anion that will form a ppt with only ONEcation:

~

. C...:hlO...ride..'.crOT

Bromide, Br-or~dide,l- I All others Solu\'lle

, .-. ',"', ,'" o.c;;)Pb.-t, ell+ Low Solubility

Step 1: add No.. c, \ to precipitate ~

Net ionic equation: '\ A8 ~t:;)T C\ l~) -7 A 3 c \ (s)(f\U 5\>~L\-lI\TO'i' ,C0») i

Filter the solution (the precipitate will remain on the filter paper).. 2--r

Step 2: add to precipitate Sr .Q ~ -\ ~ do.- S s r-:

Net ionic equation: 0( c~) -\- 00'4 (lUJ.) -7 r Vt+ CS)

Filter the solution (the precipitate will remain on the filter paper).

Step 3: add NL\ 0 tt to preciPitate~ 1-V

(**note: it doesn't matter if you add an anion that precipitates 2 ions, sincethe other ions are gone. But if you can, choose an anion that just precipitatesthe ion you want). OH - be \-\-C~ +-h a f\ po'-} "3-

Net ionic equation: 1v'\9 '-~£\Jr) -t20 It -("-1-) ~ JIllj CD I+)z. U)

Filter the solution (the precipitate will remain on the filter paper).

Do Questions in Hebden p. 90 #26-32, 36, 37