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III.5 – Separating Mixtures of Ions by Precipitation Methods (Qualitative Analysis) (# 26-39)

• One important use of solubility is in the field of qualitative analysis.

• Qualitative analysis involves the use of experimental procedures to determine what elements or ions are present in a substance. It is NOT quantitative, so it is not interested in how much substance is present.

• Using the precipitation method, you can try to isolate either cations or anions.

• Basic idea = Using the solubility table, try and find an ion that will precipitate one of the anions / cations that you are looking for.

Example: Assume we have a sample of water from Cultus Lake. We are required to perform a routine check for heavy metal contamination. The metals that we are testing for are: Ag+ and Sr2+. (For this example, we assume that these are the only two ions in solution.)

▪ Try to find an anion that will precipitate ONE of the two cations (Ag+ and Sr2+) by constructing a solubility table like below.

▪ The five columns correspond to the five groups of ions in the solubility table that can produce a precipitate. (i.e., Alkali metals, H+ and NH4

+ are omitted.)

Cl-, Br-,

I-

SO42- S2- OH

- PO4

3-, CO32-, SO3

2-

Ag+

ppt ppt ppt ppt ppt

Sr2+

ppt

ppt

This analysis will require 2 steps as maximum of 2 cations are present.

Step 1:

Step 2:

▪ Note: is it possible to go to a chemical storeroom and get some Cl- anions? No, it is not. How could you get those Cl- ions into solution?

89

↳ is the cation present?↳ yes tf PPT forms

Anion

O X

- anions-

CT,Br, I

- 5042-

S2-

OH- 17045, 355032-

( Agt ppt PPT PPT PPT PPT

cation spit og ppt sa - PPT

Add to detect Agtno transparency change = Agtabsent

gets cloudy due to Agclcs)= Agtpresent

Liz 4 LBS Fitter out PPT

Add SHEKto detect grit→ no PPT-- Sr'tabsent

→PPT (srsly) form = SRHpresent

⇒ Add soluble salt containingSpectator ions (Lit, Nat, KT. . .,H5NHe5N05)

case I 2 3 4

Nothing

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Try this one…

Example: A solution contains one or more of Ag+, Ba2+, and Ni2+. What ions could be added, and in what order, to determine which of these cations are present.

▪ Set up a table of solubilities and fill in the columns with ‘-‘ or ‘ppt’

▪ In what order should you add the different salts to check for the presence of each cation?

Examples:

90# cation = # of steps

Look vertically Cl- 5042

-

S2- OH-

poop-

groupto find

Agt ppt ppt ppt PPT PPTanion that

-

ppt - ppt PPTforms PPT

Batt

w/ Fine cation Ni" - -

PPT PPT ppt

step① Add Lid to detect Agt. If PPTCAGCI) forms then fitter.② Add Nazs = Nit

.

= (NTS) = = .

③ Add Kzsoq = Baat.

⑦

as÷

ppt ppt

②

•T⇒omefuno2SGOuestTonSN

- PPT- PPT o Next class Quiz II. I -3

& SCT II. 6 ( calculator)