Objectives. Define precipitation reactions. Determine the solubility and solubility product of different salts. Compare the solubility of different salts. Define precipitation titrations. Define how to detect end point of argentometric titrations. Precipitation reactions.
Simple rule for the solubility of salts in water (VIP)
Ksp = [A]a[B]b
CuBr(s) Cu+(aq) + Br-(aq)
Ksp = [A]a[B]b
Ksp = [Cu+] [Br-]
Ksp = [2.20x10-4 mol/L][2.20x10-4 mol/L]
Ksp = 4.0x10-8 mol2/L2 (the units for Ksp are usually omitted).
Notice that, each mol of Bi2S3, produces 2 moles of Bi3+ and 3 moles of S2-, so
The Ksp value for copper(II) iodate Cu(IO3)2, is 1.4x10-7 at 25°C. calculate its solubility at 25°C.
A salt’s Ksp value gives us information about its solubility.
However, we must be careful in using Ksp values to predict the relative solubilities of a group of salts as there are two possible cases:
AgI Ksp = 1.5x10-16
CuI Ksp = 5.0x10-12
CaSO4 Ksp = 6.1x10-5
Each of these solids dissolves to produce two ions
Salt cation + anion
We can compare the solubilities by comparing the Ksp values. CaSO4(s) > CuI(s) > AgI(s)
2. The salts being compared produce different numbers of ions. For example consider
CuSKsp = 8.5 x 10-45
Ag2SKsp = 1.6x10-49
Bi2S3Ksp = 1.1x10-73
Because these salts produce different number of ions when they dissolve, the Ksp cannot be compared directly to determine relative solubilities. First we have to calculate the solubilities,
CuS Solubility = 9.2x10-23
Ag2S Solubility= 3.4x10-17
Bi2S3 Solubility = 1.0x10-15
from which we get
(More Soluble) Bi2S3 > Ag2S > CuS (less soluble)
A) Effect of analyte and reagent concentration.
From the figure:
Disappearance or appearance of a precipitate.
Formation of a coloured precipitate, formation of a coloured complex or using adsorption indicators.
In this case, the solution contains excess of cyanide ions which precipitates as Silver cyanide when silver is added. Titration is continued until all the cyanide is transferred to soluble argentocyanide complex. The first excess of silver ion added will form the insoluble silver argentocyanide complex,
AgCN + KCN K[Ag(CN)2]
K[Ag(CN)2] + Ag+ Ag[Ag(CN)2] + K+
The end point is detected by the first appearance of turbidity. Most silver salts are soluble in silver cyanide Cl-, I-, Br-, SCN-.
Ag+ + X- AgX(s) white
2Ag+ + CrO42- Ag2CrO4(s) red
In aqueous solution, (neural or slightly alkaline 7-9 while strongly acidic medium hinders its dissociation), fluorescein partially dissociates into hydronium ions and negatively charged fluoresceinate ions that are yellow green. The fluoresceinate ion forms an intensely red colour with silver ions.
Applications of Precipitation titrations