TITRIMETRY Flashcards
analytical procedures in which the amount of analyte is determined from the amount of standard reagent required to react with the analyte completely
titrimetric methods
types of titrimetric methods
- Volumetric method
- weight or gravimetric titrimetry
- coulometric titrimetry
type of titrimetric method where it is based on the volume of reagent of known concentration that is consumed by the analyte
volumetric method
type of titrimetric method where the mass of the titrant is measured rather than the volume
weight or gravimetric titrimetry
type of titrimetric method where the “reagent” is a constant direct current of known magnitude that reacts with the analyte; the time required to complete the reaction is measured
coulometric titrimetry
reagent of known concentration that is used to carry out a volumetric analysis
standard solution
performed by slowly adding a standard solution from a buret or other volumetric measurement device to a solution of analyte until the reaction between the two is complete
titration
final - initial = ?
volume needed
point of titration when the amount of added standard reagent exactly equals the amount of analyte
equivalence point
point in the titration where there is a physical change associated with the condition of equivalence
end point
often added to the analyte solution in order to give an observable physical change at or near the equivalence point
indicator
types of indicator
visual and instrumental
type of indicator where it changes in color at the equivalence point
visual indicators
type of indicator where it responds to certain properties of the solution during titration
instrumental indicator
determines the amount of excess standard titrant to be added
back titration
highly purified compound that serves as a reference material in all volumetric method
primary standard
use to prepare standard solution
primary standard
requirements for a primary standard
- high purity
- stable in air
- absence of hydrate water
- readily available at modest cost
- reasonable solubility in the titration medium
- reasonably large formula weight
methods to prepare standard solutions
- direct method
- standardized by titration (weighted quantity of primary standard and measured volume of another standard solution)
standard acids
HCl, HClO4, H2SO4
oxidizing acid
HNO3
standard base
NaOH, KOH, Ba(OH)2
phenolphthalein indicator color
• acid - colorless
• base - pink
methyl orange and methyl red
• acid - red
• base - yellow
bromothymol blue
• acid - yellow
• base - blue
indigo carmine
• acid - blue
• base - blue/yellow
primary standard are weighed and diluted to an exact volume of solution
direct method
best method to prepare standard solution
direct method
variables that influence behavior of indicators
- temperature
- ionic strength of the medium
- presence of organic solvent
- colloidal particles
solubility rules
- all nitrates are soluble (NO3-)
- all chlorides are soluble except AgCl, Hg2Cl2, and PbCl2
- All sulfates are soluble except CaSO4, SrSO4, BaSO4, Hg2SO4, PbSO4, and Ag2SO4
- All carbonates are insoluble except the 1A elements and NH4-
- All hydroxides are insoluble except those of the 1A elements, Sr(OH)2, and Ba(OH)2; Ca(OH)2 is slightly insoluble
- all sulfides are insoluble except those of the 1A and 11A elements and NH4
ways of expressing solubility
- molar solubility - no. of moles of solute dissolved in 1L of saturated solution
- solubility - number of grams of solute in 1L of saturated solution
factors affecting solubility
- temperature
- presence of common and uncommon ion
- pH
- effect of complexing agent
particle size
- crystalline suspension
- colloidal suspension
particle size have dimension on the order of tenths of s millimeter or greater
crystalline suspension
tend to settle spontaneously and are readily filtered
crystalline suspension
tiny particles are invisible to the naked eye
colloidal suspension
show no tendency to settle from solution nor are they easily filtered
colloidal suspension
particle size is influenced by experimental variables such as
precipitate solubility, temperature, reactant. concentration, and the rate at which reactants are mixed
the net effect of these variables could accounted by a single property called
relative supersaturation (RSS)
if the RSS value is large, the precipitate is a ….
colloidal
if the RSS value is small, the precipitate is…
crystalline solid
in this process, large number of colloidal precipitate
nucleation
higher rate of nucleation happens when
there is higher RSS
this precipitation process happens when there is only smaller number of larger size particles appear
particle growth
the rate of particle growth depends on..
RSS
this determines the charge on a colloidal particle formed in gravimetric analysis
change of the lattice ion that is in excess when the precipitation is complete
can be hastened by heating, stirring, and adding an electrolyte to the medium
coagulation
a phenomena in which otherwise soluble compounds are removed from solution during precipitate formation
co-precipitation
types of co-precipitation
- surface absorption
- mixed-crystal formation
- occlusion
- mechanical entrapment
occurs when the precipitate has a large surface area
surface absorption
a contaminant ion replace analyte ion in its crystal lattice (inclusion)
mixed - crystal formation
compounds are trapped within a pocket during rapid crystal growth
occlusion
a portion of solution are trapped when crystals are fused together
mechanical entrapment
an important and widely used titrimetric reagent
silver nitrate (AgNO3)
employed for the determination of anions that precipitates as silver salts
- halides
- halide like ions
- mercaptan and certain fatty acids
halides
I-, Cl-, Br-
halide like ions
SCN-, CN-, CNO-
factor affecting end point sharpness
- reagent concentration
- end point improves as the analytical reaction becomes more complete
- the lower the value of Ksp the better will be the detection of the end point
end point improves as solution concentration become more concentration
reagent concentration
types pf argentometric titrations
- direct titration
- indirect “back” titration
the analyte solution is directly titrated against standard AgNO3 solution
direct titration
known excess of standard AgNO3 is added to the analyte solution
indirect “back” titration
what happens during back titration
the excess unconsumed of standard AgNO3 is back titrated against another standard titrant
types of direct titration
- Mohr method
- Fajan method
titration technique that determines the concentration of chloride ions in a solution by using silver nitrate as a standard solution
mohr method
limitations of mohr method
- the pH of the medium must be from 6-10
- In alkaline medium: Ag+ will precipitate as oxide
- this will interfere with the titration due to pptn of Ag2O before Ag2CrO2 where Ksp (Ag2O) < Ksp (Ag2CrO2) (+ve error)
- in the presence of ammonium salts, the pH must not exceed 8, free NH4 is released which dissolves AgCl ppt
- interfering ions (cations which give insoluble CrO4²- salts must be absent and anions such as AsO4²-, CO3²-, and PO4³- interfere by forming insoluble Ag salts in neutral or alkaline solutions
the end point is detected by using absorption indicators that is absorbed on the surface of the precipitate immediately after the equivalence point
fajan method
limitations of Fajans method
- coagulation of the precipitate must avoid sharp endpoint if surface area is large (increase absorption of indicator) — dextrine (protective coolloid) keep the ppt highly dispersed
- absorption indicators enhance the tendency of silver halides toward photodecomposition, which blackens the ppt — titration must be done rapidly in subdued light
- the titration solution must he controlled to ensure the indicator will exist in its ionized form — fluorescein (pKa = 7) operates in solution of pH 7 or more
formation of soluble coloured complex
volhard method
could be used for direct determination if Ag+ and indirect determination of halides and anions
volhard method
