ERT207 Analytical Chemistry Gravimetric Analysis and Precipitation Equilibria

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ERT207 Analytical ChemistryGravimetric
Analysis and Precipitation Equilibria

• Pn Syazni Zainul Kamal

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• CO2 Ability to classify and use separation
  techniques and gravimetric methods for mass
  determination

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Introduction

• The term gravimetric pertains to a Weight
  Measurement.
• Gravimetric method is one in which the analysis
  is completed by a weighing operation.
• Gravimetric Analysis is a group of analytical
  methods in which the amount of analyte is
  determined by the measurement of the mass of a
  pure substance containing the analyte.
• Gravimetric Methods can also be defined as
  quantitative methods based on the determining the
  mass of a pure compound to which the analyte is
  chemically related.

Analyte constituents to be determined
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There are two main types of gravimetric analyses
A) Precipitation
analyte must first be converted to a solid
(precipitate) by precipitation with an
appropriate reagent. The precipitates from
solution is filtered, washed, purified (if
necessary) and weighed.
B) Volatilization
In this method the analyte or its decomposition
products are volatilised (dried) and then
collected and weighed, or alternatively, the mass
of the volatilised product is determined
indirectly by the loss of mass of the sample.
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Example for Precipitation-

• Calcium can be determined gravimetrically by
  precipitation of calcium oxalate and ignition of
  the oxalate ion to calcium oxide.
• Ca2 C2O42- ?CaC2O4
• CaC2O4 ? CaO CO2 CO
• The precipitate thus obtained are weighed and the
  mass of calcium oxide is determined.

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Example for Volatilisation-

• The analyte or its decomposition products are
  volatilised at a suitable temperature. The
  volatile product is then collected and weighed,
  i.e. the mass of the product is indirectly
  determined from the loss in mass of the sample.
• Example
• Water can be separated from most inorganic
  compounds by ignition, the evolved water can then
  be absorbed on any one of several solid
  desiccants. The weight of water evolved may be
  calculated from the gain in weight of the
  absorbent.

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Not all insoluble precipitates are well suited
for gravimetric analysis. It is important to
consider what properties are required in order
that a precipitate be applicable for a
quantitative precipitation method-
Solubility
Filterability
Chemical Composition
Other Desirable Properties
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Solubility
The product must be sufficiently insoluble to
prevent the loss of weight.
Filterability
Precipitate formed should be adoptable to simple
and rapid filtration methods.
Chemical Composition
The product must be of known chemical composition.
Other Desirable Properties
Other factors effecting the stability and purity
of the precipitate.
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For a successful determination in gravimetric
analysis the following criteria should be met -

• The desired substance must be completely
  precipitated. In most determination the
  precipitate is of such low solubility that losses
  from dissolution are negligible. An additional
  factor is the common ion effect, this further
  decrease the solubility of the precipitate.
• E.g. When Ag is precipitated out by addition of
  Cl-
• Ag Cl- AgCl
• The low solubility of AgCl is reduced further by
  the excess of Cl- which is added force to the
  reaction to proceed towards right side.

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For a successful determination in gravimetric
analysis the following criteria should be met -
(2)The weighed form of the product should be of
known composition. (3)The product should be
pure and easily filtered. It is usually difficult
to obtain a product which is pure or which is
free from impurities. This could be reduced by
careful precipitation and sufficient washing.
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Gravimetric Analysis

• Gravimetric analysis is potentially more accurate
  and more precise than volumetric analysis.
• Gravimetric analysis avoids problems with
  temperature fluctuations, calibration errors, and
  other problems associated with volumetric
  analysis.
• But there are potential problems with gravimetric
  analysis that must be avoided to get good
  results.
• Proper lab technique is critical

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Steps in a Gravimetric Analysis

• 1. Preparation of the solution
• 2. Precipitation
• 3. Digestion
• 4. Filtration
• 5. Washing
• 6. Drying or ignition
• 7. Weighing
• 8. Calculation

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1. Preparation of analyte solution

• Gravimetric analysis usually involves
  precipitation of analyte from solution.
• 1st step prepare the analyte solution
• May need preliminary separation to separate
  potential interferences before precipitating
  analyte
• May need adjustment of solution condition to
  maintain low solubility of precipitate
• May require pH/temp/volume (of sol.) and/or other
  adjustments to maximize precipitate formation. Eg
  Calcium oxalate insoluble in basic medium

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2. Precipitation

• The precipitating reagent is added at a
  concentration that favors the formation of a
  "good" precipitate.
• This may require low concentration, extensive
  heating (often described as "digestion"), or
  careful control of the pH.
• The precipitate should
• Be sufficiently insoluble
• Have large crystals (Easier to filter large
  crystals)
• Be free of contaminants

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Precipitation process

• When solution of precipitating agent (AgNO3)
  added into testing solution (chloride solution)
  to form AgCl precipitate,
• Supersaturation the solution phase contains
  more dissolved salt than at equilibrium. The
  driving force will be for the system to approach
  equilibrium (saturation).
• - Nucleation initial phase of precipitation. A
  min number of particle precipitate will gather
  together to form a nucleus of particle
  precipitate (solid phase). Higher degree of
  supersaturation, the greater rate of nucleation

AgCl- AgCl-AgCl- AgCl-AgCl-AgCl- AgCl-AgCl
-AgCl-
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• nucleation involves the formation of ion pairs
  and finally a group of ions formed.
• - Particle growth particle enlargement
  process. Nucleus will grow and forming a crystal
  of a specific geometric shape.
• Von weimarn discover the particle size of
  precipitates is inversely proportional to the
  relative supersaturation of the sol. during the
  precipitation process.

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The von Weimarn Ratio

• von Weimarn ratio (Q S)
• S
• A measure of relative supersaturation or
  supersaturation ratio
• The lower the better
• If high, get excessive nucleation, lots of small
  crystals, large surface area
• If low, get larger, fewer crystals, small surface
  area

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• S solubility of precipitate at equilibrium
• Keep it high with high temperatures, adjusting pH
• Q concentration of mixed reactants before
  precipitation
• Keep it low by using dilute solutions, stir
  mixture well, add reactants slowly
• Can lower S later by cooling mixture after
  crystals have formed

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3. Digestion of the Precipitate

• Let precipitate stand in contact with mother
  liquor (the solution from which it was
  precipitated), usually at high temp.
• This process is called digestion, or Ostwald
  ripening. The small particles tend to dissolve
  and precipitate on the surfaces of the larger
  crystals
• Large crystal (small surface area) have lower
  free energy than small crystal (large surface
  area)
• Digestion make larger crystals, reduce surface
  contamination, reduce crystals imperfection

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4. Filtration

• Sintered glass crucibles are used to filter the
  precipitates.
• The crucibles first cleaned thoroughly and then
  subjected to the same regimen of heating and
  cooling as that required for the precipitate.
• This process is repeated until constant mass has
  been achieved, that is, until consecutive
  weighings differ by 0.3 mg or less.
• (you may see the proper technique of filtration
  in chapter 2)

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5. Washing

• Coprecipitated impurities esp those on surface
  can be removed by washing the precipitate
• Wet precipitate with mother liquor and which will
  also be remove by washing
• Need to add electrolyte to the wash liquid bcoz
  some precipitate cannot be wash with pure water,
  peptization occur.
• Eg HNO3 for AgCl precipitate

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6) Drying or ignition

• To remove solvent and wash electrolytes
• Done by heating at 110 to 120C for 1 to 2 hrs.
• Converts hygroscopic compound to non-hygroscopic
  compound
• May used high temp if precipitate must be
  converted to a more suitable form before weighing
• Eg MgNH4PO4 convert to pyrophosphate Mg2P2O7 by
  heating at 900C.

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7) Weighing

• After the precipitate is allowed to cool
  (preferably in a desiccator to keep it from
  absorbing moisture), it is weighed (in the
  crucible).
• Properly calibrated analytical balance
• Good weighing technique

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Organic Precipitates

• Organic precipitating agents have the advantages
  of giving precipitates with very solubility in
  water and a favorable gravimetric factor.
• Most of them are chelating agents that forms
  slightly insoluble, uncharged chelates with the
  metal ions.

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Organic Precipitates
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Gravimetric Analysis Weight Relationship

• in gravimetric method the analyte (solute) is
  converted to precipitate which is then weight.
• From the weight of the precipitate formed in a
  gravimetric analysis, we can calculate the weight
  of the analyte

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Gravimetric factor (GF) weight of analyte per
unit weight of precipitate. Obtain from ratio of
the F Wt of the analyte per F Wt precipitate,
multiplied by moles of analyte per mole of
precipitate obtained from each mole of
analyte GF f wt analyte (g/mol) x
a (mol analyte/mol precipitate)
f wt precipitate (g/mol) b g
analyte / g precipitate
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• Example 1
• If Cl2 in a sample is converted to chloride and
  precipitated as AgCl, the weight of Cl2 that
  gives 1 g of AgCl is???
• F wt Cl 35.453
• F wt AgCl 107.868
• GF f wt analyte (g/mol) x a (mol
  analyte/mol precipitate)
• f wt precipitate (g/mol) b
• g analyte / g precipitate

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• GF f wt analyte (g/mol) x a (mol
  analyte/moprecipitate)
• f wt precipitate (g/mol) b
• g analyte / g precipitate
• g Cl2 g AgCl x f wt analyte (g/mol) x
  a
• f wt precipitate
  (g/mol) b
• 1 AgCl x 70.906 x 1
• 143.321 2
• 0.2474 g

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• percent composition by weight of the analyte
• in the sample
• A gA x 100
• gsample
• gA grams of analyte (the desired test
  substance)
• gsample grams of sample taken for analysis

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EXAMPLE 2 A 0.3516 gm sample of commercial
phosphate detergent was ignited at a red heat to
destroy the organic matter. The residue was then
taken up in hot HCl which converted P to H3PO4.
The phosphate was precipitated with Mg2 followed
by aqueous NH3 to form MgNH4PO4.6H2O. After being
filtered and washed, the precipitate was
converted to Mg2P2O7 (FW222.57) by ignition at
1000ºC. This residue weighed 0.2161 gm. Calculate
the percent P (FW 30.974) in the sample.
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GF f wt analyte (g/mol) x a (mol
analyte/mol precipitate) f wt
precipitate (g/mol) b g analyte /
g precipitate

• g P 0.2161 g x 30.974 x 2
• 222.57 1
• 0.0601g
• A gA x 100
• gsample
• 0.0601 g x 100
• 0.3516 g
• 0.1709

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• Orthophosphate (PO43-) is determined by weighing
  as ammonium phosphomolybdate (NH4)PO4.12MoO3.
  Calculate the percent P and the percent P2O5 if
  2.1771g precipitate (ppt) were obtained from a
  0.3588g sample.
• F wt P 30.97, F wt P.molybdate 1876.5,
  F wt P2O5 141.95

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• g P 2.1771g x 30.97 x 1 mol
• 1876.5 1 mol
• 0.0359g
• P 0.0359g x 100
• 0.3588g
• 10.01

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• g P2O5 2.1771 g x 141.95 x 1 mol
• 1876.5 2 mol
• 0.0823g
• P2O5 0.0823g x 100
• 0.3588g
• 22.94

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Precipitation Reactions

• Solubility of a compound concentrations of a
  soluble species at equilibrium with its insoluble
  form.
• If the compound is sparingly soluble, it will
  produce cation anion.
• Eg PbCl2 slightly dissolved in water. So PbCl2
  has a specific solubility, s solid phase aq
  aqueous phase
• PbCl2 (s) Pb2 (aq) 2Cl- (aq)

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• The equilibrium constant for the reaction is
  known as solubility product constant.
• Ksp (PbCl2) Pb2Cl-2
• Concentration of any solid (PbCl2) is constant
  and is combined in the equilibrium constant to
  give Ksp

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• Example
• Calculate the concentration of Ag and Cl- in a
  saturated solution of AgCl, and the molar
  solubility of AgCl. Ksp for AgCl at 25C is 1.0 x
  10-10.

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• Solution
• AgCl Ag Cl-
• s s ssolubility
  (M)
• Ksp Ag Cl-
• (s)(s)
• 1.0 x 10-10
• s 1.0 x 10-5 M Ag Cl-
• Molar solubility of AgCl is equal to
  concentration of Ag or Cl- so molar
  solubility of AgCl 1.0 x 10-5 M

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Common ion effect

• If there is an excess of one ion over the other
• - the concentration of the other is suppressed
• - solubility of precipitate is decreased

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• Example
• Calculate the molar solubility of AgCl in (a)
  water and (b) in 0.10M KCl solution. Ksp for AgCl
  is 1.7x10-10.

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• Solution
• Solubility of AgCl in water
• AgCl Ag Cl-
• s s ssolubility(M)
• Ksp Ag Cl-
• (s)(s)
• 1.7x10-10
• s 1.3x10-5 M
• Solubility of AgCl in water 1.3x10-5 M

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• b) Solubility of AgCl in 0.10M KCl
• AgCl Ag Cl-
• s s0.10
• Ksp Ag Cl-
• (s)(s0.10)
• 1.7x10-10
• Since s is small compared to 0.10M, it can be
  neglected
• Ksp (s)(0.10)
• 1.7x10-10
• s 1.7x10-9 M
• Solubility of AgCl in the presence of 0.10M KCl
  is 1.7x10-9

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• Precipitation only occur when ionic product
  greater than Ksp value
• If ionic product equal Ksp, all the ions will be
  in the solution without formation of precipitate