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Gravimetric analysis describes a set of methods in analytical chemistry for the
quantitative determination of an analyte based on the mass of a solid.
In most cases, the analyte must first be converted to a solid by precipitation with an
appropriate reagent. The precipitate can then be collected by filtration, washed, dried to
remove traces of moisture from the solution, and weighed. The amount of analyte in the
original sample can then be calculated from the mass of the precipitate and its chemical
composition. When the precipitating reagent is added, there is a period of induction
(time taken before the first sign of precipitate is seen) and then nucleation (when a
minimum amount of ions come together to form a stable solid) occurs. Large crystals
are preferred to small crystals as they are more easily filtered, digestion (heating of the
solid and mother liquor [liquid containing the solid]) encourages the formation of large
crystals.
For filtration, a vacuum pump is used (large conical flask connected to a tap via an
aspirator), sintered crucible are used (small crucible with very high melting point,
inert) which can withstand high pressures used in vacuum filtration. A rubber
policeman (glass rod with a hard rubber scrapper on one end) is also used to transfer
residues of solid or precipitate onto glass surfaces. The precipitate is rinsed with cold
distilled water and then heated in an oven to remove all traces of water and then cooled
in a dessicator (air tight container which contains silica to absorb any water vapour
present) before weighing.
In other cases, the volitilisation method would be used. For example with a hydrated
salt, the aim is to determine the number of moles of water of crystallisation present. It
would be heated multiple times to remove the water ( and the difference in mass can
then be used to determine the amount of water present and thus the number of moles of
water needed for one mole of the anhydrous salt.
NB Sometimes the element being analysed is in one form, but must first be converted to a
different form before quantitative analysis can take place. Usually an acid would be used to
convert the analyte which is a solid to an aqueous solution which can then be precipitated by an
known amount of precipitating reagent. Either HCl or HNO3 is used, if the substance is a
carbonate, the acid can be dilute, otherwise the concentrated version of the acid is used.
Gravimetric method of calibrating a pipette
The mass of a small empty beaker is measured and recorded using an analytical
balance.
The temperature of the water used is measured and recorded.
Then using 10ml pipette, the liquid is drawn up and run into the small beaker and the
mass of the beaker is measured and recorded.
The previous step is repeated until there are 5 mass readings with water present.
The relationship between density of water and its temperature is well known and the
temperature recorded is used to obtain the density of the water used.
The mean mass of the water delivered by the pipette is converted to volume using the
relationship of density with mass and volume.
The standard deviation of the data is calculated.
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Thus the mean volume and the standard deviation comprises the capacity of the pipette
used.
Checkpoint A
1. State briefly what steps one would take to determine the amount of water in a
sample of hydrated magnesium sulphate crystals
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2. Imagine you are given a sample of sea water and asked to determine the
amount of chloride ions present. You decide to use the precipitation method,
suggest a suitable reagent to use to form the precipitate……………………….
Diagram of SOME apparatus used in gravimetric analysis
The sintered glass filter can withstand the high pressures used in vacuum
filtration.
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Uses of gravimetric analysis in quality control
Measurement of the “essential” elements in plant foods (phosphorus, for
example, is converted into the insoluble salt, magnesium ammonium
phosphate)
Estimation of pollutants in the air, such as sulphur dioxide (by conversion to
insoluble barium sulphate)
Estimation of sulphur dioxide (used to prevent microbial spoilage) in soft
drinks, such as orange juice.
Estimation of chloride ions in water supplies
(by conversion to insoluble silver chloride).
Worked Example of gravimetric analysis question
A compound of Iron (Fe) and Chlorine (Cl) is soluble in water. An excess of
Silver Nitrate (AgNO3) was added to precipitate the chloride ion as silver
chloride. If a 134.8 mg sample of the compound gave 304.8 mg of AgCl, what
is the formula of the compound?
Answer:
Note 1000 mg = 1g Ar Fe = 56, Cl = 35.5 Ag = 108
determine # mol AgCl = mass AgCl / molar mass AgCl = 0.3048 / (143.5)
= 0.0021
 Ratio AgCl : Cl- = 1: 1 therefore # mol Cl- = 0.0021
 Mass of Cl- = # mol x relative atomic mass of Cl = 0.0021 x 35.5 = 0.0746g
 Mass of iron ions = mass of sample – mass of Cl- ions = 0.1348 – 0.0746
= 0.0602 g
 # mol iron = mass of iron / relative atomic mass iron = 0.0602 / 56 = 0.0011
 Ratio of mol of iron : chloride = 0.0011 : 0.0021  1: 2
 Therefore formula of compound = FeCl2
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Checkpoint B
1. A 2.00g sample of limestone was dissolved in dilute hydrochloric acid and
all the calcium present in the sample was converted to Ca2+(aq). Excess
ammonium oxalate solution, (NH4)2C2O4(aq), was added to the solution to
precipitate the calcium ions as calcium oxalate, CaC2O4(s). The precipitate was
filtered, dried and weighed to a constant mass of 2.43g. Determine the %
calcium in the limestone sample. Note: Ca2+(aq) + C2O42-(aq)  CaC2O4(s)
Ca = 40, N = 14, C = 12, H = 1, O = 16, Ag = 108, Cl = 35.5, Fe = 56
2.
Note the dry precipitate would have the formula CaC2O4
END OF GRAVIMETRIC ANALYSIS