GRAVIMETRIC M METHODS OF A ANALYSIS
S.O. 3.1, 3.2, 3.3, 3.4, 3.5.
+. $plain the principles upon which gravimetric analyses are based +.- escribe the functions of various pieces of basic equipment used in gravimetric analysis +.+ /arry out e$periments based on gravimetric determinations +. 1erform calculations based on data d ata obtained from gravimetric analyses +.2 /ite e$amples based on the use of gravimetric analysis in quality control
GRAVIMETR GRAVIMETRIC IC ANALYSIS ANALYSIS
Gravimetr Gravimetric ic analysis analysis is a macroscopic macroscopic,, quantitati quantitative ve method method involving involving the accurate accurate weighing weighing of a compound. compound. This technique is used to determine the mass of an analyte or analyte or some compound chemically related to it and usually involves large samples. The compound or element that is weighed should be in as pure form as a s possible after some form of chemical treatment has been carried out. The compound or element is usually precipitated usually precipitated from solution, volatilized or or adsorbed onto onto a surface. Mass measurements are made with an analytical balance (± 0.00g!, an instrument that yields highly accurate and precise data. Traditional gravimetric analysis techniques are essentially manual in characteristic and labor intensive. "owever, recent technology now allows gravimetric analysis to be performed automatically. # typical e$perimental procedure to determine an un%nown concentration of an analyte in solution is as follows& quantitatively (and completely! precipitate the analyte from solution. 'deally, the precipitate is of such low solubility that losses from dissolution are negligible. #n additional factor is the common ion effect, this further reduces the solubility of the precipitate.) collect the precipitate by filtering and wash it to remove impurities dry the solid in an oven to remove solvent, allow to cool weigh the solid on an analytical balance The weighed form of the product should be of %nown composition. The product should be *pure* and easily filtered calculate the analyte concentration in the original solution based on the weight of the precipitate •
•
•
•
•
ADVANTAGES
OF GRAVIMETRIC ANALYSIS
•
#ccurate and precise when using modern analytical balances.
•
uring precipitation reactions, possible sources of errors are reliably chec%ed, since filtrates can be tested for completeness of precipitation. #lso, precipitates can be e$amined for the presence of impurities.
•
eterminations can be carried out with relatively ine$pensive e quipment.
•
1ossible to obtain repeatability of results.
•
t is an absolute method, i.e. it involves direct measurement without any form of calibration or standardi3ation being required and results are determined from e$perimental data. DISADVANTAGES OF GRAVIMETRIC ANALYSIS
•
Time consuming.
•
4aborious.
•
5elatively large quantity of sample is required.
• •
6ot suitable for samples that undergo thermal decomposition. The time consuming nature of gravimetry limits applications to a small number of determinations. USES OF GRAVIMETRIC ANALYSIS •
#nalysis of standards to be used for the testing and7or calibration of other instrumental techniques.
•
#nalyses requiring high accuracy.
•
8sed in quality control.
•
Measurement of the 9essential: elements in plant foods (phosphorus, for e$ample, is converted into the insoluble salt, magnesium ammonium phosphate!.
•
stimation of pollutants in the air, such as sulphur dio$ide (by conversion to insoluble barium sulphate!.
•
stimation of sulphur dio$ide (used to prevent microbial spoilage! in soft drin%s, such as orange ;uice.
•
stimation of chloride ions in water supplies (by conversion to insoluble silver chloride!.
•
etermining the moisture content of soils, cereals, grains etc
•
etermining the percentage 6a"/<+ in antacids
•
etermining the percentage sulphates7sulphites present in a sample
-
GRAVIMETRIC PRECIPITATION REACTIONS
Gravimetric analysis is suitable for precipitation reactions when the precipitate is& •
"ighly insoluble such that no appreciable loss occurs when filtered
•
5eadily separated from the mi$ture by filtration
•
#ble to be washed and impurities must be soluble
•
8nreactive with components of the atmosphere
•
To Obta! La"#$" Pa"t%&$'(
. -. +. . 2. >. ?. @.
8se elevated temperatures 8se dilute solutions =low addition of precipitating reagent with constant stirring p" control for precipitates whose solubility are p" dependant =low cooling igestion =tirring #dding an electrolyte to the solution E)AMPLES OF GRAVIMETRIC ANALYSIS REACTIONS
P"$%*tato! "$a%to!' A etermination of the concentration of sulphate ions by the precipitation of barim sulphate Vo&at&t+ A etermination of the percentage of water of crystalli3ation present in copper (! sulphate
#dsorptionA /alcium o$ide adsorbing water in the laboratory preparation of ammonia, silca gel or anhydrous calcium chloride in the laboratory preparation of carbon dio$ide and o$ygen.
EUIPMENT USED IN GRAVIMETRIC ANALYSIS 1. -%/!$" 0!!$& '%to! 0&a' 2. Bilter paper 3. Cacuum pump 4. F&t"ato! 0&a' '%to! 0&a' 5. /rucibleAsinteredAglass, silica . /rucible holder . "eat proof mat 6. -!'$! b"!$", o7$!, 0"!a%$ 8. esiccator 19. #nalytical balance
+
2
HOME :OR;
. efine the following terms& #nalyte, Matri$, #dsorption, #bsorption, igestion, 1recipitate, 5eprecipitation, Mother 4iquor, =upersaturated, "eating to constant mass, /oAprecipitation,
-. # certain barium halide e$ists as the hydrated salt DaE-.-"-<, where E is the halogen. The barium content of the salt can be determined by gravimetric methods. # sample of the halide (0.->20 g! was dissolved in water (-00 cm+! and e$cess sulfuric acid added. The mi$ture was then heated and held at boiling for 2 minutes. The precipitate (barium sulfate! was filtered off, washed and dried. Mass of precipitate obtained F 0.-2++ g. etermine the identity of E. +. hen a sample of impure potassium chloride (0.200g! was dissolved in water and treated with an e$cess of silver nitrate, 0.@0- g of silver chloride was precipitated. /alculate the percentage H/l in the original sample. . escribe the use of the following pieces of apparatus in gravimetric analysis& i. Duchner funnel(suction flas%! vii. /rucible holder ii. Bilter paper viii. "eat proof mat iii. Cacuum pump i$. Dunsen burner, oven, furnace iv. Biltration flas% (suction flas% $. esiccator v. /rucibleAsinteredAglass $i. #nalytical balance vi. /rucibleA silica
2. Gravimetric methods of analysis depend on the isolation of a product of a completed reaction and the careful measurement of its mass. Therefore, for a gravimetric method to be both accurate and precise, the precipitated form of the material isolated must meet certain criteria. a! $plain how each of the following characteristics of the precipitate meets the criteria for gravimetric analysis& i. =olubility ii. =i3e of crystals iii. 1urity of crystals iv. Molecular Mass v. /omposition of the precipitate b! # +00 mg solid sample containing chloride ions is dissolved in water. The chloride is then precipitated by addition of silver nitrate to the solution of the sample. The precipitate is then collected, washed, dried and found to have a mass of 200 mg. i. rite a balanced equation for the reaction between silver nitrate and the chlorideA containing sample. ii. /alculate the mass of chloride present in the sample. iii. /alculate the percentage chloride present in the sample. iv. =tate T< assumptions made in the calculation. c! =tate T< advantages and T< disadvantages of gravimetric analysis.
>
