College of Health Sciences Education
3rd Floor, DPT Building
Matina Campus, Davao City
Telefax: (082)
Phone No.: (082)300-5456/300-0647 Local 117
Cherry Mae H. Arangcon
Name: _____________________________
1-BS PHARMACY
Year and Course: ____________________
PH
CHEM 2
Subject title: ________________________
Name of Professor: ___________________
04-24-2021
Date Performed: ____________
04-26-2021
Date Submitted: ____________
9182
Code number: _______________
Score: ______________________
ACTIVITY 5
PRINCIPLE OF GRAVIMETRIC ANALYSIS
I.
INTRODUCTION
The chemical reactions in gravimetric analysis take place in accordance with the
established laws and theories of chemistry. Gravimetric analysis, the measurement of
the weight of a substance in a sample or calculation of the weight of a substance in a
sample from the weight of a chemically equivalent amount of some other substance.
In pharmaceutical quantitative analysis, the substance to be measured gravimetrically
is separated from other substances composing the sample by physical methods,
purified and weighed without chemical change. For example, the quantity of
amobarbital in amobarbital tablets is determined by extracting, purifying further by
complete removal of solvent and weighing the amobarbital in a sample of the
powdered tablets. When the other components of the sample are such that
separation by physical means of the substance being measured is not possible or
convenient, chemical reaction is employed to convert the substance to a chemically
equivalent amount of some other substance which can be separated, purified and
weighed.
II.
1.
2.
3.
III.
OBJECTIVES
Identify one example of drug products assayed by gravimetric analysis.
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Calculate problem solving involving gravimetric analysis.
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Understand the principle behind the gravimetric analysis.
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LEARNING ACTIVITY
a) For this activity, your task is to answer the questions given.
Drug Product Assayed by
Gravimetric Analysis
Assay of Sodium Chloride
Accurately weight two samples of 0.2 to 0.3g of NaCl.
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Phone No.: (082)300-5456/300-0647 Local 117
Completely dissolve the sample in 100 to 125ml of distilled water. Add about 1ml of nitric
acid slowly while stirring. Test the solution by touching a strip of blue litmus paper with the
moist stirring rod. If it is not acid in reaction, add sufficient acid to make it so. Measure out
5ml in excess of the amount of silver nitrate solution theoretically required to precipitate all
the chlorine as silver chloride. Add the silver nitrate solution in small successive portions,
stirring constantly with a glass rod. Cover the beaker with a watch glass and heat the mixture
to boiling point with constant stirring. Turn down the flame and digest the mixture, without
boiling, for 10 minutes. Then, add 1 to 2 drops of silver nitrate solution to the hot supernatant
liquid to test for complete precipitation. If a precipitate forms, add 5ml more of silver nitrate
solution, stir, allow to settle and test as before. Set the beakers aside, away from the direct
sunlight to allow the precipitate to settle.
During intervals of the procedure above, carefully clean and prepare 2 Gooch crucibles for
filtration. After the crucible have been prepared, heated to constant weight and weighed, fit
them into the suction flask. Decant most of the supernatant liquid into the Gooch crucible,
using gentle suction to hasten filtration, guiding the liquid into the crucible. Then, wash the
precipitate 3 times with 15ml portions of nitric acid.
When the filtrate is free with silver nitrate, wash the precipitate twice with 5ml portions of
distilled water to remove most of the nitric acid retained by the precipitate from the former
wash solution. Dry the crucible to constant weight at 110 to 120 degree Celsius in an electric
oven until successive weighing differ by no more than 0.0002. Allow the crucible to cool in a
desiccator before weighing. The weight of the crucible and precipitate minus the weight of
the crucible gives the weight of the silver chloride obtained from the sample.
QUESTIONS
1. Why should the solution be acidified with nitric acid?
The solution should acidified with nitric acid to prevent the precipitation of other
substances insoluble in water but soluble in nitric acid, such as carbonates, oxides, and
phosphates. The acid is also helps to coagulate any colloidal silver chloride.
2. Why add silver nitrate in excess in the precipitation of the chloride?
The silver nitrate is added in excess to drive the reaction to complete the common-ion
effect, ensuring complete precipitation.
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3rd Floor, DPT Building
Matina Campus, Davao City
Telefax: (082)
Phone No.: (082)300-5456/300-0647 Local 117
3. Why is the precipitate washed with water slightly acidified with nitric acid?
The reason for washing the precipitate with water slightly acidified with nitric acid is
for the precipitates makes it possible to remove harmful impurities or to extract valuable
components that are contained in the pore liquid. Furthermore, if you wash with nitric
acid, a high electrolyte concentration is maintained and the AgCl particles stay clotted
together. It is also to ensure that the coagulated or the larger pieces of the salt do not
break back up into the colloid to the very small particles.
4. Why is it best to wash the precipitate by decantation before it is transferred to the Gooch
crucible?
It is best to wash the precipitate by decantation before it is transferred to the Gooch
crucible to hasten filtration that helps in guiding the liquid into the crucible.
5. Calculation. A 0.3056g sample of a soluble chloride was analyzed gravimetrically for Cl, and
0.7265g of AgCl was obtained. Calculate the Cl content of the sample. (Kindly show your
solution.)
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3rd Floor, DPT Building
Matina Campus, Davao City
Telefax: (082)
Phone No.: (082)300-5456/300-0647 Local 117
6. Kindly illustrate the assay of sodium chloride employing gravimetric analysis. (Refer to the video
attached in LMS for your guide.)
College of Health Sciences Education
3rd Floor, DPT Building
Matina Campus, Davao City
Telefax: (082)
Phone No.: (082)300-5456/300-0647 Local 117
IV.
CONCLUSION
One example of drug products that are assayed by gravimetric analysis is the assay of
Sodium Chloride. In applying a solution of silver nitrate to an aqueous solution of chloride
ions, a precipitate of silver chloride was produced in this assay. The precipitate was gathered
and measured after proper filtration. In calculating problem solving involving gravimetric
analysis, one must always
remember is to calculate the gravimetric factor first before
calculating the percent content required. Furthermore, gravimetric analysis works on the basis
that the mass of an ion in a pure compound can be measured and then used to calculate the
mass percent of the same ion in a specified amount in an impure compound. The ion under
investigation has completely precipitated. The precipitate must be a completely pure
compound.