gravimetric - copy

7/27/2019 Gravimetric - Copy

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Gravimetric assays-Gravimetric analyses is the procedure forisolation

& weighing of an element or compound in aspure

a form as possible(orpure derivative form).

-In case of derivative, equivalent Wt. of the

substance serves as the basis for calculating the

result.

-A definite portion is taken(weight or volume) from

bulk substance or sample being examined.


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-The element or compound is separated(pure or

purederivative) from the weighed or measured

portion.

-Separation techniques: Physical(frequently)Chemical

-The weight of the constituent element or compoundin the sample is calculated from the weight of the

product.


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The gravimetric assays may be grouped into

following categories:

1)Weighing the active ingredient after separation,

2)Precipitation & weighing of derivative of the

active ingredient.

3)Weighing of the residue after ignition of the

sample.


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1)Weighing the active ingredient after separation:

-Active principle separated, dried & weighed.

Estrone injection: Determination ofEstrone

C18H22O2

-An elaborate purifcation procedure is involved.

-The estrone is converted to water soluble derivative.

-Trimethylacethydrazide ammonium chloride is used

for derivatisation.(Girards reagent for carbonyl

Compd.).


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-The aqueous extract contains only ketonic material.

-The aqueous extract is then decomposed with acid

to regenerate estrone.

-Extracted into chloroform.

-The solvent is removed & residue weighed.


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Caffeine & sodium benzoate injection:

Determination ofCaffeine C8H10N4O2

-The caffeine is extracted into chloroform in

alkaline condition.

-The chloroform extract is evaporated to

dryness.

-The residue is weighed.


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2)Precipitation & weighing of derivative of the

active ingredient:

Thiamine hydrochloride: Determination of the

percentage purity of C12H17ON4 SCl,HCl

-Depends upon the precipitation ofinsoluble

thiamine silicotungstic acid.

-In the slightly acidified sample solution, a solution

of silicotungstic acid is added.


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-The precipitating reagent is a complex silicate with

some what variable degree of hydration.

SiO2,12WO3 ,xH2O.

-Thiamine silicotungstate is of constant composition.

2C12H17ON4 SCl,HCl + [SiO2 ,12WO3 ] +6H2O

(C12H17ON4 SCl,HCl),[SiO2 (OH) 2 ,12WO3 ]4H2O

674.6g C12H17ON4 SCl,HCl 3858g of residue.

0.1929g C12H17ON4 SCl,HCl 1g of residue.


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Anticoagulant citrate phosphate dextrose

solution:Determination of C6H12O6.H2O .

-The sample solution isboiled with Fehling

solution.

-Cu2O precipates out, washed, dried to constant Wt.

1.0 mg of cuprous oxide(Cu2O) 0.496mg

C6H12O6.H2O


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Wool alcohol: Determination of cholesterol

-3 hydroxyls(Cholesterol) form an insoluble

molecular addition complex with the steroidal

saponin, digitonin.

-The Complexes have very low solubilities.

-Are stable & strongly crystalline.


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C27 H46O + C56H92O29 C83H138O30Cholesterol Digitonin Complex

386.3g cholesterol 1616g of complex

0.239g cholesterol 1g of complex.


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-The weighted sample is melted on a water-bath,

mixed thoroughly & cooled.

-Weighed sample is dissolved in warm ethanol(90%),

filtered immediately through aNo.2 sintered glass

crucible.

-Residue on filter is washed with the warm

ethanol(90%).

-The filtrate is diluted quantitatively in a V.F.

-In to a pipetted amount , digitonin(0.5%) is added.


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-Forcomplete complexation, the mixture is warmed

to 60.

-Then the mixture is filtered through the driedNO.2

sintered glass crucible.

-Theppt. is washed with ethanol(90%), acetone &

carbon tetrachloride successively.

-Then after draining ,ppt. is dried to constant weight

at 105C.


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3)Weighing of the residue after ignition of the

sample.

Sodium aurothiomalate:Determination of the

percentage of Au.

-It is of undefined constitution,

-Contains variable proportion of moisture.


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-So that a portion of the substance is dried over

phosphorous pentoxide at maximum pressure of

5mm of mercury.

-Thepercentage of gold is calculated W.R.T. dried

substance.

-The organic matters are oxidised to volatile

products by sulphuric acid.

-Sulphuric acid is reduced to sulphur dioxide &hydrogen sulphide.

-Hydrogen sulphide reduces gold salts to metallic

old.


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-Metallic gold isprecipitated.

-Further treatment & digestion with nitric acid.

-The stage ensures complete decomposition of

organic matter&

oxidation of inorganic mattere.g. sulphur(from

theomalic acid).

-Finally the sample is filtered, heated & ignited to

the constant weight.


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Cognate determinations:

Sodium aurothiomalate injection,

Aurothioglucose.


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Zinc oxide & salicylic acid paste:Determination of

ZnO.

-Weighed amount of the paste is gradually heated in

a porcelain crucible.

-After thorough charring , the substance is heated to

constant Wt. removing all carbonaceous materials.

-The Wt. of the residue represent the quantity of

ZnO the weigh of the paste taken for the assay.


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Cognate determination:

Silica in colloidal silicon dioxide,

Aluminum oxide in Aluminum monostearate etc.


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Alfonso R. Gennaro: Remington: the Science and

Practice of Pharmacy,

USP,

Beckett/Stenlake

Practical pharmaceutical chemistry

gravimetric - copy

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