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Back Titration In this technique. An accurately known amount of a reagent is added to analyte in such a way that some excess of the added reagent is left. This excess is then titrated to determine its amount and thus: mmol reagent reacted = mmol reagent taken - mmol reagent titrated mmol reagent reacted = mmol reagent taken - mmol reagent titrated Mmol NaOH reacted= 2 Mmol Aspirin Mmol NaOH titrated= mmol HCl Mmol NaOH reacted= 2 Mmol Aspirin Mmol NaOH titrated= mmol HCl 2 mmol Aspirin= Mmol NaOH taken - mmol HCl Mmol aspirin = ½ ( 0.5 x 30) – ( 0.5 x 20) 2 mmol Aspirin= Mmol NaOH taken - mmol HCl Mmol aspirin = ½ ( 0.5 x 30) – ( 0.5 x 20) Therefore, (the no. of mmol reagent reacted can be related to the no. of mmol of analyte from stoichiometry of the reaction between the two substance) can be determined since we know mmol reagent added and mmol reagent titrated.
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Lab 2
Assay of Aspirin using Back Titration
Aspirin is a weak acid that also undergoes slow hydrolysis; i.e., each aspirin molecule reacts with two hydroxide ions. To overcome this problem, a known excess amount of base is added to the sample solution and an HCl titration is carried out to determine the amount of unreacted base. This is subtracted from the initial amount of base to find the amount of base that actually reacted with the aspirin and hence the quantity of aspirin in the analyte.
Why do we use back titration ?
Back TitrationIn this technique. An accurately known amount of a reagent is added to analyte in such a way that some excess of the added reagent is left. This excess is then titrated to determine its amount and thus:
mmol reagent reacted = mmol reagent taken - mmol reagent titrated
Mmol NaOH reacted= 2 Mmol AspirinMmol NaOH titrated= mmol HCl
2 mmol Aspirin= Mmol NaOH taken - mmol HClMmol aspirin = ½ ( 0.5 x 30) – ( 0.5 x 20)
Therefore, (the no. of mmol reagent reacted can be related to the no. of mmol of analyte from stoichiometry of the reaction between the two substance) can be determined since we know mmol reagent added and mmol reagent titrated.
Calculation of chemical factor and concentration of aspirin
1 mole of aspirin = 2 moles of NaOH180 gm of aspirin = 2 Equivalent no. of NaOH180 gm/2 of aspirin = 1000 ml of 1N NaOH90/2 gm of aspirin = 1000 ml of N/2 NaOH0.045 gm of aspirin = 1ml of N/2 NaOH
0.045 gm of aspirin = 1ml of N/2 NaOH standard solution
CH3COOC6H4COOH + 2NaOH ---> CH3COO.Na + HOC6H4COO.Na + H2O
NaOH + HCl ----> NaCl + H2O
excess1
2
Heat?
Conc. of aspirin = ( ml of NaOH in excess x f" - ml of HCL titrant consumed x f") X F X f
1 -Take 10 tablets of aspirin 300mg and weight it using sensitive balance.
2 -Grind the tablets using mortar and pestle .
3- accurate weight of powder equivalent to 0.5gm was taken and 30 ml 0.5 M NaOH is added to it.
10 tablets x 300mg = 3000 mg =3 gm (3.6 g) weigh you get it from the balance
I want to take weight equivalent to (0. 5g) (??? gm)
???= this is the weight which must be taken from the grind tablets it is equivalents to 0.5gm aspirin.
Preparation of aspirin tablet solution
Indicator Indicators can be used to tell you when you have reached the Equivalence Point in a Titration.However, different indicators must be used for different types of titrations. pKa (indicator) = pH at Equivalent Point of Titration
To choose the best indicator for a particular titration ,
Strong Acid-Strong Base Titration Curve:
The best indicators are the ones which have the pH at EP
within their Transition Range.
So the best indicators for the SA-SB Titration would be:
*Bromthymol Blue (Range 6.0 – 7.6)
*Phenol Red (Range 6.6 – 8.2)
*Neutral Red (Range 6.8 – 8.0)
as these all have pH =7 within their transition ranges
• The substance used in the pharmaceutical field should be almost pure so that they can be safely used. It is rather difficult to obtain an almost pure substance
• We find substances and chemicals with varying degree of purity. Purity of a substances depend upon several factors such as their methods of manufacture and ways of crystallization of purification processes.
• In pharmaceutical field one deals with large number of drugs, chemicals and other substances which are used in formulations.
• All such materials need to be pure. However, it is almost impossible to get an absolutely pure material, as impurities, may be in traces, do get incorporated in materials either during manufacture. Purification or in storage.
Purity of a substance
• Since it is not possible to avoid impurities, it is necessary to have substances which can be considered as of reasonably pure quality.
Permissible impurities in pharmaceutical substances
• The pharmacopoeial committee takes some points into consideration with regards to the problem of impurities in substances
Pharmacopoeia:1 -British Pharmcopoeia
2 -Unitated state Pharmcopoeia 3 -European Pharmcopoeia
Test for purity• Pharmacopoeias of various countries prescribe tests for
purity for substances which are to used for medical purpose. • The so called ( test for purity) are as a mater of fact tests for
detecting impurities in the substances.• Pharmacopoeias fix limits of tolerance for these impurities.
The governing factor for these is to determine how much impurity is likely to be harmful or bring about technical and another difficulties when the substance is used.
• Limit tests are quantitative or semi quantitative tests designed to identify and control small quantities of impurity which are likely to be present in the substance.
Limit test
• Limit tests must give some form of selective reaction with the trace impurity.
Limit test
• The sensitivity of some tests and the amount tolerated is such that a negative result (indicating the presence of only small permissible quantity of impurities) can be used as control.
• In many cases, a standard prepared from a prescribed amount of the impurity which is being tested, is compared with the sample under examination and the control is thus exercised.
• When comparing turbidity it should be done against black background.
• When comparing colored it should be done against white background.
Nessler cylindersNessler cylinders (also named Color comparison cylinders, Color comparing cylinders) are laboratory tube with a fixed volume , made of glass with optically plane bottom. On the walls there are marks of the nominal stroke volume (usually 100 ml) and possibly one half-way mark (then usually 50 ml)
To minimize differences in the subjective impression of the color of the solution of the substance to be analyzed, cylinders of a series should have the same characteristics height, diameter and thickness of glass.
Basically there are three types of used cylinders:high with a capacity of 50 ml (about 300 mm in height, approximately 21 mm in diameter)low with a capacity of 50 ml (about 175 mm in height, approximately 25 mm in diameter)high with a capacity of 100 ml (about 375 mm in height, approximately 24 mm in diameter)low capacity of 100 ml (about 210 mm in height, approximately 34 mm in diameter)These parameters (except volume), however, can vary significantly between two cylinders.
