Pharmaceutical Analytical Chemistry 1. By Dr. Mohammed El-Sayed Abd El-Ghaffar Hammouda

Transcription

1 Pharmaceutical Analytical Chemistry 1 By Dr. Mohammed El-Sayed Abd El-Ghaffar Hammouda

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3 Some Applications of Neutralization Reactions General Methods 1. Direct Titration: For acids and bases having ionization constant more than Residual (Back) Titration: In general residual titrations are used for: Volatile substances which may be lost during titration. Compounds which are insoluble in water, where the rate of reaction is slow, such as: zinc oxide and acetylsalicylic acid. Substances which require heating with the standard reagent such as: HCHO. Some substances, their quantitative reaction proceed rapidly only in presence of excess reagent, such as lactic acid.

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5 3. Displacement Titration: For salts of very weak acids and very weak bases which can t be determined by direct titration but they can be titrated by displacement method in which the very weak acid or base is displaced from its salt by a strong acid or base. Example: sodium carbonate, sodium salicylate and sodium borate 4. Differentiating Titration (Double Indicator method): Two or more end points are produced, such as mixture of strong and weak acid and mixture of strong and weak base.

6 Some Applications in Acid-base Titrations Titration of mixture of HCl & acetic acid # NaOH. 1- At the equivalence point of HCl, with M.O There will be a solution of acetic acid and NaCl and so the equivalence point is acidic. (i.e M.O give HCl only because HCl suppress ionization of CH 3 COOH by common ion effect ) 2- After the equivalence point their will be CH 3 COOH/CH 3 COONa buffer which suppress the ph break for HCl (that occurs in case of titration of strong acid against strong base). Then ph.ph will determine both acetic acid and hydrochloric acid (Total mixture) since sodium acetate will make ph suitable for color change of ph.ph.

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8 Mixture of carbonate and bicarbonate: 1- Total Alkali By direct titration with HCl using M.O as indicator (Ep 1 ) Na 2 CO 3 + 2HCl = 2NaCl + H 2 O + CO 2 NaHCO 3 + HCl = NaCl + H 2 O + CO 2 2-Half carbonate By titration another sample with HCl using ph.ph as indicator (EP 2 ) To obtain total Carbonate = 2Ep 2 Bicarbonate = Ep 1 2Ep 2

9 Conclusion Representative Diagram Titration Curve Equations Titration of (CO3 --, HCO3 - ) HCl 1 Na 2 CO 3 + HCl NaCl + NaHCO 3...Half neutralization Sample (ph>10) Titrant ( ph 8.3 ) 2 NaHCO 3 + HCl NaCl + CO 2 + H 2 O...Complete neutralization Titrant ( ph 3.8 ). Na 2 CO 3 + 2HCl 2NaCl + CO 2 + H 2 O Using M.O. Using ph.ph. ph ph 7-- ph.ph. 7-- M.O. (2A) ml E.P (M.O.) mls added of standard HCl (A) ml E.P (ph.ph.) mls added of standard HCl CO3 -- sample 14 CO3 -- sample 14 (A) ml of standard HCl (A) ml of standard HCl HCO 3 - E.P. HCO 3 - ph range of ph.ph. (A) ml of standard HCl 7 7 E.P. CO 2 + H 2O ph range of M.O. ph 0 ph 0 E.P1. (M.O.) CO3 -- +HCO 3 - & E.P2. (ph.ph.) HCO3 - = EP1-2EP2 1 CO3-- 2

10 Determination of Boric acid Boric acid (H 3 BO 3 ) is a very weak monoprotic acid (ka 5.8x10-8 ), so it can t be titrated against a standard alkali. But if a sufficient amount of Glycerol, Mannitol or similar organic polyhydroxy compound is added to boric acid, it will be converted into a relatively stronger monoprotic acid that can be directly titrated against standard NaOH using ph.ph. indicator. (very weak acid) { ph near 7 } (relatively stronger acid) { ph 4.2 }

11 Then: [Glyceroboric complex] H + NaOH [Glyceroboric complex] Na + H 2 O Notes: 1) The reaction between glycerol and boric acid may be 2:1 (as shown in the previous equation) or 1:1 (as shown in the following equation):

12 2) M.O. indicator is unsuitable for the titration of glyceroboric acid against NaOH because the ph of glyceroboric acid is about 4.2 (i.e. the titration curve will start at ph 4.2 and go upward) so the vertical part of the titration curve will be above the ph-range of M.O. 3) If the titration is done directly on boric acid without adding glycerol, the end point will appear very rapidly due to the hydrolysis of the produced sodium metaborate leading to the formation of NaOH again giving pink color with ph.ph. H 3 BO 3 + NaOH NaBO 2 + 2H 2 O ph.ph. indicator. Sod. metaborate

13 Determination of Borax: Borax is hydrolyzed in water as follows: Na 2 B 4 O H 2 O 2 NaOH + 4 H 3 BO 3 1 st method Tit. 0.1 N HCl {E.P 1 } 2 nd step add glycerol Tit. 0.1 N NaOH {E.P 2 }

14 The liberated NaOH can be titrated against 0.1 N HCl using M.O. indicator without interference of boric acid as it is a very weak acid. E.P 1 2 NaOH (from borax) Then or borax can be again determined through its boric acid liberated by hydrolysis. That is achieved by adding glycerol to the above titrated solution (i.e. the solution obtained after E.P 1 ) and then titration against 0.1 N NaOH using ph.ph. indicator. E.P 2 4 H 3 BO 3 (from borax)

15 Determination of mixture of boric acid and borax: 1- The mixture is titrated with HCl using M.O as indicator borax (Ep1) 2- Glycerol is added and then titrated against NaOH using ph.ph as indicator boric acid originally present + boric acid from borax (Ep2) 3- Boric originally present = Ep2-2Ep1

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17 Determination of Sodium benzoate or Sodium Salicylate ( Biphasic Titration) The aqueous solution of sodium salicylate (water soluble as it is salt) is mixed with ether in a separating funnel, then the aqueous layer is titrated with standard HCl. When HCl is added to the aqueous layer, salicylic acid is liberated and is extracted into the ether layer (oil soluble) (require shaking). At the equivalence point only NaCl is present and the first drop of acid will produce color change of the indicator.

18 Salicylic acid is removed into the organic layer because it is a strong acid and give acid reaction with any usual indicator.

19 Residual (back)titrations 1- Volatile substances which may be lost during titration e.g. ammonia. 2- Compounds which are insoluble in water, where the rate of reaction is slow, such as: zinc oxide and acetylsalicylic acid. 3- Substances which require heating with the standard reagent such as: HCHO.

20 1. Determination of insoluble oxides or carbonate Example: ZnO Dissolve in a known excess of HCl with gentle heating, the excess acid is back titrated against standard base using M.O as indicator. NH 4 Cl is added to prevent precipitation of Zn(OH) 2 ZnO + 2HCl = ZnCl 2 + H 2 O

21 2. Determination of Formaldehyde: It Depends on oxidation of HCHO into HCOONa. By treating a sample with a measured excess of NaOH and neutralized H 2 O 2. The solution is heated and the excess base is titrated with standard H 2 SO 4 HCHO + H 2 O 2 + NaOH = HCOONa + 2H 2 O HCOOH

22 Modification of molecule producing titrable acid or base Many compounds are not titratable with acid or base, but by modification of the molecule, they are transformed into an acid or base which is titrable. In other cases, the compound may be too weak acid or base to be titrated, so they converted into more stronger acid or base.

23 1- Ketones and Aldehydes: Example: Benzaldehyde sample is treated with an excess of hydroxylamine hydrochloride C 6 H 5.CHO+H 2 N.OH.HCl C 6 H 5.CH=NOH+ H 2 O + HCl Benzaloxime The liberated HCl is titrated with standard NaOH using bromophenol blue as indicator (end point is green). The liberated HCl is titrated with standard NaOH. Both free HCl and combined HCl will be titrated, so a blank must be done in the same manner.

24 2. Determination of Alpha Amino Acids: RCH(NH 2 )COOH. These are carboxylic acids having an NH 2 group on a carbon atom adjacent to COOH group, so they usually exist as zwitterions and therefore they are less acidic than most carboxylic acids. These compounds can be titrated after elimination of the basic character of the amino group. So they react with HCHO to form Schiff s base, in which the NH 2 is no longer basic and COOH has its usual acidic character. RCH(NH 2 )COOH + HCHO RCH(N=CH 2 )COOH + H 2 O

25 3. Determination of ammonium salt (eg. NH 4 Cl) ممكن نلغيه There are two methods to determine ammonia in ammonium salts: Distillation Method. Formaldehyde Method (Formol Titration).

26 a. Ammonium salt + NaOH = sodium salt + H 2 O + NH 3 Ammonia is distilled and absorbed in a known volume of standard acid. The excess acid is determined against standard alkali. b. Ammonium salt+ excess NaOH= sodium salt+ H 2 O+ NH 3 After distillation of ammonia, the excess alkali is determined by titration with acid. (This method gives false results due to reaction of hot alkali with glass).

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28 b- Formaldehyde method (formol titration): NH 4 Cl+ HCHO = hexamethylene tetramine (hexamine) + HCl 6 HCHO + 4 NH 4 Cl = (CH 2 ) 6 N 4 + 6H 2 O+4 HCl The liberated HCl is titrated against standard alkali Note: Hexamine is very weak base and not affect indicator. Formaldehyde should be neutralized.

29 4- Determination of Nitrogen by Kjeldahl method: قرائة للشفوى It was developed by a Danish chemist named Johan Kjeldahl who first described it in This method is used for nitrogen analysis in a wide variety of materials as calculating the protein content in human and animal food, fertilizer, waste water and fossil fuels.

30 First Step: Digestion It Depends on oxidation of the organic materials by digestion with conc. H 2 SO 4 into CO 2 and H 2 O. The amino nitrogen is converted into ammonium bisulphate NH 4 HSO 4. K 2 SO 4 is added to raise the boiling point of the mixture during digestion thus speed the decomposition. A metal catalyst such as mercury is added to accelerate the digestion, but copper and selenium are more preferred for safety and environmental reasons

31 Second Step: Neutralization and Distillation of NH 3 After complete digestion of the sample, excess sodium hydroxide is added to the mixture to neutralize the remaining sulfuric acid and release the ammonia formed as the nitrogen containing molecule was oxidized. The ammonia is then distilled over into a measured excess of a standard acid.

32 Third Step: Titration As the ammonia dissolves in the acid trapping solution, it neutralizes some of the HCl it finds there. The excess acid can then be "back titrated with a standard NaOH solution. In this way the amount of ammonia distilled off from the digestive solution can be calculated, and hence the amount of nitrogen in the protein determined.

33 Digestion : N(in protein) + H 2 SO 4 (NH 4 )HSO 4 + CO 2 + H 2 O Neutralization & Distillation of NH 3 : (NH 4 )HSO 4 + 2NaOH NH 3 (g) +Na 2 SO 4 +2H 2 O Collection of NH 3 in excess HCl : NH 3 + HCl NH 4 Cl Titration of unreacted HCl with NaOH : HCl + NaOH NaCl+H 2 O

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35 Some Pharmaceutical Applications of neutralization Titration 1- Determination of Aspirin (Analgesic drug): (Back Titration) Principle: The method depends on boiling aspirin (acetylsalicylic acid) with a known excess of standard NaOH, where aspirin is hydrolysed into acetic acid and salicylic acid which react with sodium hydroxide giving sodium acetate and sodium salicylate. Then the remaining unreacted part of NaOH is back titrated against standard HCl using ph.ph. indicator. 1 ry standard NaOH + HCl NaCl + H 2 O 2 nd standard

36 2- Determination of Ibuprofen (non steroidal antiinflammatory drug): By acid-base titration (it is the official method for the analysis of Ibuprofen as mentioned in the British and European Pharmacopeias). Principle: Dissolve ibuprofen in methanol and then direct titration against NaOH using ph.ph as indicator.

37 Note: The aim is to determine the purity of ibuprofen in raw materials. If the ibuprofen content is in the range % by mass, the sample is shown to be sufficiently pure to be use in pharmaceutical preparations such as tablets and ampoules.

38 Non Aqueous Titrations Most of the titrations are performed in the aqueous media, means water is used as solvent. There may be difficulty if reactant is insoluble in water or reactant is reactive with water or the analyte (sample) is either too weak acid or too weak base. Those too weak acids or bases cannot be titrated in aqueous solution due to the amphoteric behavior of water (i.e water can react as an acid on titration with a base and act as a base on titration with acid). So water will compete with the sample if it is weak acid or weak base. The simple solution for this problem is to replace water as solvent with another non aqueous solvent, so this type of titration is named Non Aqueous Titrations.

39 Theory of Non Aqueous Titrations: The acidity of weak acids can be enhanced in presence of a basic solvent, this because basic solvent has a higher affinity to take up protons from the acid. So acetic acid behaves as a strong acid in ammonia solution (basic solvent). Also the basicity of weak bases can be enhanced in presence of acidic solvent. This is called the leveling effect of the solvent. By using this concept, the strength of weak acidic or weak basic drugs can be enhanced by dissolving in the appropriate solvent to enhance its strength and then can be titrated by acid-base titration (non aqueous titration). Note: Moisture should be avoided in non aqueous titration to increase the sharpness of the end point. Also temperature should be kept constant during titration due to higher coefficients of expansion of organic solvents.

40 Types of solvents in non aqueous titrations: 1- Protogenic solvents: These are acidic solvents and used to enhance the basicity of weak bases. Examples: glacial acetic acid. 2- Protophilic solvents: These are basic solvent and used to enhance the acidity of weak acids. Examples: pyridine, ethylenediamine and dimethylformamide (DMF). 3- Amphoteric solvents: These solvents behave as acid as well as base depending on the substance dissolved in it. They can accept or donate protons. Examples: water, alcohol. 4- Aprotic solvents: These solvents neither accept proton nor donate proton. They are used in dissolving the drugs especially those are insoluble in water. Examples: benzene, carbon tetrachloride.

41 Selection of solvent: The selection of solvent in non aqueous titration based on: 1- Solubility of drug: The weak acidic or basic drug should be soluble in the solvent which at the same time must be miscible with the titrant. 2- Nature of drug: The solvent is used according to the nature of drug, whether it is weak acid or weak base. 3- Unreactivity: The solvent should be unreacted with the drug.

42 Types of Non aqueous titrations: 1- Acidimetry: It involves the quantitative determination of weak bases by non aqueous titration. 2- Alkalimetry: It involves the quantitative determination of weak acids by non aqueous titration

43 Samples: Acidimetry Basic drugs such as: Alkalimetry Acidic drugs such as: Ephedrine, Adrenaline, Caffeine, Acyclovir. Nalidixic Flurouracil. acid, Solvent: Titrant: Protogenic solvents such as: glacial acetic acid Perchloric acid HClO 4 Protophilic solvents such as: DMF Sodium methoxide. Indicator: Crystal violet (0.5% in glacial acetic acid) Color change from violet to yellowish green. Thymol blue (0.5 % in methanol) pink to blue

44 1- Determination of Ephedrine: Principle: Dissolve ephedrine in glacial acetic acid, add few drops of crystal violet indicator and titrate with standard perchloric acid. Glacial acetic act as a base and so accept the proton from perchloric acid to form the onium ions which is the actual titrating species. The onium ions react with acetate ions liberated after protonation of ephedrine. The overall reaction: HClO 4 + CH 3 COOH CH 3 COOH ClO - 4 onium ion C 10 H 5 NO + CH 3 COOH C 10 H 15 N + HO + CH 3 COO - ephedrine protonated ephedrine CH 3 COOH 2+ + CH 3 COO - 2CH 3 COOH

45 2- Determination of Acyclovir: (Antiviral drug) Principle: Dissolve acyclovir in glacial acetic acid, and titrate against standard perchloric acid using crystal violet as indicator. 3- Determination of Nalidixic acid: Principle: Dissolve nalidixic acid in DMF and titrate against sodium methoxide using thymol blue as indicator.