Assay of Aromatic Spirit of Ammonia for Ammonium Carbonate (1)

Assay of Aromatic Spirit of Ammonia for Ammonium Carbonate

I.Principles Involved - Residual titration with blank determination - Type of reaction: Neutralization - Titration conditions: - Away from light - Cool temperature

- Two indicators used: - Phenolphthalein - endpt: colorless solution (8.0-10.0)

Uses of Aromatic Ammonia Spirit for Ammonium Carbonate - Respiratory stimulant - Used for treating syncope (fainting) - Has antacid and carminative properties - Liquefying expectorant - Also called “smelling salt”

Official Requirement Aromatic Ammonia Spirit is a hydroalcoholic solution that contains, in each 100 ml, not less than 1.7 g and not more than 2.1g total NH3, and Ammonium Carbonate corresponding to not less than 3.5 g and not more than 4.5 g (NH4)2CO3.

II. Reason for Important Steps 1.Transfer 10.0 ml to a flask about 300-ml capacity  Away from light - Aromatic ammonia spirit, USP is light sensitive

(USP 35 NF 30, 2012, p. 2188)

II. Reason for Important Steps 2. Add 30 ml of 0.5 N NaOH, and boil until the mixture, replacing the water lost by evaporation until the vapor no longer turn moistened red litmus paper blue. Addition of NaOH with boiling liberates NH3. - Detected by red litmus to blue (Svehla, 1996, p. 160) (NH4)2CO3 + 2NaOH → 2NH3↑ + H2O + Na2CO3

(Soine & Wilson, 1961, p. 311) (Svehla, 1996, p. 160)

II. Reason for Important Steps  Ammonia - removed from the solution - unstable in aqueous solution - Affects pH of solution

- Ammonium carbonate decomposes into ammonia and ammonium bicarbonate. (NH4)2CO3 → NH4HCO3 + NH3

NH3 + H2O

↔

NH4OH

II. Reason for Important Steps Water - added to compensate loss due to evaporation Boiling: - Eliminates CO2 -

Eliminates carbonate/bicarbonate buffer region

II. Reason for Important Steps 3. Cool, dilute with 100 mL of cold, carbon dioxide-free water. CO2 is soluble in water and produces a very weak acid known as carbonic acid, H2O + CO2 -→ H2CO3 H2CO3 + H20↔ H30+ + HCO3- (K1 = 5.0 X 10-7) HCO3- + H20 ↔ H30+ + C032- (K2 = 4.6 X 10-11) CO2 lowers the pH of the solution and interferes with the end point due to C032 -/ HCO3- buffer region. (Christian, 2004, pp. 279, 280)

II. Reason for Important Steps 4. Add about 6 drops of phenolphthalein TS. Add just enough 0.5 N sulfuric acid  VS to discharge the color of phenolphthalein Phenolphthalein (pH 8.0 -10.0) - used to detect the first endpoint. - used to get an approximation of where the second endpoint will occur. - is colorless beyond the first endpoint and does not interfere.

II. Reason for Important Steps Addition of H2SO4 enables complete neutralization of NaOH. Consequently, the phenolphthalein endpoint detects the transformation of carbonate to bicarbonate

2NaOH + H2SO4

→

Na2SO4 + H2O

NaOH + H2CO3 ↔ Na2CO3 2Na2CO3 + H2SO4 → Na2SO4 + 2NaHCO3 (Skoog, et.al., 2014, p. 383)

II. Reason for Important Steps 5. Add methyl orange and titrate with 0.5 N sulfuric acid VS Methyl orange (pH 3.2-4.4) yellow to faint pink - Used for bicarbonate to carbon dioxide - used as indicator for the second titration to visualize the neutralization of the NaHCO3, a weaker base produced in the neutralization of Na2CO3 which is outside the pH transition range of phenolphthalein.

2NaHCO3 + H2SO4 → Na2SO4 + 2CO2↑+ 2H2O (Knevel & Digangi 1977, p. 67) (Christian, 2004, p. 280)

II. Reason for Important Steps

Further titration with H2SO4 is performed to completely neutralize NaHCO3 which liberates carbon dioxide gas from the buffer system HCO3-/CO2, leaving only HCO3- in the solution. (Soine & Wilson, 1961, p. 207) (Christian, 2004, p. 280)

II. Reason for Important Steps 6. Perform a blank determination A blank determination is performed to account for the presence of sodium carbonate in the sodium hydroxide solution which might have interfered with the endpoint.

Sodium hydroxide reacts with CO2 forming Na2CO3 and adsorbed water Carbonate ion in the solution decreases the sharpness of endpoints.

III. Chemical Reactions 1. (NH4)2CO3+ 2NaOH → 2NH3↑+ Na2CO3 + 2H2O

2.2Na2CO3 + H2SO4 → 2NaHCO3 + Na2SO4 3. 2NaHCO3 + H2SO4 → Na2SO4 + 2H2O + 2CO2↑ 2Na2CO3 + 2H2SO4 → 2Na2SO4 + 2H2O + 2CO2↑ Factor: 2, 2 H+ ions were transferred in the overall equation

IV. Sample Computation Sample weight (g) = N T × VT ×    (/)   × 1000

= 0.5N × 30 mL × 96.09 (/) 2 × 1000 = 0.7207g Na2CO3

IV. Sample Computation Since Official Requirement: 3.5 23 = 0.7207 23 100 

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4.5 23 = 0.7207 23 100   = 16.02 – 20.59mL

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References:  Ammonia spirit, aromatic (Inhalation-systemic). (2004, July 31). Retrieved from www.drugs.com/mmx/ ammonia-spirit-aromatic.html. Christian, G. (2004. Analytical Chemistry (6th ed.). USA: John Wiley and Sons Inc., pp. 279-280 Harris, D.C. (2010). Quantitative Chamical Analysis (8th ed.). USA: W.H. F reeman and Company, p. 223. Knevel, A., & DiGangi, F. (1977). Jenkins’ Quantitative Pharmaceutical Chemistry (7th ed.). New York : McGraw-Hill, p. 67. Laitinen, H. , & Harris W . (1975). Chemical Analysis an Advanced Text and Reference (2nd ed.). New York : McGraw- Hill, p. 111. Mosby (2017). Mosby’s medical dictionary (10th ed.). Missouri: Elsevier, p.134 Skoog, D. A., West, D. M., Holler, J. F., Crouch, S. R.(2014). Fundamentals of Analytical Chemistry . USA : Brooks/Cole, Cengage Learning, p. 386, 383. Soine, T.O., & Wilson, C. O. (1961). Rogers’ Inorganic Pharmaceutical Chemistry (7th ed.). Philadelphia: Lea & Febiger, pp. 207, 311, 317, 501. Svehla, G. (1996). Vogel's Qualitative Inorganic Chemistry (7th edition). Essex, England: Longman Group Limited. p. 160, 212 United States Pharmacopeial Convention, Inc. (2012). United States Pharmacopeia 35th Revision and National Formulary 30th Edition. Maryland, USA: USPCI. p. 2188.