Nonaqueous titration

2. Nonaqueous titration

3. Nonaqueous titration is the titration of substances dissolved in nonaqueoussolvents. • It is the most common titrimetric procedure used in pharmacopoeialassays and serves a doublepurpose: • it is suitable for the titration of very weak acids and very weak bases, and it provides a solvent in which organic compounds are soluble.

4. Theory • The theory is that water behaves as both a weak acid and a weak base; thus, in an aqueous environment, it can compete effectively with very weak acids and bases with regard to proton donation and acceptance, as shown below: • H2O + H+ ⇌ H3O+ • Competes with RNH2 + H+ ⇌ RNH3+ • or • H2O + B ⇌ OH- + BH+ • Competes with ROH + B ⇌ RO- + BH+

5. The effect of this is that the inflection in the titration curves for very weak acids and very weak bases is small, because they approach the pH limits in water of 14 or 0 respectively , thus making endpoint detection relatively more difficult.

8. A general rule is that bases with pKa < 7 or acids with pKa > 7 cannot be determined accurately in aqueous solution.

9. Substances which are either too weakly basic or too weakly acidic to give sharp endpoints in aqueous solution can often be titrated in nonaqueous solvents. The reactions which occur during many nonaqueous titrations can be explained by means of the concepts of the Brønsted-Lowry theory. According to this theory an acid is a proton donor, i.e. a substance which tends to dissociate to yield a proton, and a base is proton acceptor, i.e. a substance which tends to combine with a proton. When an acid HB dissociates it yields a proton together with the conjugate base B of the acid:

10. Alternatively, the base B will combine with a proton to yield the conjugate acid HB of the base B, for every base has its conjugate acid and, every acid has its conjugate base.

11. It follows from these definitions that an acid may be either: * an electrically neutral molecule, e.g. HCl, or * a positively charged cation, e.g. C6H5NH3+, or * a negatively charged anion, e.g. HSO4-. A base may be either: * an electricially neutral molecule, e.g. C6H5NH2, or * an anion, e.g. Cl-. Substances which are potentially acidic can function as acids only in the presence of a base to which they can donate a proton. Conversely basic properties do not become apparent unless an acid also is present.

12. Nonaqueous solvents used • Aprotic solvents are neutral, chemically inert substances such as benzene and chloroform. • They have a low dielectric constant, do not react with either acids or bases and therefore do not favor ionization.

13. The fact that picric acid gives a colorless solution in benezene which becomes yellow on adding aniline shows that picric acid is not dissociated in benzene solution and also that in the presence of the base aniline it functions as an acid, the development of yellow color being due to formation of the picrate ion.

14. Protophilic solvents are basic in character and react with acids to form solvated protons. • HB + Sol. ⇌ Sol.H+ + B- • Acid + Basic solvent ⇌ Solvated proton + Conjugate base of acid

15. weakly basic solvent has less tendency than a strongly basic one to accept a proton. Similarly a weak acid has less tendency to donate protons than a strong acid. As a result a strong acid such as perchloric acid exhibits more strongly acidic properties than a weak acid such as acetic acid when dissolved in a weakly basic solvent. On the other hand, all acids tend to become indistinguishable in strength when dissolved in strongly basic solvents owing to the greater affinity of strong bases for protons. This is called the leveling effect. Strong bases are leveling solvents for acids, weak bases are differentiating solvents for acids.

16. Protogenic solvents are acidic substances, e.g. sulfuric acid. They exert a leveling effect on bases.

17. Amphiprotic solvents have both protophilic and protogenic properties. • Examples are water, acetic acid and the alcohols. They are dissociated to a slight extent. The dissociation of acetic acid, which is frequently used as a solvent for titration of basic substances, is shown in the equation below: • CH3COOH ⇌ H+ + CH3COO- • Here the acetic acid is functioning as an acid. If a very strong acid such as perchloric acid is dissolved in acetic acid, the latter can function as a base and combine with protons donated by the perchloric acid to form protonated acetic acid, an onium ion: • HClO4 ⇌ H+ + ClO4- • CH3COOH + H+ ⇌ CH3COOH2+ (onium ion)

18. Since the CH3COOH2+ ion readily donates its proton to a base, a solution of perchloric acid in glacial acetic acid functions as a strongly acidic solution.

19. When a weak base, such as pyridine, is dissolved in acetic acid, the acetic acid exerts its levelling effect and enhances the basic properties of the pyridine. It is possible, therefore, to titrate a solution of a weak base in acetic acid with perchloric acid in acetic acid, and obtain a sharp endpoint when attempts to carry out the titration in aqueous solution are unsuccessful.

20. HClO4 + CH3COOH ⇌ CH3COOH2+ + ClO4- • C5H5N + CH3COOH ⇌ C5H5NH+ + CH3COO- • CH3COOH2+ + CH3COO- ⇌ 2CH3COOH • Adding HClO4 + C5H5N ⇌ C5H5NH+ + ClO4-

21. Visual indicators The following indicators are in common use:

22. Potentiometric titration • The end point of most titrations is detected by the use of visual indicator but the method can be inaccurate in very dilute or colored solutions. However under the same conditions, a potentiometric method for the detection of the equivalence point can yield accurate results without difficulty. The electricalapparatus required consists of a potentiometer or pH meter with a suitable indicator and reference electrode. The other apparatus consists of a burette, beaker and stirrer.

23. محاسبات کمی و مفاهیم ریاضی تعادلات در تیتراسیبون های غیر مایی الف) کامل شدن واکنش های خنثی شدن: باز ضعیف B را در محلول آبی اسیدی شده و در محلول اسید فورمیک اسیدی شده مقایسه کنیم: B + H3O+ ↔ BH+ + H2O K= [BH+]/[B][ H3O+] = Kb/Kw B + HCOOH2+ ↔ BH+ + HCOOH K = [BH+]/[B][HCOOH2+] = K’b/K’s تعریف K’b و K’s چیست؟ برای یک اسید ضعیف واکنش خنثی شدن چگونه است؟

24. ب)تاثیر قدرت اسیدی یا بازی حلالها بر رفتار ماده حل شده: *تعدادی از حلالهای آمفوتر، از جمله فرمیک اسید، استیک اسید و سولفوریک اسید، به نحو قابل توجهی پروتون دهنده بهتری هستند تا پروتون پذیرنده، و ینا براین به عنوان حلالهای اسیدی ده یندی می شوند. به واسطه افزایش تمایل آن به واکنش با حلال، ترکیبی چون آنیلین یه نحو محسوسی در استیک اسید بک باز قوی تراست. C6H5NH2 + HOAc ↔ C6H5 NH3+ + OAc- K’b از Kb بزرگتر است. C6H5NH2 + H2O ↔ C6H5 NH3+ + OH-

25. *حلالهایی مانند اتیلن دی آمین و آمونیاک مابع تمایل شدیدی به جذب پروتون دارند و بنابراین به عنوان حلالهای بازی رده بندی می شوند. در این محیط ها، خواص اسیدی یک ماده حل شده تقویت می شود. *آب و الکلهای آلیفاتیک، مانند متانول واتانول، نمونه های از حلالهای آمفوتر خنثی هستند.

26. ج) تاثیر ثابت دی الکتریک بر رفتار مواد حلشده: ثابت دی الکتریک یک حلال تعین کننده ظرفیت آن برای جداسازی ذرات با بار مخالف است. در یک حلال با ثابت دی الکتریک بالا مانند آب جداسازی یک یون با بار مثبت از یک یون با بار منفی نیاز به حداقل کار است. HA + SH ↔ SH2+ + A- Or B + SH ↔ BH+ + S- ثابت تفکیک اسید استیک در آب حدود 10-5 است در حالیکه در اتانول 10-10 است. هنگامی که واکنش تفکیک شامل جدا شدن بارها نباشد، قدرت یک اسید یا باز چندان تحت تاثیر ثابت دی الکتریک قرار نمی گیرد: BH+ + SH ↔ SH2+ + B A- + SH ↔ HA + S-

27. انتخاب حلالهای آمفوتر برای تبتراسیون های خنثی شدن: • ثابتخود پروتون کافتی آن، کوچک باشد. • خواص آن به عنوان یک دهنده یا پذیرنده پروتون: در تیتراسیون یک باز ضعیف، حلال با تمایل به دادن پروتون و در تیتراسیون یک اسید ضعیف یک حلال با خاصیت پذیرنده پروتون مناسبی هستند. • ثابت دی الکتریک آن هر چه بزرگتر باشد بهتر است.

28. مثال:درصد NH4+ واکنش نکرده را در نقطه هم ارزی در تیتراسیون NH4+ ،0.2F با الف) NaOH ،0.2F در محلول آبی و ب) با C2H5ONa 0.2F در اتانول بی آب محاسبه کنید. ثابت های خود پروتون کافتی برای آب و اتانول به ترتیب برایر با 1x1o-14 و 8x10-20 و ثابت تفکیک اسیدی برای NH4+ در آب تقریبا یرابر با 6 x 10-10 و در اتانل برابر با 1x10-10 است.

29. تا جلسه بعد