•Nonaqueous titration •Nonaqueous titration

• Nonaqueous titration is the titration of substances dissolved in nonaqueous solvents.

• It is the most common titrimetric procedure used in

pharmacopoeial assays and serves a double purpose:

• 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.

• Nonaqueous titration is the titration of substances dissolved in nonaqueous solvents.

• It is the most common titrimetric procedure used in

pharmacopoeial assays and serves a double purpose:

• 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.

Theory 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+

• 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+

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.

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

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:

HB ⇌ H+ + B-

acid proton base

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.

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.

Nonaqueous solvents used 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.

• 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.

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.

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

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.

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

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)

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.

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.

HClO4 + CH3COOH ⇌ CH3COOH2+ + ClO4

- C5H5N + CH3COOH ⇌ C5H5NH+ + CH3COO-

CH3COOH2+ + CH3COO- ⇌ 2CH3COOH

Adding HClO4 + C5H5N ⇌ C5H5NH+ + ClO4-

Visual indicators Visual indicators

• The following indicators are in common use:

• The following indicators are in common use:

Potentiometric titration 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 electrical apparatus 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.

• 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 electrical apparatus 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.

محاسبات کمی و مفاهیم ریاضی تعادالت در تیتراسیبون های غیر مایی

محاسبات کمی و مفاهیم ریاضی تعادالت در تیتراسیبون های غیر مایی

: شدن( خنثی های واکنش شدن کامل الفضعیف محلول Bباز در و شده اسیدی آبی محلول در را

: کنیم مقایسه شده اسیدی فورمیک اسیدB + H3O+BH ↔ +H + 2O

K= [BH+H [[B[/[3O+K = [bK/w

B + HCOOH2

+BH ↔ +HCOOH + K = [BH+HCOOH[[B[/[2

+’K = [b’K/s

چیست؟ Ks’و Kb’تعریف است؟ چگونه شدن خنثی واکنش ضعیف اسید یک برای

: شدن( خنثی های واکنش شدن کامل الفضعیف محلول Bباز در و شده اسیدی آبی محلول در را

: کنیم مقایسه شده اسیدی فورمیک اسیدB + H3O+ ↔ BH+ + H2O

K= [BH+[/[B[[ H3O+[ = Kb/Kw

B + HCOOH2

+ ↔ BH+ + HCOOHK = [BH+[/[B[[HCOOH2

+[ = K’b/K’s

چیست؟ K’sو K’bتعریف است؟ چگونه شدن خنثی واکنش ضعیف اسید یک برای

حل ب( ماده رفتار بر حاللها بازی یا اسیدی قدرت تاثیرشده:

اسید،* فرمیک جمله از آمفوتر، حاللهای از تعدادیتوجهی قابل نحو به اسید، سولفوریک و اسید استیکینا و پذیرنده، پروتون تا هستند بهتری دهنده پروتون . شوند می یندی ده اسیدی حاللهای عنوان به براین

ترکیبی حالل، با واکنش به آن تمایل افزایش واسطه بهباز بک اسید استیک در محسوسی نحو یه آنیلین چون

. تراست قویC6H5NH2 + HOAc ↔ C6H5 NH3

+ + OAc-

K’b ازKb. است بزرگترC6H5NH2 + H2O ↔ C6H5 NH3

+ + OH-

به * شدیدی تمایل مابع آمونیاک و آمین دی اتیلن مانند حاللهاییبندی رده بازی حاللهای عنوان به بنابراین و دارند پروتون جذب . شده حل ماده یک اسیدی خواص ها، محیط این در شوند می

. شود می تقویتاز* های نمونه واتانول، متانول مانند آلیفاتیک، الکلهای و آب

. هستند خنثی آمفوتر حاللهای

حل ج( مواد رفتار بر الکتریک دی ثابت شده: تاثیربار با ذرات جداسازی برای آن ظرفیت کننده تعین حالل یک الکتریک دی ثابت

. با یون یک جداسازی آب مانند باال الکتریک دی ثابت با حالل یک در است مخالف. است کار حداقل به نیاز منفی بار با یون یک از مثبت بار

HA + SH ↔ SH2+ + A-

OrB + SH ↔ BH+ + S-

حدود آب در استیک اسید تفکیک اتانول 5-10ثابت در حالیکه در 10-10استاست.

باز یا اسید یک قدرت نباشد، بارها شدن جدا شامل تفکیک واکنش که هنگامی: گیرد نمی قرار الکتریک دی ثابت تاثیر تحت چندان

BH+ + SH ↔ SH2+ + B

A- + SH ↔ HA + S-

:انتخاب حاللهای آمفوتر برای تبتراسیون های خنثی شدن:انتخاب حاللهای آمفوتر برای تبتراسیون های خنثی شدن

خود پروتون کافتی آن، کوچک باشد. ثابت•خواص آن به عنوان یک دهنده یا پذیرنده پروتون: •

در تیتراسیون یک باز ضعیف، حالل با تمایل به دادن پروتون و در تیتراسیون یک اسید ضعیف یک

حالل با خاصیت پذیرنده پروتون مناسبی هستند.ثابت دی الکتریک آن هر چه بزرگتر باشد بهتر •

است.

خود پروتون کافتی آن، کوچک باشد. ثابت•خواص آن به عنوان یک دهنده یا پذیرنده پروتون: •

در تیتراسیون یک باز ضعیف، حالل با تمایل به دادن پروتون و در تیتراسیون یک اسید ضعیف یک

حالل با خاصیت پذیرنده پروتون مناسبی هستند.ثابت دی الکتریک آن هر چه بزرگتر باشد بهتر •

است.

NH4درصد مثال:در + ارزی هم نقطه در را نکرده واکنش

NH4تیتراسیون + ،0.2F ) الف و NaOH ،0.2Fبا آبی محلول در

با( .C2H5ONa 0.2Fب کنید محاسبه آب بی اتانول دربا برایر ترتیب به اتانول و آب برای کافتی پروتون خود های ثابت

1x1o-14 8وx10-20 برای اسیدی تفکیک ثابت NH4وآب + در

با یرابر با x 10-10 6تقریبا برابر اتانل در .است 1x10-10و

بعد جلسه تا