4

13D Gravimetric Titrations Mass (weight) or gravimetric titrations differ from their volumetric counterparts in that the mass of titrant is measured rather than the volume. Therefore, in a mass titration, a balance and a weighable solution dispenser are substituted for a buret and its markings. 13D-1 Calculations Associated with Mass Titrations The most common way to express concentration for mass titrations is the weight concentration, cw, in weight molar concentration units, Mw, which is the number of moles of a reagent in one kilogram of solution or the number of millimoles in one gram of solution. Thus, aqueous 0.1 Mw NaCl contains 0.1 mol of the salt in 1 kg of solution or 0.1 mmol in 1 g of the solution. The weight molar concentration cw(A) of a solution of a solute A is computed using either one of two equations that are analogous to Equation 4-2:

where nA is the number of moles of species A and msoln is the mass of the solution.

13E Titration Curves in Titrimetric Methods

Titration Curves

Reagent volume vs. concentration-related variables

13E Types of Titration Curves

13E-2 Concentration Changes during Titrations

①

②

③

① ②

③

13F Precipitation Titrimetry

13F-1 Precipitation Titration Curves Involving Silver Ion

• Standard solution: AgNO3

• Analyte: halide ions

• Reaction product: solid silver halide

• Titration curve: pAg vs. VAg+

■ The Effect of Concentration on Titration Curves

■ The Effect of Reaction Completeness on Titration Curves

• Ksp 小 → 반응의 완결성 大 →당량점에서의 pAg 변화 大

• Ksp >10-10 → 종말점 검출이 어려움

13F-2 Titration Curves for Mixtures of Anions

서로 다른 용해도를 가지는 침전을 형성하는 혼합물의 적정

Ex) 0.1000M AgNO3 용액으로 혼합용액 (0.05 M I- + 0.0800 M Cl-, 50 mL) 적정 Ksp(AgI) = 8.3 ⅹ 10-17

Ksp(AgCl) = 1.82 ⅹ 10-10

→ AgI가 먼저 침전 됨

• Curve A: Cl-/I- mixture → 용해도 차이가 큼

→ 두 개의 당량점이 명확하게 구분됨

• Curve B: Cl-/Br- mixture → 용해도 차이가 작음

→ 두 개의 당량점이 명확하게 구분되지 않음

13F-3 Indicators for Argentometric Titration

•Three methods of end points detection in titration with silver nitrate

1) Chemicals: three chemical indicators (this section)

2) Potentiometry: measuring the potential between a Ag electrode and reference electrode (Section 21C)

3) Amperometry: measuring the current generated a pair of silver microelectrodes (Section 23B-4)

•chemical indicators에 의한 end point 검출: 색 변화 또는 혼탁물의 생성/소멸 관찰

■ Chromate Ion; The Mohr Method

Analyte: Cl-, Br-, CN-

Indicator: CrO42-

pH = 7 ~ 10

• Silver ion concentration at equivalence point

• Chromate ion concentration require to initiate formation of Ag2CrO4

• 붉은색 침전(Ag2CrO4)을 뚜렷하게 관찰하기 위해 과량의 Ag+와 CrO42- 용액을

첨가 → positive systematic error 발생 → error를 보정하기 위해 ① Cl-가 들어있지 않은 calcium carbonate suspension을 이용하여 blank titration을 하거나 ② 또는 일차표준등급인 NaCl을 이용하여 AgNO3를 표준화 함

■ Absorption Indicators: The Fajans Method

Fluorescein (HFl) H+ + Fluoresceinate (Fl-) (yellow green)

AgCl(s) + Ag+ + Fl- AgCl(s)AgFl (red)

Before the equivalence point : Cl- excess (particles are “-” charge) → Fi- are repelled → yellow green in solution

After the equivalence point : Ag+ excess (particles are “+” charge) → Fi- are adsorbed → red color of AgFl in the surface layer of solid

Primary adsorption layer

Cl-

AgCl Cl-

Cl-

Cl-

Cl-

Cl- Cl-

Cl-

Fl-

Fl-

Counter-ion layer

AgCl

Ag+

Ag+

Ag+

Ag+

Ag+ Ag+

Ag+ Ag+

Fl-

Fl-

Fl- Fl-

Fl-

v

■ Iron(III) Ion: The Volhard Method

Analyte: 1) direct determination of SCN-

2) indirect determination of halide ion

Indicator: Fe(III)

pH = Strong acidic condition (①Fe(III)의 hydrated oxide 침전 방지, ②carbonate, oxalate 및 arsenate의 방해작용 제거)

Direct determination of SCN- Indirect determination of halide ion

• Volhard법에서 지시약의 농도

• Indirect 법으로 halide ion 적정 시 주의할 점