CHAPTER 26: GRAVIMETRIC

ANALYSIS, PRECIPITATION TITRATIONS, AND

COMBUSTION

Harris: Quantitative Chemical Analysis, Eight Edition

THE GEOLOGIC TIME SCALE AND GRAVIMETRIC ANALYSIS

In the 1800s, geologists understood that new layers (strata) of rock are deposited on top of older layers.

To measure Pb, he fused (Section 28-2) each mineral in borax, dissolved the fused mass in acid, and quantitatively precipitated milligram quantities of PbSO4.

Geologic ages deduced by Holmes in 1911

Geologic period Pb/U(g/g) Millions of years Today’s accepted value

Carboniferous 0.041 340 362-330Devonian 0.045 370 380-362Silurian 0.053 430 443-418Precambrian 0.125-0.20 1 025-1 640 900-2 500

Ag+ + Cl- AgCl(s) (27-1)

26-1 Examples of Gravimetric Analysis

A few common organic precipitants (agents that cause precipitation) are listed in Tabled 27-2.

• 침전형성 과정의 mechanism : 불확실• 침전의 particle size : 여러 가지 실험조건에 따라 영향 .                1) 온도 2) 침전의 solubility                3) reactants concentrations     4) 반응물이 섞일 때의 속도 ⇒ 위 변수의 영향 → 정성적 설명 可 → relative supersaturation (RS) 과 입자 크기가 관계

Q : the concentration of the solute at any instantS : the equilibrium solubility

• 침전의 크기는 average relative supersaturation와 반비례 관계 (Q – S)/S 大 → colloidal solid 생성 ( 침전크기 小 )               小 → crystalline solid 생성 (침전크기 大 )

  • 침전반응은 느림 → 침전제 첨가 시 → 순간적으로 supersaturate ( Q > S )                이 경우 불안정 하므로 → precipitate 생성

■ Mechanism of Precipitate Formation

• 입자 크기에 미치는 RS의 효과 → precipitation mechanism 설명 • 침전은 ① nucleation   particle growth② 두 과정에 의해 형성 • freshly formed precipitate의 입자크기는 ① , ②중 어느 쪽이 우세한 가에 의존

• nucleation 시 ⇒ 최소수의 이온 , 원자 또는 분자가 모여 안정한 고체형성 (nuclei) ⇒ 먼지 입자와 같은 suspended solid의 표면에서도 nuclei가 생김 ⇒ further precipitate → 다른 핵의 발생에 의하거나 nuclei에 다른 고체가 deposition 되어 생성

• nucleation 우세 : 입자의 크기가 작은 , 많은 수의 침전 보임 particle growth 우세 : 입자 크기 大 , 입자 수 小

• nucleation의 속도 : RS에 대해 지수함수적으로 증가 particle growth의 속도 : RS에 직선적으로 증가 at high RS → nucleation rate particle growth ≫ → 많은 수의 작은 입자 생성 at low RS → nucleation rate particle growth ≪ → crystalline suspension 생성

■ Experimental Control of Particle Size • minimize supersaturation ((Q-S)/S : 小 → 입자 크기 大 ) 조건 1) elevated temperature ( to increase S)   2) dilute solution (to minimize Q)   3) slow addition of ppt. agent with good stirring (to lower the Q)    • environment의 acidity에 S 의존하는 침전의 경우 → pH 조절 → 침전 생성 동안 S 를 大 → size 大 ex) CaC2O4 acidic environment → 침전 덩어리 생성 여기에 NH4OH 加 → 침전 종결 • S is very small (Q S)≫ → colloidal suspensions 생김 ex) Fe(OH)3 , Al(OH)3 , Cr(OH)3,  most heavy metal의 sulfides → colloid.

Homogeneous Precipitation

In homogeneous precipitation, the precipitant is generated slowly by a chemical reaction (Table 27-3).

3HCO2- + Fe3+ Fe(HCO2)3 · nH2O(s)↓ (27-4)

Fe(III) formate

Precipitation in the Presence of Electrolyte

To understand why, we must discuss how tiny colloidal crystallites coagulate (come together) into larger crystals.

Demonstration 26-1 Colloids and Dialysis

Colloids are particles with diameters of ~1-500 nm.

You can demonstrate the size of colloidal particles with a dialysis experiment in which two solutions are separated by a semipermeable membrane that has pores with diameters of 1-5 nm.3

(Collecting biological samples by microdialysis was discussed at the opening of Chapter 25.)

Ca2+ + 2RH CaR2(s)↓ + 2H+

Analyte N-p-Chlorophenyl- Precipitate cinnamohydroxamic acid

Mn2+ + 6CN- Mn(CN)64-

Impurity Masking agent Stays in solution

Postprecipitation: Impurities might collect on the product while it is standing in the mother liquor. (it usually involves a supersaturated impurity that does not readily crystallize.)

Th breaking up of the product is called peptization, results in loss of product through the filter.

-Some impurities can be treated with a masking agent to prevent them from reacting with the precipitant.

Product Composition

A hygroscopic substance is one that picks up water from the air and is therefore difficult to weigh accurately.

Ignition (strong heating) is used to change the chemical form of some precipitates.

In thermogravimetric analysis, a substance is heated, and its mass is measured as a function of temperature.

26-3 Examples of Gravimetric Calculations

Grams of Mg in analyte

Grams of Mg2P2O7 formed

26-4 Combustion Analysis

A historically important form of gravimetric analysis was combustion analysis, used to determine the carbon and hydrogen content of organic compounds burned in excess O2 (Figure 27-4).

Gravimetric Combustion Analysis

Combustion Analysis Today9

C, H, N, S CO2(g) + H2O(g) + N2(g) + SO2(g) + SO3(g) 95% SO2

1050oC/O2

Cu + SO3 SO2 + CuO(s)

Cu + 1/2O2 CuO(s)

850oC

850oC

A key to elemental analysis is dynamic flash combustion, which creates a short burst of gaseous products, instead of slowly bleeding products out over several minutes.

The sample is thermally decomposed (a process called pyrolysis) in the absence of added O2.

26-5 Precipitation Titration Curves

26-6 Titration of a Mixture

26-7 Calculating Titration Curves with a Spreadsheet

26-8 End-Point Detection