Harris: Quantitative Chemical Analysis, Eight Edition
CHAPTER 26:
GRAVIMETRIC
ANALYSIS,
PRECIPITATION
TITRATIONS, AND
COMBUSTION
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
Carboniferous
Devonian
Silurian
Precambrian
Pb/U(g/g)
0.041
0.045
0.053
0.125-0.20
Millions of years
340
370
430
1 025-1 640
Today’s accepted value
362-330
380-362
443-418
900-2 500
26-1 Examples of Gravimetric Analysis
Ag+
+ Cl-  AgCl(s)
(27-1)
A few common organic precipitants (agents that cause precipitation) are listed
in Tabled 27-2.
Q : the concentration of the solute at any instant
S : the equilibrium solubility
• 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
• 침전반응은 느림 → 침전제 첨가 시
→ 순간적으로 supersaturate ( Q > S )
이 경우 불안정 하므로 → precipitate 생성
• 침전의 크기는 average relative supersaturation와 반비례 관계
(Q – S)/S 大 → colloidal solid 생성 (침전크기 小)
小 → crystalline solid 생성(침전크기 大)
■ Mechanism of Precipitate Formation
• 입자 크기에 미치는 RS의 효과 → precipitation mechanism 설명
• 침전은 ① nucleation ② particle growth 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 S를 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)↓
Fe(III) formate
(27-4)
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.)
-Some impurities can be treated with a masking agent
to prevent them from reacting with the precipitant.
Ca2+
Analyte
Mn2+
Impurity
+
2RH

N-p-Chlorophenylcinnamohydroxamic acid
+
6CNMasking agent

CaR2(s)↓
+
2H+
Precipitate
Mn(CN)64Stays 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.
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
1050oC/O2

CO2(g) + H2O(g) + N2(g) + SO2(g) + SO3(g)
95% SO2
o
Cu + SO3 850C
Cu + 1/2O2
850oC

SO2 + CuO(s)
CuO(s)
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