Chapter 12

Gravimetric methods of Analysis

Gravimetry Gravimetric methods are quantitative methods that are based on determining the mass of pure compound to which the analyte is chemically related.

Precipitation gravimetry: the analyte is separated from a solution of the sample as a precipitate and is converted to a compound of known composition that can be weighed.

Volatilization gravimetry: the analyte is separated from other constituents of a sample by conversion to a gas of known chemical composition. The weight of this gas then serves as a measure of the analyte concentration.

Electrogravimetry: the analyte is separated by deposition on an electrode by electrical current. The mass of this product then provides a measure of the analyte concentration. Gravimetric titrimetry: measuring the mass of a reagent of known concentration (in a solution) required to react completely with the analyte. Atomic mass spectrometry: use a mass spectrometer to separate the gaseous ions formed from the elements making up a sample of matter. The concentration of the resulting ions is then determined by measuring the electrical current produced when they fall on the surface of an ion detector.

12A PRECIPITATION GRAVIMETRY In precipitation gravimetry, the analyte is converted to a sparingly soluble precipitate. This precipitate is then filtered, washed free of impurities, converted to a product of known composition by suitable heat treatment, and weighed. Procedure for gravimetry based on precipitation

1) Preparation of sample solution

2) Precipitation : Gravimetric precipitating agent should react specifically or at least selectively with the analyte.

rR + aA = RrAa (solid)

3) Digestion

4) Filtration and washing

5) Drying, ignition and weighing

6) Computation of results analyte precipitate

Ex. Precipitation method for determining calcium in natural water (AOAC)

2NH3 + H2(COO)2 → 2NH4+ + (COO)2

2–

Ca2+ + (COO)22– → Ca(COO)2 (s)

The CaC2O4 precipitate is filtered using a weighed filtering crucible, then dried and ignited. This process converts the precipitate entirely to calcium oxide.

Ca(COO)2 (s) → CaO (s) + CO (g) + CO2 (g)

∆ After cooling, the crucible and precipitate are weighed, and the mass of calcium oxide is determined by subtracting the known mass of the crucible. The calcium content of the sample is then computed.

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12A-1 Properties of Precipitates and Precipitating Reagents

Requirements of the ideal product of a gravimetry : 1) easily filtered and washed free of contaminants (very pure)

2) of sufficiently low solubility that no significant loss of the analyte occurs during filtration and washing 3) unreactive with constituents of the atmosphere

4) of known chemical composition after it is dried or, if necessary, ignited

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12A-2 Particle Size and Filterability of Precipitates

• gravimetric analysis ->large particle precipitates demand

→ Easy to filter, wash free of impurities, purer than fine particles

■ Factors That Determine the Particle Size of Precipitates

■ 침전에 의해 형성된 고체의 입자크기는 매우 다양함

• Colloidal suspensions (dia.=10-7 ~ 10-4 cm) → 가시광선을 산란시킴(Tyndall effect) → 가라앉지 않음 → 쉽게 여과되지 않음

Tyndall effect : particles of colloidal dimensions scatter visible radiation, the path of the beam through the solution can be seen by the eye.

• Crystalline suspensions: particles with dimensions on the order of tenths of a millimeter or greater. → spontaneously settle down → easily filtered

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Color plate 6 The Tindall effect. The photo shows two cuvettes: the one on the left contains only water while the one on the right contains a solution of starch. As red and green laser beams pass through the water in the left cuvette they are invisible. Colloidal particles in the starch solution in the right cuvette sctter the light from the two lasers, so the beams becomes become visible.

• 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 ⇒ relative supersaturation (RS) 과 입자 크기가 관계

Q : the concentration of the solute at any instant S : 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 ② particle growth 두 과정에 의해 형성 • freshly formed precipitate의 입자크기는 ①, ②중 어느 쪽이 우세한 가에 의존

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

Nucleation is a process in which a minimum number of atoms, ions, or molecules join together to give a stable solid.

• 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

If nucleation predominates, a large number of very fine particles is produced. If particle growth predominates, a smaller number of larger particles is obtained.

Mechanism of Precipitate Formation Filterability : large particle size

Precipitation mechanism 1) Induction period (few min) : A precipitant(counterion) is added

2) Nucleation : form small aggregates : Nucleus has 4 molecules

3) Particle growth to form larger crystal

4) Adsorption

5) Electrostatic attraction to coalesce : electric double layer

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http://hishamezzat.weebly.com/docx2.html part_3a_gravimetry-_2013.doc

Stage 1 Ions in supersaturated solution

Primary nuclei ( non filterable)

Stage 2 Colloidal particles

Stage 3 Fine crystals Stabilized Colloid

Final form Coarse crystals Crystalline aggregate Colloidal aggregate e.g. PbSO4 e.g. BaSO4 Gel [e.g. Fe(OH)3 ] Curd [e.g. AgCl ]

Three stages of precipitation process. The three paths of precipitation are competitive and a given precitate may contain all forms with one form predominating.

Particle size greater than 10,000 nm Particles readily retained by filter paper

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A supersaturated solution is an unstable solution that contains a higher solute concentration than a saturated solution. As excess solute precipitates with time, supersaturation decreases to zero (see color plate 5).

Color Plate 5. Crystallization of sodium acetate from supersaturated solution. A tiny seed crystal is dropped into the center of a petri dish containing a supersaturated solution of the compound. The time sequence of photos taken about once per second shows the growth of the beautiful crystals of sodium acetate.

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■ Experimental Control of Particle Size

• minimize supersaturation ((Q-S)/S : 小 → 입자 크기 大) 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 ex) Fe(OH)3 , Al(OH)3 , Cr(OH)3, most heavy metal의 sulfides → colloid.

12A-3 Colloidal Precipitates

1) not retained on ordinary filtering media. 2) Brownian motion prevents their settling under the influence of gravity. but the individual particles of most colloids → coagulate, agglomerate → filter 可

■ Coagulation of colloids

• coagulation process의 촉진 : ① heating, ② stirring, ③ adding an electrolyte to the medium

• Colloidal suspension이 안정하고, 자발적으로 coagulation 되지 않는 이유 → surfaces에 cation, anion이 adsorption(흡착)되어 “+”, “-”의 전하를 갖는다.

AgCl 입자 + Cl- (Cl- 이온 과잉인 용액에서) AgCl 입자 + Ag+ (Ag+ 이온 과잉인 용액에서)

Cl- AgCl–Cl- AgNO3 첨가

excess Cl-

AgCl AgNO3 첨가

AgCl–Ag+

AgNO3 첨가

excess Ag+

• Colloidal suspension에 먼저 흡착되는 ion은 용액에 과잉 존재하는 lattice ion 임

• AgCl 침전을 이용하여 Colloidal suspension의 성질 설명

excess of neither ion

- charge + charge minimum charge

Silver chloride lattice

Primary adsorption layer

Counter ion layer

Electric double layer

Figure 12-1 A colloidal silver chloride particle suspended in a solution of silver nitrate. 17

AgNO3 과잉용액에서의 colloidal AgCl

Repulsive force에 의해 colloid의 coagulation 방해

excess of silver ion 의 용액 → solid surface에 Ag+이 primary adsorption layer 형성. surrounding the charged particle : counter layer의 용액층 (negative ion, NO3

-).

⇒ primary adsorbed ion (particle에 흡착) & counter ion (soln) → electrical double layer 형성 (repulsive force) ∴ coagulation of a colloid 위해 repulsive force를 없애야 한다. 응집 : 입자간의 인력에 의하므로 electrical double layer에 의해 不可 ① AgCl 침전 생성시 excess Ag+ 可 → repulsive force 감소 (단위 입자당 Cl-이온량이 감소하므로) ∵ Ag와 Cl의 chemical equivalence에서 coagulated colloid is observed. ig. a large excess of Ag+ 可 → agglomeration process revered.

② heating (with stirring) i) 흡착이온의 수 감소 → double layer의 두께 감소 →coagulation 가능 ii) colloidal particle 운동에너지 > repulsive force →coagulation 가능 ③ suitable ionic compound 加에 의한 electrolyte concentration을 증가시키는 방법 → center-ion layer의 shrinking 표면 전하 중화됨

⇒ Peptization : coagulated colloid → returns to its original dispersed state Ex) 침전 세척을 위해 pure water 사용시 세척 → 엉키게 한 이온 resolution → counter ion layer 大 → repulsive force are reestablished → particle을 detach → peptization. ⇒ Peptization이 일어나지 않게 세척하는 방법 Volatile electrolyte 로 agglomerated colloid 세척 ↳ heating 으로 제거 가능 Ex) AgCl ppt + dilute HNO3 로 세척 → ppt 110℃에서 dry → HNO3 제거

Treatment after precipitation Peptization: a process by which a coagulated colloid returns to its dispersed state.

■ Practical Treatment of Colloidal Precipitates

Practical Treatment of Colloidal Precipitates Digestion : following ppt, for a period of standing with heating the precipitate in contact with its mother liquor accelerate the ripening of crystalline particles.

Also the digestion period results in some improvement in the internal perfection of the crystal structure [sometimes called ripening], here some internal foreign atoms may be expelled. Growth of larger nuclei or crystallites can be encouraged by digestion, a process which involves heating the solid and mother liquor for a certain period of time. During digestion, small particles dissolve and larger ones grow. Digestion of the product is an important practical process and you will find that most if not all gravimetric analysis involve a digestion period.

• colloid : hot, stirred solution to which sufficient electrolyte → coagulation • coagulated colloid의 filterability의 증가를 위해 ⇒ 뜨거워진 그 용액 속에서 그대로 수시간 방치 (digestion 과정) → 약하게 결합된 물이 침전에서 빠져 나옴 → 거르기 쉬운 denser mass가 됨

12A-4 Crystalline Precipitates

■ Methods of Improving Particle Size and Filterability

• 거르기와 정제하기 쉬움

• 입자크기와 filterability를 어느 정도 조절할 수 있음

Q : minimizing S : maximizing

ex) dilute soln. & slow addition of reagent (Q 小) hot solution (S 大)

• RS 조절

• Digestion (without stirring)

particle size 大

•침전형성 후 일정시간 방치 → yields a purer, more filterable product

• 침전의 dissolution과 recrytallization에 의해 일어남 → 온도를 높이면 속도 증가

• Colloidal 침전의 purity와 filterability를 향상시키는데도 효과적임

12A-5 Coprecipitation Coprecipitation is a process in which normally soluble compounds are carried out of solution by a precipitate. Contamination of a precipitate by a second substance whose solubility product has been exceeded is not coprecipitation. Four types of coprecipitation: equilibrium processes: 1) surface adsorption, 2) mixed-crystal formation, kinetics of crystal growth 3) occlusion, and 4) mechanical entrapment.

(a) (b)

(c) (d)

A: surface adsorption B: inclusion-isomorphic carrying C: inclusion-mechanical entrapment in crystals D: occlusion-mechanical entrapment in colloidal aggregate.

Joseph J. Klingenberg and Kenneth P. Reed, Introduction to Quantitative Chemistry, Reinhold Publishing Corporation, New York, 1965. p. 76

■ Surface Adsorption

• Coagulated colloids: specific surface area 가 큼 → adsorption 이 일어남

Crystalline solids: specific surface area 가 작음 → adsorption이 매우 작음

• Lattice ion과 counter-ion이 주로 coprecipitation 됨

Ex) AgCl 침전 시 → Ag+ 와 NO3- 또는 다른 counter-ion 이 흡착됨 → 가용성

AgNO3가 AgCl과 coprecipitation 됨

Minimizing Adsorbed Impurities on Colloids

① Digestion: water expel denser mass (smaller specific surface area)

② Washing: volatile electrolyte로 세척 → 흡착된 counter-ion과 교환 됨 → 흡착된 nonvolatile electrolyte 제거

Ex) Cl-를 가하여 Ag+ 정량시 → primarily adsorbed species는 Cl- 임 → 산성용액 으로 세척 → counter-ion layer가 H+로 바뀜 → 건조 시 휘발성 HCl 제거

⇒ 충분히 세척하여도 어느 정도는 오염됨

→ AgCl에 흡착된 Ag+ : 1~2 ppt 이하 오차

→ trivalent Fe 또는 Al의 hydrous oxide에 coprecipitation된 heavy-metal hydroxide: 허용 값 보다 큰 수 %의 오차 생김

Reprecipitation • 걸러진 고체를 다시 녹여 재 침전 시킴(시간이 걸림)

• coprecipitation된 heavy-metal (Zn, Cd, Mn) hydroxide을 강하게 흡착하는 Fe(III) 또는 Al의 hydrous oxide 침전의 경우 필요함

Washing Upon washing the precipitate with water, part of the adsorbed electrolyte is removed. The electrolyte concentration in the supernatant liquid may fall below the coagulation value (minimum amount of electrolyte necessary to cause flocculation), the precipitate may pass into colloidal solution again. Reprecipitation In a reprecipitation, a sought-for precipitate is washed, dissolved in pure solution and repreprcipitated. This method is satisfactory in decreasing the surface-adsorbed and occluded impurities, it may also improve the purity of precipitates suffering from postprecipitation. During the second precipitation, the amount of contaminant adsorbed will be lower since its concentration in the solution is less. As a result, less will be enclosed within the crystalline structure of the precipitate when it is again formed. But reprecipitation is of little value in dealing with contaminants carried by simultaneous precipitation or by isomorphic inclusion. Reprecipitation is also unsatisfactory whenever the contamination arises from the reagent used for precipitation process itself. It is often necessary for such precipitates as the hydrous oxides of iron(III) and aluminum, which have extraordinary tendencies to adsorb the hydroxides of heavy-metal cations such as zinc, cadmium, and manganese.

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■ Mixed-Crystal Formation

• Lattice ion과 replacing ion의 성질: 같은 하전, 비슷한 크기(~5% 이내), 같은 crystal class를 가져야 함

SO42-, Pb2+, OAc-

BaCl2 첨가

Ba(Pb)SO4

Ex)

Ex) MgKPO4 in MgNH4PO4, SrSO4 in BaSO4, MnS in CdS

• 침전이 형성되고 나면 제거하기가 특히 힘 듬

• Colloidal suspension과 crystalline 침전에서 모두 나타남

• 최종 침전단계 이전에 interfering ion 제거

• Mixed-crystal을 만들지 않는 침전제 사용

■ Occlusion and Mechanical Entrapment

• 생성 이유 ⇒ crystal 성장속도가 빠름 → 불순물을 갖는 용액방울을 결정이 둘러쌀 때 발생

Occlusion Mechanical Entrapment • 감소시키는 방법

⇒ 침전생성 속도를 느리게 함 (낮은 supersaturation) → 최소화 됨

⇒ Digestion: 가온하여 digestion 시킴 → 용해와 재침전이 빨라짐 → pocket이 열림 → 불순물이 용액 속으로 방출 → occlusion과 mechanical Entrapment가 감소

■ Coprecipitation Error

• positive error : contaminant 이 정량 목적 이온이 아닐 경우 발생 ex) Cl- 정량시 AgCl이 AgNO3 흡착. • positive or negative error : contaminant가 정량 목적 이온을 포함할 경우 ex) positive : Ba2+ 정량시 BaSO4 침전에 Ba(NO3)2 occlusion. ex) negative : BaSO4 ppt에 BaCl2 흡착.

12A-6 Precipitation From Homogeneous Solution

침전제가 느린 화학반응을 통하여 용액 내에서 생성 → local reagent excess가 일어나지 않음 → 침전이 생성되는 동안 RS 낮게 유지 → 순도가 높고, 거르기 쉬운 침전 생성

Ex) urea: homogeneous generation of OH-

• 염기를 직접 가할 경우 → 심하게 오염, 거르기 힘 듬, 부피가 큼, 젤라틴 형태

• 염기를 homogeneous generation 할 경우 → 순도가 높음, 거르기 쉬움, 부피가 작음, 조밀한 형태

: slow

12A-7 Drying and Ignition of Precipitates

• after filtration ppt의 weight가 const될 때까지 heating solvent, volatile electrolytes 제거 weighing

알려진 조성을 갖는 화합물이 될 때까지 ignition decompose the ppt weighing

• AgCl : 110 ℃이상에서 물 완전히 제거 • Al2O3 : 1000 ℃이상에서도 물 완전히 제거되지 않음 (urea로 homogeneously 얻어진 Al2O3 → 650℃에서 건조) • Calcium oxalate : 온도에 따라 조성이 달라짐 135 ℃부근 → CaC2O4·H2O 225 ℃부근 → CaC2O4 450 ℃ → CaCO3로 분해 800 ℃이상 → CaO로 분해

mass loss vs. T

12B Calculation of Results From Gravimetric Data

Mass of Crucible

12C Applications of Gravimetric Methods

① inorganic anions & cations, ② neutral species: water, sulfur dioxide, carbon dioxide, iodine ③ organic → lactose in milk products salicylates in drug preparations phenolphthalein in laxatives(설사약) nicotine in pesticides cholesterol in cereals, benzaldehyde in almond extracts

12C-1 Inorganic Precipitating Agents

• 불용성 염 또는 hydrous oxide 형성 함

• 각 침전제는 많은 원소들과 반응 (선택성이 좋지 않음)

12C-2 Reducing Agents

12C-3 Organic Precipitating Agents

⇒ inorganic species의 분석을 위한 유기시약 ⇒ organic agents : more selective than the inorganic agent.

• Two type of organic reagents

① forms slightly soluble, non-ionic products (coordination compounds)

② forms products in which the bonding between the inorganic species and the reagents is largely ionic.

유기시약들 중 한 분자 내에 배위결합을 할 수 있는 작용기를 2개 이상 가지고 있음 (chelating agent) → cation과 결합 시 5- 또는 6-membered ring 형성(chelates)

• Metal chelates의 특성

⇒ Relatively nonpolar → low solubility in water, high solubility in organic liquids

⇒ Low density

⇒ Often intensely color

Chelating reagents

8-hydroxyquinoline

dimethylglyoxime

Sodium tetraphenylborate

Cation의 종류 및 pH에 따라 용해도가 넓은 범위에서 변함 → pH를 조절하면 침전형성 시 선택성을 얻을 수 있음

Ni2+ 이온만을 침전시킴 (붉은색)

⇒ Salt 형태의 침전 형성(chelate 아님)

⇒ 차가운 mineral acid 용액에서 K+ 및 NH4+와

선택적으로 침전 형성

⇒ Mercury(II), rubidium, cesium이 방해 작용

12C-4 Organic Functional Group Analysis

some reagents → react selectively with certain organic functional groups.

12C-4 Volatilization Gravimetry

Water direct determination: 가열시 방출되는 수증기를 포집 → 질량측정 indirect determination: 가열 시 질량감소로부터 수분 측정 (많이 사용됨) Carbon dioxide

Sulfides and sulfites 산성조건에서 H2S 또는 SO2 기체 방출 → 흡수제에 포집하여 무게 측정

Carbon and hydrogen in organic compounds H2O 또는 CO2 기체를 적당한 흡수제에 포집하여 무게 측정

침전 가열 → 기체방출 → 기체포집(흡수제) → 질량측정 → 침전의 질량감소 측정