Chapter 24. Coulometry 1) Electrogravimetry: sample soln.+ Pt전극(wire, gauze) 직류전압 加 ┏ 일정전압법 (Constant applied cell potential) ┃ 일정전류법 (Constant electrolysis current) ┗일정작업전극전위 (Constant working electrode potential)  정량성분 , 전해석출  weighing (금속의 순도, 합금성분의 정량에 利用) 2) Coulometry: sample soln. + 일정 전위의 전압 加 (전류효율100%)  소요된 전기량 측정  

Electrogravimetry 일정전위분해법 : 황산구리 용액에 Pt wire 및 Pt gauze (150cm2)전극을 0.0220M Cu2+ 200 ml + 1.00 M H+ 용액에 담궈 (cell of 0.50 Ω) 직류전압 加  전기분해 Cathode : Cu2++2e-  Cu 석출 Eo = 0.34 V Anode : 1/2O2+2H++2e- = H2O Eo = 1.23 V  석출 전 후의 전극무게 측정  정량 Deposit 의 physical characteristic에 영향을 주는 요소: ① Current density ② Temp. ③ Complexing agent 의 존재  실험적으로 결정   Electrolysis : Eapp=EC-Ea+Πc+Πa-IR (24-1)

24A-1 Current-Voltage relationships during an electrolysis. ▪ Initial Thermodynamic Potential of the cell Standard potential data for the two half-reactions in the cell under consideration are Using the method shown in Ex 22-6, the thermodynamic potential for this cell can be shown to be -0.94 V in 0.0220M Cu2+ 200 ml + 1.00 M H+ solution. When a potential of -2.5 V is applied to the cell, Iinit = 1.5 A. (See Fig.14-1a) ▪ Current changes during an electrolysis at constant applied potential; It = Ioe-kt , k = 25.8DA / Vδ Typically, D = 10-5 cm2/s, δ = 2 x 10-3 cm

1-2. Constant current electrolysis : By maintaining the current : 전해 동안 주기적 전압증가 要. Fig. 24-2

Eapp = (Ea+ηa)-(Ec+ηc)+IR Ec = EoCu2+,Cu + 0.059/2log[Cu2+] ex) 0.01M Cu2+ 99.9% 전해에 必要한 voltage EC = 0.34 + 0.059/2*log[10-2]=0.28 V 전해 종료시 EC = 0.34+0.059/2*log[10-5] = 0.19 V (저하) ηa ,ηc  H+ 의 accumulation  depolarizer 첨가 즉 Cu2++H2O = Cu+1/2O2(g)+H+ NO3-+10H++8e- = NH4++3H2O Electrode reaction 의 속도에 의존 (Pt에서 거의 무시) IR 강하  이론적인 加 전압 값보다 약간 더 큰 전압 加

1-3. Controlled potential electrolysis (limited cathode potential electrolysis) 장치: ( working elec.의 전위  reference에 대해 측정  이론 값 만큼 전위 加  ) Reference 전극에 대해 일정한 전위를 설정  저항 조정 working electrode의 전위가 일정하게 조절

적용 : 두 종류의 금속  수소보다 양성 , 전극전위 비슷 분리 곤란(일정전류법)  Controlled potential electrolysis 사용 전류 I  시간 t 에 대해 지수 함수적으로 감소 I = Ie-kt . logI = -2.303kt+logI0 Ct/C0 = it/io = e-(DA/Vδ)t = 10-0.43(DA/vg)t v : 용액의 volume δ : 확산 층의 두께 Ct/C0 = e-ktk =mA/V m : mass transfer coefficient = D/δ k = 25.8DA/(Vδ)

1-4. Mercury cathode electrolysis 음극: 백금대신 수은->전해 Mn++ne+Hg  M(Hg)  원소의 분리에 利用 ex) [Ag+] = 10-6M, [Cu2+] = 0.1M 일 때 EC = 0.08+(-0.35) =0.45V EC-ηC = 0.45-0.07 = 0.38(V) 구리 EC =0.31V이므로 0.38V volt 에서 Ag+을 정량적 석출 可能   1-5. Internal electrolysis 양극 : 석출금속보다 음의 단위를 가진 금속 음극: 백금 두 극 사이에 격막을 놓으면 전지 형성내부전해법

24B An Introduction to coulometric methods of analysis Coulometry Analyte를 정량적으로 다른 산화상태로 변화시키는데 필요로 하는 전기량 2. Controlled-potential coulometry idt = Q = nFw/M F ; 964877±1.6(≒96500)C/g당량 Q를 측정  w계산 w : 산화 환원 물질의 무게 , M : formula weight, Q : coulombs의 수

24B-1 Units for Quantity of Electricity The quantity of electricity or charge or charge is measured in units of coulombs(c). 24B-2 Type of Coulometric Methods ┏ Constant potential coulometry ┗ Constant current coulometry

2-1. Constant potential coulometry Q = Idt = I0e-kt dt = I0(1+10-kt)/2.3033k ≒ I0/2.303k I = I0e-kt, logI = -2.3033kt +log I0 식에서 I0와 kt를 구하여 Q값 계산   2-2. Constant current coulometry, (전기량 적정) I = constant이므로 Q = It에서 t측정하여 Q계산 정량물질의 선택적 전해가 어렵다. 전해에 의해 생긴 제2 물질 적정하여 정량

2-3. Coulometers 종류 H2, O2 coulometer Ag coulometer H2, N2 coulometer Cu coulometer H2 coulometer 적산 coulometer   1. H2-O2 coulometer 뷰렛모양의 전해용기에 K2SO4 용액을 담고  Pt를 전극으로 하여 전기분해  발생 전 H2-O2 의 부피 측정 (16.80ml/1mg)

2. Ag coulometer Cathode : Pt crueible containing AgNO3 solon Anode : Ag 전극 Pt+석출 전 Ag  무게 측정  Q계산 3. N2-H2 coulometer Hydrazine sulfate  전기분해  수소, 질소 발생  gas 양 측정  전기량 계산 4. 적정 전기량계 양극 : 은 KBr+K2SO4용액 전해 음극 : Pt 2Ag+2Br-+2H2O  2AgBr+H2+2OH- OH-을 0.01 NHCl로 적정하여 구한다.

24C Controlled-potential coulometry 24C-1 Instrumentation ▪ Cells Fig 24-5 illustrates two types of cells that are used for potentiostatic coulometry. ▪ Potentiostats A potentiostat is an electronic device that maintains the potential of a working electrode at a constant level relative to a reference electrode. Fig 24-6 is a schematic of an apparatus for controlled=potential coulometry, which contains a somewhat different type of potentiostat. ▪ Integrators As shown in Section 3E-3, analog integrators can be constructed from operational amplifier circuits.

Cl3CCOO- + H+ + 2e-  Cl2HCCOO- + Cl2 24C-2 Applications Controlled-potential coulometric methods have been applied to more than fifty elements in inorganic compounds. Cl3CCOO- + H+ + 2e-  Cl2HCCOO- + Cl2

24D Coulometric titration 24D-1 Electrical Apparatus Fig 24-7 is a conceptual diagram of a coulometric titration apparatus showing its main components. Fig 24-8 is a photo of one of a number of automated coulometric titrators on the market. The instrument in Fig 24-8 is specifically designed for the Karl Fischer determination of water. ▪ Cells for coulometric Titrations A typical coulometric titration cell is shown in Fig 24-9. Note that the apparatus shown in Fig 24-10 provides either hydrogen or hydroxide ions depending on which arm is used.

24D-2 Applications of Coulometric Titrations ▪ Neutralization Titrations Both weak and strong acids can be titrated with a high degree of accuracy using hydroxide ions generated at a cathode by the reaction. ▪ Precipitation and Complex-Formation Titrations A variety of coulometric titrations involving anodically generated silver ions have been developed(see Table 24-1).

▪ Coulometric Titration of Chloride in Biological Fluids The accepted reference method for determining chloride in blood serum, plasma, urine, sweat, and other body fluids is the coulometric titration procedure. (ncl-)u = t u /t s X (ncl-)s

▪ Oxidation-Reduction Titrations Table 24-2 indicates the variety of reagents that can be generated coulometrically and the analyses to which they then have been applied.

① Variation in the current source of error ▪ Comparison of Coulometric and Volumetric Titrations There are several interesting analogies between coulometric and volumetric titration methods.  Titration error ① Variation in the current source of error ② Departure of the process from 100% current efficiency ③ Error in the measurement of current ④ Error in the measurement of time ⑤ Titration error due to the difference between the equivalence point & end point 정밀 : 0.01%

실험 및 이론: 1) Pt gauze를 nitric acid  rinsed-dried  weighing 2) Deposition : Eapp=(Ea+ηa)-(EC+ηC)+iR EC=E°cu2++0.059/2log[Cu2+]   @ 0.01M-Cu2+ 99.9% 전해에 필요한 Voltage EC=0.34+0.059/2log[10-2]=0.28V ηa : electrode reaction의 속도에 의존  Pt에서는 거의 무시 ηe : H+의 accumulation  (depolarizer) (stirring) Cu2++H2O <-> Cu+1/2O2(g)=2H+ 특정 이온만의 분리 다양한 기술 필요

* IR 강하로 인해 이론적인 加 전압 값보다 약간 더 큰 전압은 加해야 한다. H+의 방해감소 (depolarizer첨가)  H2 gas발생(spongy식의 전착) (Cu2+의 경우) Cathode : Cu2++2e-=Cu Anode : 1/2O2+2H+=2e- <-> H2O   NO3-+10H+=8e- <-> NH4++3H2O Cu  H2발생 無

3-1. Electrolysis at controlled potential Copper, ion 혼합물 정량 시   Working elec.의 전위  reference에 대해 측정  이론값 만큼 加  보정해 주며 전압 加 3-2. Electrolytic separations Easily reduced metals from small currents of less easily reduced material

3-3. Controlled-potential coulometry Analyte  chemical state의 chrage  q를 측정  phy. constant에 따라 양을 계산 w=QM/nF w : 산화, 환원전 물질의 무게 F : 96.4877±1.6coulomb/g M : 물질의 분자량 Q : cell을 지나는 전하의 수   q=it