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คาบที่ 4 - โพลาโรกราฟีกระแสตรง PowerPoint PPT Presentation


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คาบที่ 4 - โพลาโรกราฟีกระแสตรง. การค้นพบเทคนิคโพลาโรกราฟีกระแสตรง ( Direct current polarography) เครื่องมือ - โพลาโรกราฟ โพลาโรแกรม และการตีความเพื่อใช้ประโยชน์ในการวิเคราะห์ ตัวอย่างการประยุกต์ใช้เทคนิค DCP ข้อจำกัดของเทคนิค DCP และแนวทางแก้ไข. การค้นพบเทคนิค.

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คาบที่ 4 - โพลาโรกราฟีกระแสตรง

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4

4 -

  • (Direct current polarography)

  • -

  • DCP

  • DCP


4

(Direct current polarography) Miroslav Heyrovsky (electrocapillary study) (dropping mercury electrode)

Prof. Heyrovsky

Electrocapillary curve


4

-

voltammetric cell SCE (linear ramp) 2- 5 mV/s

(potential waveform) DCP

DCP


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half wave potential limiting current

E1/2

Il -

DCP


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DCP

Detection limit 10-5M


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Ilkovic equation

Id = 708 n D1/2 C m2/3 t1/6

n = , D = diffusion coefficient, C= , m= , t=

(charging current)

Ic = 0.00567 Ci (Ez-E) m2/3 t-1/3

Ci = , Ez = zero charge

DCP background current


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DCP

Ic = K Ci (Ez-E) m2/3 t-1/3

( Ci)

scan rate ( Ez-E)

( m2/3)

(t-1/3)

(a) DCP (b) Tast DCP

Tast DCP sampled current DCP


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5 - /

  • (normal pulse polarography, NPP)

  • (differential pulse polarography, DPP)


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5 - /

(square wave voltammetry, SWV)


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ic if (pulse) if -


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Timing:

waiting period 2-4 s

pulse width 5-100 ms

current sampling 2-20 ms at nearly end of pulse

Advantages:

increase if proportion

short period of pulse = less analyte consumption = very thin diffusion layer = better sensitivity

5-10 times more sensitive than DCP

1 1


4

NPP

Timing:

waiting period 2-4 s

pulse width 5-100 ms

current sampling 2-20 ms at nearly end of pulse

Advantages:

increase if proportion

short period of pulse = less analyte consumption = very thin diffusion layer = better sensitivity

5-10 times more sensitive than DCP

1 1


4

Advantages:

increase if proportion

short period of pulse = less analyte consumption = very thin diffusion layer = better sensitivity

25-50 times more sensitive than DCP

Detection limit about 10-8 M

Peak polarogram/voltammogram: improve selectivity = better resolution between 2 adjacent peaks (Ep > 50 mV can be resolved)

Resolution: W1/2 = 3.52 RT / nF

Pulse amplitude & scan rate affect sensitivity and resolution

i = i2 i1

Potential waveform DPP


Dcp npp

DCP & NPP

10-3M Fe3+ 0.1 M HCl

(a) DCP, (b) NPP (c) DPP

DPP sensitivity

  • Block diagram


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DPP sensitivity ,


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  • square wave stair case 2 forward reversepulse anodic cathodic

  • i = i1 (- i2) = i1 + i2

Stair case

Squarewave

Squarewave waveform


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i = i1 (- i2)

= i1 + i2

  • DP 3-4 ()


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4


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