Revista Mexicana de Física 23 (1974) FAI_ FA6

AN IMPROVEO ELECTROMETER FDR MEASURING REDOX

POTENllALS IN BIOLOGICAL MEOIA ANO FDR GENERAL

EL ECTR00l911 CAL MEASUR91ENTS

Claude ~larmasse

Biophysies Lahoralory

Facultad de Ciencias. Universidad Nacional Autónoma de México

.Il irieo 20. D. F.

(Recibido: agosto ~. 1974)

FAI

ABSTRACT: The construuion of an elecuometer with an input impedance

of lOs Mº convenient for the measurement of redox polentialsin biological media is described.

The measurements oC oxidation-reduction potentials in biologicalmedia present special problems because many oC the systems oC interest arenot very electroactive and particularly .siuggish"l. This in tuen imposes\'cry stringent conditions on the electrode system and the valuc of input im-pedance oí the elcctronic chain. Manual and autfWnatictechniqucs with inputimpcdances in the range lOO-IODO MO have been described2•3; in both cases,the measuring technique involvcs a potentiometric balancing of the inputcircuit for a relatively shon (ime. While this prescn(s che advantage of fumish-ing a high precision (for instance, the standard error oC (he redox potential oCa sample of human blood subjected ro Creezing was found2 (o be as lov.: as

FA2 Marmasse

0.65 mV), ir: is time-eonsuming when done manually, and implementation DE(he automatic installation previously described3 requires much space (withrhe necessary accessories, (wo standard-size racks) and in addition is rathercostly. In many instances, need arises for a compact, versatile and directreading instrumente The purpose oí the.present mper is to discuss [he techni.cal requirerJIents ol such ao instrument and to describe briefIy one practicalimplementadon _using operational amplifiers, which has been thoroughly testedover the last six years.

By far, (he most important condition to be mer: relates to me input ¡m-pedance DE (he electronic chain, and this lor a number DE reasaos. In [hefirst place, as (he monitored systems are not very electroacdve, great caremust be taken not to polarize the electrodes. In the second place, becauseof the sluggishness of the electron transfer reactions involved in the electro-chemical cell eonsidered, one has ro study the kinedcs of the phenomenonand me redox potential sought is obtained as me Iimiting value of me observedpotential. Now, in many occasions, hours are necessary to obtain su eh anequilibrium (for numerieal examples, see ref.2 and 4). Clearly, an apparatusleft eonnected (even for a short period of time, say 10 s every 5-10 minutes)to a polarizadon prompt eleetrochemical eell, should have an extremely highinput impedance. A number of preliminary experiments (realised in [he con-ditions described in reí. 2 and 5) indica,ed ,ha' a value of 10' MO was a

RI

R6

R9

R,

RIO

R8

Fig. l. Block diagram of the decuometer.

A" ¡mproued eleclromeler ••• FA3

desirable minimum. Therefore, a value oE lOs Mn was selected in order tohave an added coefficient oE security. Otller desirable features inelude highcommon mode rejection ratio, zero-suppressing and extended-scale capabili-ties, low-power consumpdori, mechanical ruggedness, ability to perform in anot necessarily very dry lab atmosphere and, if possible, under field con-ditions, and finaUy interphasing.

The general principie of the apparatus can be gathered froro Fig. l.The signal is fed to an impedance transformer Z of gain unity followed by acontinuously variable multiplier M. In these conditions, the unknown high-impedance input voltage is lineady transformed (in a known ratio) in a low-impedance voltage. 1'0 this voltage is added, by means of the adder A, aknown voltage generated by a suitable source (e. g. a mercury battery) andthe continuously variable amplifier G (referred to in this text as the generator);the sum is measured with a convencional microammeter. A version of the ap-paratus, using operational amplifiers of the Philbrick-l'Jcxus- Teledyne Companyis showo 00 Fig. 2 aod Table 1.

1

z

1

M A

Fig.2. Example of an implementation of rhe cle<.'uomerer.See tex[ and Table l.

F A4 \I.um.l ~St'

TABLE 1

Lis[ of componcnrs. Sce {ex[ and Fig. ].

Z

,~,G,A

Ri

R2

R3,R4,Rli

R5,R7,RIO

R(,

R8

R9

•,~11

] FT -2 B

QS-IOa

IOK. 10 turns. 0.1% lineariey

500'\OK

iOK:t 5%; sdeel R'\ < RIO

1K trimmcr

lOOK. 1.0 Curns. 0.1% linc:uiey

sce t('xl; typical "atue: 20K for 1000 01\'

all seal,'

100 j.i./\, Simp ..••on

mercury bac[(:ry.

Th<.' consrruction of the apparatus calls ror a (('w comnH'nts:

a) rhe common should remain floatinJ!;;

o) all ¡:round loops shouid he a\"Oided;

e) [hcr{' must he no switching: done on tI){, high input sid", and

in gen{'ral switching should be h'pt [o él minimurn. :\Iso,[fR' input conncc[ors (BNC) are dircctly soldcrt.:d to tll(' corre-

sponding: pins uf rhe impeJance transformer Z.

The calibration of da' instrument can be pcrformcd in a numher of way~."11)('pToceJurc shown in Table 2 has been found convcnienL

This instrumcnc has beco found highly sensitivc and reliable: irslineariry is berrer (han 0.1% and rhe noisc of dH,' imp(:dancc rransformer. whichis [he mosc cricical e1{'menc of che chain. is inferior Co a few microvolts(measured wich che inpucs shorc-eircuiced). In order Co obcain p{'ak per.

formances, a warming time of approximacely 20 minuces is required. lt canbe considerably decreased by leaving syscematically [he apparacus in a scand.

by condicion.: chis is easily achieved by using supp.y voltages of :t 8V; fo,measurements. cJJ.ese \'oltagcs are swicched Co cheir nominal values (:t I~V).

An improued electrometer ••• FAS

TABLE 2

Calibration o( the instrument

Step ConfigurationB to:

(see Fig. 3)e' to: Operation

A' and D

2 e and D

3 A' and D

4 A and D

S F. and 11

6 A'

e Trim Z-transformer

Trirn adder

Adjust the generator so that the micro-arnmeter registers 50 divisions. Set themultiplicr gain to 1.00

Conncet /1 to A and balance the adder(the mieroarnmeter must register100 divisions)

Using a referenee voltage, adjust the loadof the adder (potentiometer R). Often aeonvenient value is su eh that a fulI de-flection of the microarnrneter corresponds10 1,00 V

F. Set multiplier to a very highgain and trim it

Leave multiplier set to a very highgain and check trim of Z-transfürmer

Trim generator11

G

F

G

G

D8

7

Sorne caution must be cxercised in the rnanipulation of an electro"I1lctcr af this type, bccause of its great sensitivity to electrostatic fieldswhich is tlue 10 the presence of a FET input stage in [he impedance uans-formcr. CarcfuI shielding of rhe cquipmcnr and care ro avoid rhe productionof rribodcctriciry (synrhcric fiber c1Nhes are great offenders in this respect)arc sufficienr to eliminare an}' serious problem.

Alrhough this apparatus was specifically dcsigned for dIe measuremcntof redox pott'ntials in biological media, its use is much more general. Inparticular the prescnce of a very stable variable power supply ( o( the po-rentiostaric type) wirh a low ourput impedancc is ideal for many electro-

FA6 .\tarmasse

e E

,Z MA

F

Bl G'

H

D

Fig.3. Labeling of the lie points used foc (he trimming and (he calibrationof (he elecuometer.

chemical ser ups; in [har case, ir is clearly convenient to conocer the ap-

paratus in (he configuration: G [O E (see Fig. 3) .Finally, lct us note char (he incorporarían of an electrometer oC this

eype in the automatic equipment previously described3 foc the multichannel

recording of rcdox potentials in biological media could permit to raise sig-ni£icantly (he performance s of (he lauce tcchnique.

REFERENCES

l. 'Ji. M. Clark, Oxidation- redu cliorJ poten tial s 01 organ; e sy slem s.(Williams and Wilkins, Baltimore 1960).

2. C. Marmasse and 11. J. Grosz, Nature, 202 (1964) 94.3. C. Marmasse, Bioteeh. J.lioeng., 8 (1966) 189.4. C. Marmasse, C. R. Aead. Se. Paris, sér. D. 263 (1966) 419.5. C. Marmasse, C. R. Aead. Se. Paris, sér. D. 276 (1973) 3215.

RESUMEN

Se describe la construcción de un electrómetro con una impedancia deentrada de lOS~tn muy conveniente para la medición de potenciales redox en

medios biológicos.