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An inexpensive, very high impedance digital voltmeter for selective electrodes



Design and construction of an instrument that exceeds in accuracy and input impedance most expensive commercial pH meters and potentiometers. Keywords (Audience): First-Year Undergraduate / General
§;r Ene:rpeli:siire, !isr!í *!i3ll lmpeda*ce Elglta! Lrci?melier
6ñry (.1,rr =¡.i¡: :15 !; ié::¡r 'Jv ñ"r'.,: lci
ti¡! .J-r"vr,rLi i-¡*.¡Ji!'¡J-ruat
Frori.ia Sla¡c Universi'y, fall.liarsec. FL 32:10€
'j'i,," imi,.n.»cc of ¡nosL .o¡irer.:.1 tsl¡r:, clectrodas i: i¡,
11,c. ;(x:ir' or iClfl, {\e.'lecüodesr¿lc};;in'
,j.¿.r.t!.i1!301,..1no¡c I iolln¡or.r ilt ord.r I o r,rchsür!
lh r di,i.,enc., pol.',lirlbc§\¿er ! s.isirs ¡nd a rrle¡elce
rle.trd,. {,¡!! }rlr ¡ r;if.h hiÍ|rr ¡j,lju: in\xdrnc€ ¡hr.:' Ii'e
.i' .ade $jth the hi{1..1 i t,..j¡nce (i.?, >> i0, C).'Ih€
¡.¡ n¡, rbr ll,ir;s se.¡) i)i i'isu¡c i sLer? fassuniirg pu¡;ly 1.-
' ,... ¡.(l i l./n"l
,. D,-iu; the 3cl¡ar (uc]:nosn) e!€(tf..le prrcnt:¿l¡ 8., thc
.lecl¡..1€ r€srsl.rn.?, i,, ths poten¿;al read rl the ;oltmeter:
r¡. l|e ¡crislLrr.É ol'"li! v6lt,nete¡
-r'r:4 ¿.!ual pol.-rtli.lisI¡ighe¡¡h¿r rhe r,e¡sr¡!,.d .n,1 by s
;. rr ",r!.al to th,r r¡Uc bet§r.r) the irip!dr:,"c otth loi'úg.
",,,r(e ¿i d 111rt.f th¡ m.asuring i¡¡struú€Ii
1,, rr<.ljce, vc:._ ,t¿rs dc nor beLrle l;k. ¡esistors, a3 far
:r! ii,?,,: ir.iie ¿a.'c ji "cnc€, Ded, bur r¡rhe. drain smnll cur-
ia,rt.r (in,)ris i)ia: r!ricnls) \hich dev¿iop Ec.oss th1 ele.
irod¿s.a rolrDl;ár [ropo.tio¡i3l to their rcsi:tance. T]pic¿l
¡rJt qurlill pH ¡Jerersa¡e r¿te(iNith ¡n inpuiofr:.t cü.renr
¡ I l,rss tlraD l0-r:' ,i. it ca¡ be eásilv calculatcd th¡i suci a
:is,rrt aer,¡eien* art,.ior o'rrr ic 1.,¿ !'rhcn nte.sui¡,í 10, r,rv
. . ,:r ¿ typrcai. r-,r'b¡,ñe clrc ¡úqe )f toe 9 res¡s.. n.e
TÉa ccnst¡u.lior of inei!.ns,\ e iliillal ph nete.s \ras ¡¿.
... il!'iliusi rlcd io d.rail i, Til,s "rouit^*Á¡,.1 Tiris ,)ape¡
¡escribes ¡n rnJl¡'r¡.-¡c)r L rsh ir¡ pr.eeds, borh in accu¡¿cJa¡d
ir frr n¡rped¿nc€. üe meters descli)rd ju rote 2 as §€ll a.
f o![ com,¡re¡.ia! pH mer,:rs a»d Dotcnrirmc(c¡. rt ¿ cost o1'
abrüi Sr50 1ll.j -: I ). r:hji s;in ple comoac'" d:rir al l'oltmete¡ hal
il I '^ r re¿olulior, on a i.2 i'sral€. a,rd an i¡)put offsot cutr:nt
oi;rss lha¡r I(l- a.r\ \". rpplied itsucccss¡ully in lhe co¡rst
olr .rud!inlolvinjli-lrellerelopmcntof ¿ u.¡riuú.selecti\t
¿!.c!¡oáe of tlic rcaied wirt §pe,3 \rhere th. ve.) hish iD-
!.d¡L.e of rho elef¡oCe5 c¡used illaccuúLc.e¡dins on corr-
iha ilrtrrn,¡h¿ (FrS. ::), úorsisis es5¿¡tial¡J of only t1'.
t,11.,: a rcry hish,rnped:r1co hlbrid opc{lional 0mpljlicr
(li')i8-01/4, Ilrcr'§ii) used ¡s a lol'rE¡: follo$e¡, áDd a .trl,
ir,ti1. Lfi d'-r,l'r p-nel nrcrer (llJr-.1;00, T.xmrte). Thr
-.r\t,li',". is 'ircd \1i1\ a ,nnriñunr i¡!\rr oirseL óur¡ent of
i0 L' Á, §l¡ilc the di! a\', oi Lhe ¡ur )ze!)iDs t..r,e, is rated
,.:i,1. A p¡c.ij;o.r iI ('0lt'r oi-¡c¿rl;oK *: rljrl;ts ^nd is sctlo
oI..rtc¡l tl3i¡99Vlullsc¡,ci0.l nrv resoiuric¡,). A 10i0 f')
resi:ro¡ !,rotr(G thc rp!:. i ,.r-u,ro v¡.;.Li! porcDli,)nrci.¡
¿dirsls r1,,,.¡,.,rtf*: of lhc ot)r¡alioral añpi¡¡-ici.,\ pcicn-
: i¡rn¡':, t:.,¡ tilj tiIe .q1rl¡{:,. ,r't¡c,rxiIi ir D¡ovid.d itr rhe
Ci:rI ry ir-»clr,ret¿r. r', fili!r ie r)h..d or tl,c i¡repo(er suppl)
¡¡rd. il¡ oiC$ 14 rnjlliln ir r :r,ira. ., l)o,r.,r n,!,Dlrsepár¡tcd f¡on
llt dirir,ri dr:p'¿y l¡.di ih: otx.'rli ,, ¡; .rf.¡ lill{.rr c n,rist,¡!l
-riI :¡\':,,!r.rr(.trlf.r(1, rnt.,:,'\'r,, zf..rr :.u.. s,
Care §ás t¡iier¡ to prevent boih leakrge of tl¡e sis¡ral tosrouDd
¿ño r, frequcn.l nois?. An n-¡ls cor¡nector rrith
Telicr¡, insulator sfls used for th€ hish inpcdancc inpul, thr
sif¡r¿l.Drlh from lhc cor)noctor to the input rcs;stor ro lhe
a¡nlli¡cr pin (as all in solid copper Nire h air, a¡rd all lhe
crirical surr¿cÉs vere c¡reiuul degreased. The ope¡itiorr.rl
sinpliherwas !»ounted upsirle downorr ¡cooljns f¡rn slued on
Fi.L-t 1. A fedrvoriñoté¡ ¿lrowr a finire cúr.nt to for, which iñt¡oducés ¿n
rrgÚo2 Jiih imp4rl¡ñc. diqilál vo¡lmor6.l
I Der¡jf,r,Íi,-, A J:¡l"j' P,i,icplrs.l Et-'ci.¡'. tirsrrn.!r.i.r;
2ra ¿o.. '.r ú. 5.uó'lc'5 Co.,¡.!i) ch'l.C.r:ir. 197S. r. 2Us
?'/, 6r'n¡ l). rod,¡" (i.rhld. a.j í,o(t, t., I H.., C}ttt'.
Erúc 5e, ü¡,1rt82).
tr crior¡rrr. ( -ir¡r rr . .r.'¡J.xrJ t-jj!,,,r , ¡1d rj n, i. .j. c. tc t,!
' zo: P.w9 §uo9ry. I5 v, li0m^
trh f.!r.n!o¡¡ re.,2( ( ii
oro¡¡¡oiJl Ar'f,[¡'e,. (it I nsaa ^
l'¡'.'! 1r,.¡ r0 Oillir$ 1984
Shrcc lhc inln,t ol ihc S500 o¡rftntion¡l ¿¡rtliiilr is iDle¡-
Mll)r l) tu)r c.l.i l b!' Z¡rrer (lir)dcs, no c\u¡o iD¡r:; prccrutio¡s
h¡»d Iinr or sold cri¡it a¡c nce(l« I t¡'t,r¡ver¡t hr..rkdosn dr¡e
ro (lis.h.r¡lc oist¡tic e)c.r¡i.ity, as it iu usu¡llr Ihe llsc ñith
ii0S rleliu,!s.'1be ¡)pxt 10¡o 1l rcsillor ii »ccdrrl (o lrcverL
,)cc¡sirrlll out r)l-r¡¡gc potenti¡ls lronr iDdu.iig dcslrt¡ciive
currenl Ildlr ;r lhc i.tenril t) «n ccl ion .ü.uitry.
'l'hedcvice i:s¡.bI.ñilhin l¿ssihaD¡ minrte¡ficrl¡rni.g
or the po\yer. The noise level k su(h ¡s to pro.lnce rn occa-
sion¡loD..digit I1!¡ctual-ioD ir the lcrslsigDiñcant ii{urc. The
inped.¡ce w¡s leslcd by mc¡sr¡ring ¡ constnDt loltagc ('1.5
V, \!irh r¡¡d airhuut a Ll0r r l) re"Lr, r i ser(uJ in se,írs on
the inpui. The me¿s¡rred sigo.rl droppcd I.3 ú¡V the in-
scrlion ol Lha resislor, shich ú¡respo¡tds to a cu¡.enl of
3.10-ri A, equilalent to an n¡peda¡r.c oi5.rou f).
¡'isure 3 proves how the impeda¡rcc olthe loltnleter influ-
ences ihe measureDen t. The u¡¡»;um-sele.tive electm(lewas
used riih tbis voh.neter and vith a Corniog 130 pH met€r:
bolh instruments have 0.1 nV resoluiion, and bolh gite re-
producible and readinss, bul rhe Cor¡iig instru-
meDt, which hns a ratcd i¡put oflset cu¡¡ent ol 10-r! A, gave
.ead;ngs consistently 5 to 10 mV loser than ou¡ voltneter.
This rese¡rch pas suppo¡ted by acont.¡ctwith lhe Oitice
ofHe¡lih a¡rd Envi.onme¡t. USDOE. The aurho¡ idshes to
thank G. R. Choppin for his help in prep¿.ai;on ol this ar-
Fiqúe 3. Uraryl scl€clivo €lóclrode Polenlial (vs. SCE)8, Urdiuñ(v¡l .oÉ
ce¡r¿lid rrH 4, phrhalaio buÍer, 0.09.1 ¡l
a PVC boa¡d, and the case ivas coo¡ecied io Lhe output to
mi¡¡imizé leak¿se. It was also placed in á sepárate shicldins
bor in order to p.oteci it f¡óm Íoise from the display
I rFqF=?,FlfEr é
ii ib i1 tl Y--¡ii'.,¡ tl
H-!!" E ll tt"il \</
Die analytischen Chemiker entwickeln Elektroden, deren Potentiale sich in Abhängigkeit von Konzentrationsänderungen eines bestimmten Analyten in der Lösung oder in der Gasphase ändern. Diese Instrumente reichen von einfachen galvanischen Zellen der Art, wie wir sie im letzten Kapitel behandelt haben, über ionenselektive Elektroden, die etwa die Größe Ihres Kugelschreibers haben, bis zu ionensensitiven Feldeffekttransistoren, die gerade einige hundert Mikrometer groß sind und die man in einem Blutgefäß unterbringen kann. Die Messung von Zellspannungen zur Gewinnung chemischer Informationen heißt Potentiometrie.
This review presents the significant developments in the field of ion-selective electrodes (ISE). Exciting advances in all areas of ISE methodology have been reported and are covered. Specifically, we review work that has been published between the Fall of 1983 and the Fall of 1985. A manual search of major analytical journals and a computer search of Chemical Abstracts have been employed to collect the presented information. We apologize in advance for any papers that are not included because they were inadvertently missed in our search.
In this paper, pulsed streaming potentials generated in plastic microfluidic channels are used for the label-free detection of some model analytes. The microchannels are fabricated with the commodity plastic cyclic olefin copolymer (COC), and the detection signal arises from a change in the surface charge upon analyte adsorption on the modified microchannel surface. The role of the surface modification is to confer the microchannel with a predetermined charge and a particular specificity toward the adsorption of the target analyte. In this work, several target probes displaying different levels of specificity were investigated. Heparin and streptavidin were detected by adsorption on microchannel surfaces modified with protamine and biotin, respectively, whereas bovine serum albumin (BSA) and methylene blue (MB) showed nonspecific adsorption on almost any modified or unmodified COC microchannel surface. The magnitude of the streaming potential was found to be proportional to the liquid pressure and the surface charge of the microchannel in accord with the Smoluchowski equation. Because the relative polarity of the streaming potential is determined by the surface charge, the most straightforward detection with this method occurs when the charge is reversed upon analyte adsorption. This strategy was used for the species described in this work, and the lowest concentrations detected were approximately 0.01 units/mL for heparin (below clinical relevance), approximately 10 (-9) M for BSA, and approximately 10 (-6) M for MB. Unlike the conventional method of steady flow, in this work, the streaming potentials were measured under pulsed conditions of flow and using nonreference electrodes. This approach removes the need of special electrolytes as it is usually required when using reference electrodes, and at the same time, it mitigates the interference of electrochemical drift from the electrodes. Relative standard deviations of approximately 1-2% and measuring times of approximately 10 s are readily attained with this experimental setup. The on-channel modification of the surface was carried out by UV-photografting methods given the significant UV transparency of COC.
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