Ci a ion: González-A jona, D.;
Roldán González, E.; López-Pé ez, G.;
Domínguez Pé ez, M.M.;
Cale o-Cas illo, M. Coulome e om
a Digi ally Con olled Gal anos a
wi h Pho ome ic Endpoin De ec ion.
Senso s 2022,22, 7541. h ps://
doi.o g/10.3390/s22197541
Academic Edi o s: Eladio
Du án A anda, JoséLuis Ál a ez
Macías and Juan Daniel
Mozo Llamaza es
Recei ed: 6 Sep embe 2022
Accep ed: 1 Oc obe 2022
Published: 5 Oc obe 2022
Publishe ’s No e: MDPI s ays neu al
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Copy igh : © 2022 by he au ho s.
Licensee MDPI, Basel, Swi ze land.
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dis ibu ed unde he e ms and
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A ibu ion (CC BY) license (h ps://
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senso s
A icle
Coulome e om a Digi ally Con olled Gal anos a wi h
Pho ome ic Endpoin De ec ion
Domingo González-A jona *, Emilio Roldán González, Ge mán López-Pé ez, Manuel Ma ía Domínguez Pé ez
and Ma ina Cale o-Cas illo
Depa men o Physical Chemis y, Facul y o Chemis y, Uni e si y o Se illa, 41012 Se illa, Spain
*Co espondence: [email p o ec ed]
Abs ac :
In his wo k, a coulome e was de eloped om a digi ally con olled gal anos a . A
simple colo ime e based on a RGB LED was used as a ligh emi e coupled o ligh de ec o s,
while ligh dependen esis ance (LDR) and pho odiodes ha e been de eloped as endpoin de ec o s.
Bo h ha dwa e and so wa e ha e been adap ed om he o iginal gal anos a design. Rega ding
he ha dwa e, new elec ical signal condi ione s ( il e s and ol age di ide s) we e included o
op imize he wo king sys em. The so wa e was de eloped based on an open sou ce A duino UNO
mic ocon olle . The di e en a iables ha con ol he i a ion p ocess a e managed by an add-in
module o Excel da a acquisi ion so wa e ha is eely a ailable. A s udy o he possible a iables
ha in luence he i a ion p ocess has been ca ied ou . The sys em was es ed wi h wo classical
coulome ic i a ions such as iodome y ( hiosul a e, asco bic acid) and acid/base (po assium acid
ph hala e as s anda d). The de eloped sys em is e sa ile as di e en endpoin colo indica o s can
be employed (s a ch and phenolph halein o he in es iga ed eac ions). Di e en expe imen al
a angemen s ha e been s udied: he na u e o he elec odes (P , Ag), ype o cells ( wo sepa a e
compa men s o a single compa men ), and ligh de ec o s (LDR, pho odiode). The in luence o
se e al expe imen al pa ame e s (bo h elec ical, ligh , and in eg a ion ime) was s udied and chosen
o ob ain he bes pe o mance o he comple e sys em. Rep oducibili y esul s below 1% can be
ob ained unde con olled condi ions. In he case o acid/base i a ions, he p esence o a mosphe ic
ca bon dioxide was de ec ed, whose in e e ence was mainly a ec ed by he s i ing a e and he
i a ion ime.
Keywo ds: elec ochemical ins umen a ion; gal anos a ; coulome y; op ical de ec o
1. In oduc ion
One o he mos common analy ical echniques o quan i ica ion in a chemical labo-
a o y is olume ic i a ion. The unknown concen a ion o a compound in a sample is
ob ained by adding an amoun o eac an ha has a as , i e e sible, and quan i a i e
eac ion wi h i . The amoun o eac an added is s opped when he compound unde s udy
in he sample is consumed. Thus, a eliable me hod o de ec ing he endpoin needs o
be p o ided. Mo eo e , he concen a ion o he eac an solu ion o be added has o be
known p ecisely and i s concen a ion kep s able, among o he analy ical conside a ions.
Elec ochemis y, h ough elec olysis echniques, can be employed o he con olled
p oduc ion o nume ous eagen s in si u, which can be used as i an agen s. This is an
especially adequa e me hodology o he case o uns able o di icul o manipula e eac-
an s due o pa icula eac ion condi ions [
1
,
2
]. Measu emen s o he elec ic cu en can be
pe o med wi h g ea p ecision, allowing o accu a e de e mina ion a e y low concen a-
ion le els. In coulome ic me hods based on Fa aday’s law, he numbe o coulombs ha
passed h ough he cell a e quan i a i ely ela ed o he amoun o compound elec olyzed.
The cha ge associa ed wi h he elec ons is used as he p ima y s anda d eagen , gene a ing
he i an in con olled quan i ies wi hin he elec oly ic cell, which a oids he necessi y o
Senso s 2022,22, 7541. h ps://doi.o g/10.3390/s22197541 h ps://www.mdpi.com/jou nal/senso s
Senso s 2022,22, 7541 2 o 15
p epa a ion and he s o age eagen s anda d solu ions. In any case, he elec ochemical
eac ion has o be a single and as p ocess, wi h a well-de ined s oichiome y.
The i s quan i a i e applica ion o elec ic cha ge was pe o med a he beginning
o he 20 h cen u y, bu i was no un il 1938 ha a succession o pape s appea ed wi h
he e m “coulome ic” in he i le [
3
]. These pape s, oge he wi h he ad en o elec-
onic ins umen a ion and he nume ous and asso ed elec o-gene a ed chemicals, led
o he widesp ead applica ion o he coulome ic analysis. A. J. Ba d published an ex-
ensi ely e iew on coulome ic analysis [
4
] and oge he wi h chap e s 11 and 15 in his
popula ex book, bo h cons i u e an excellen s a ing poin o s udy and a ich sou ce
o e e ences [
5
]. The coulome ic echnique shows some impo an ad an ages o e he
classical i a ion me hods such as high sensi i i y, he usage o aa e y low amoun o
eagen equi ed, and/o uns able compounds as i a ion agen s, no p ima y s anda d is
needed and he main eac an (elec ons) can be p ecisely con olled. Ne e heless, some
d awbacks ha can be o e come a e also p esen such as a mo e complex ins umen a ion,
he non-comme cial a ailabili y o speci ic expe imen al de ices, speci ic aining o he
labo a o y specialis , and no mally, a highe budge ha dwa e [6–8].
A li e a u e e iew om 2000 o he p esen on he aspec s o he “coulome y a
cons an cu en ” was ca ied ou . A ound hi y ela ed a icles we e e e enced as an
indica ion o he use ulness o he cons an -cu en coulome y. In he las six yea s, some
a icles based on he use o coulome y, om he analy ical poin o iew, we e published
desc ibing he applica ion o he de ec ion and quan i ica ion o a a ie y o compounds
om me als, pha maceu icals, wa e , an ioxidan s, gases, and ce i ied ma e ials [
9
–
18
].
Fu he mo e, nume ous pape s on coulome y ha e been published in educa ion jou -
nals due o hei ins uc ional cha ac e (see ecen pape s on coulome y o chemical
educa ion [19–21] and he e e ences he ein).
Two me hods can be dis inguished based on he elec ical a iable con olled du ing
he elec oly ic p ocess, wi h ei he he elec ical po en ial o cu en being kep cons an .
In cons an -po en ial elec olysis, he edox eac ion o in e es is con olled by he po en ial
alue, a oiding o he possible elec ochemical side eac ions. When no compound is le o
be consumed by he elec olysis, he p ocess ceases. Thus, no sepa a e signaling/endpoin
de ec ion me hod is necessa y, as he cu en dec eases exponen ially o a ze o alue. The
ime o comple e eac ion is la ge and i is usually di icul o disce n he endpoin om
he esidual cu en [
5
]. Mo eo e , he ins umen a ion is mo e complica ed, mo e ex-
pensi e, and can be applied o a limi ed numbe o subs ances [
22
]. On he o he hand,
cons an -cu en elec olysis allows o a g ea e numbe o subs ances o be con e ed
and i p o ides a s aigh o wa d ela ionship be ween he elec ic cha ge passed and he
elec olysis ime. Thus, he elec olysis ime is di ec ly p opo ional o he amoun o com-
pound gene a ed by he edox eac ion. Mo eo e , he necessa y elec onic ins umen a ion
is simple (a gal anos a ). Thus, a cons an cu en and when he edox eac ion o in e es
eaches comple ion, he gal anos a modi ies he applied po en ial o keep he cu en
cons an , which ine i ably leads o side eac ions. The e o e, a sys em indica ing he end o
he elec olysis ime o he s udied edox p ocess is necessa y.
In any case, o analy ical pu poses, a 100% cu en e iciency is necessa y wi h bo h
me hodologies. One way o p o iding an e ec i e means o con ol, minimizing he side
eac ions, is by moni o ing he e olu ion o he elec ical a iable ha is no kep cons an
du ing he elec olysis. Thus, exponen ial changes in he e olu ion o he cu en when
ope a ing in po en ios a ic mode o sudden changes in he po en ial in gal anos a ic mode
can be indica i e o side eac ions, which lowe he p ocess e iciency. An addi ional
complica ion in designing an elec olysis cell a ises when he chemical componen s o med
a bo h elec odes can eac each o he in he bulk solu ion. In his case, he solu ion a ound
each elec ode has o be sepa a ed by an elec ically conduc ing memb ane.
The coulome ic sys em de eloped in his p ojec was based on an adap a ion o he
o iginal p o o ype o a digi al gal anos a [
23
], used in he cha ac e iza ion o p ima y
and seconda y ba e ies. Ini ially, he same digi al mic ocon olle (A duino UNO) [
24
] was
Senso s 2022,22, 7541 3 o 15
u ilized due o i s low cos , use - iendly p og amming en i onmen , and open so wa e–
ha dwa e en i onmen ha uns on Windows, Mac OsX, and Linux ope a ing sys ems [
25
].
The inpu /ou pu da a by se ial communica ion we e de ec ed/p ocessed by using he ee
Da a Acquisi ion ool PLX-DAQ [
26
] embedded as a con olle in an Excel ype sp eadshee ,
as p e iously desc ibed [
27
]. Thus, he inpu pa ame e s o he ins umen con ol and he
da a acqui ed can be pe o med ia sp eadshee , a oiding he need o upload he so wa e
o he mic ocon olle e e y ime an expe imen al un is s a ed. Mo eo e , he ange o
di e en inpu pa ame e s is alida ed in he sp eadshee , while he signal o he op ical
de ec o is also g aphically moni o ed du ing he ini ial elec olysis p ocess.
Ne e heless, some adap a ions o he o iginal gal anos a ha dwa e/so wa e a e
equi ed o pe o m coulome ic i a ions. Thus, he ba e y connec o s a e now elec ical
e minals o he elec olysis cell. The e minal used o ead he ba e y po en ial du ing
i s cha ac e iza ion is now employed o moni o he s a us o he elec olysis cell o he
de ec ion o inco ec elec ode connec ions, he po en ial obs uc ion o he anode/ca hode
sepa a ion memb ane, and e en o eco ding possible changes ha may be due o he
exis ence o side eac ions.
Di e en elec ochemical echniques can be used as endpoin de ec o s. These a e
based on checking he ab up change o an elec ical a iable a he endpoin . An addi ional
elec ic ci cui wi h a pai o elec odes isola ed om he elec oly ic gene a o sys em is
used: po en iome y (elec ic po en ial agains a e e ence elec ode), ampe ome y, and
dead-s op [
28
]. The use o an independen elec ic ci cui , combined wi h he mic ocon-
olle ’s di icul y in eading di ec ly nega i e alues o ol ages, complica es he endpoin
de ec o design.
On one hand, he spec ome ic de ec o s a e based on a signi ican change in colo
(UV–Vis), ei he by he addi ion o an indica o o by he colo change in he elec oly ic so-
lu ion i sel . No wi hs anding i s e sa ili y, he implemen a ion o he spec opho ome ic
echnique is gene ally mo e expensi e. A possible way a ound such budge a y cons ain s
esul ing om using spec ome y wi hou a big loss in lexibili y can be esol ed by he
use o a colo ime e o he isible de ec ion [
10
,
20
,
25
]. In his wo k, a simple colo ime e
was de eloped based on a RGB LED as a ligh emi e , coupled o a isible ligh de ec o as
a pho o esis o (LDR/pho odiode).
The comple e sys em was es ed o classical iodome ic and acid/base i a ions
( hiosul a e, asco bic acid, pe chlo ic acid, and po assium hyd ogen ph hala e). S a ch
and phenolph halein we e used as he isible endpoin indica o s, espec i ely. Mul iple
a angemen s o elec ode ma e ials (P , Ag), elec oly ic cells ( wo sepa a ed hal -cells o
a single compa men ), ligh de ec o s (LDR, pho odiode), and se e al combina ions o
expe imen al pa ame e s (bo h elec ical, ligh and in eg a ion ime) ha e been explo ed
o ob ain he bes pe o mance om he de eloped coulome ic sys em. Some impo an
ad an ages can be achie ed wi h he p oposed design. On one hand, he usage o a
mic ocon olle allows o e y p ecise con ol o he elec olysis ime, and on he o he
hand, he open sou ce sys em educes he ha dwa e budge equi ed o he i a ion de ice.
The e sa ili y o he sys em allows o he de elopmen o o he ypes o applica-
ions using gal anos a ic me hods [
3
], o example, he measu emen o he hickness o
me allic coa ings. The p oposed sys em allows he po en ial di e ence applied be ween he
elec odes o be moni o ed simul aneously wi h he gal anos a ic p ocess. An app eciable
change in he po en ial alue will indica e ha he elec olysis o he me allic coa ing has
been comple ed. The e o e, aking in o accoun Fa aday’s laws, he densi y o he me al
and he su ace a ea exposed, he elapsed ime will be p opo ional o he hickness o he
me al coa ing. This would equi e a comple e change in he design o he gal anos a ic cell,
he so wa e, and he s udy o he possible a iables in ol ed.
Senso s 2022,22, 7541 4 o 15
2. Ma e ials and Me hods
2.1. Ha dwa e
(a) Gal anos a : The coulome e ha dwa e se up mainly cons i u es he adap a ion o
he o iginal gal anos a p o o ype [
23
,
27
], a lexible op ical endpoin de ec ion sys em, and
an A duino UNO employed as a mic ocon olle o handling he en i e sys em.
The o iginal gal anos a design was based on a scale/in e e ope a ional ampli ie .
The cu en anges we e manually selec ed (jumpe s) by a combina ion o he esis o s.
Swi ching on/o o he gal anos a ope a ion was digi ally con olled. A di e en ial
ope a ional ampli ie (OA) was used o ead he ba e y’s ac ual ol age. The ac ual cu en
alue wi hin he ull scale selec ed was con olled by applying a DC bias ol age. This DC
po en ial was digi ally gene a ed by using a pulse wid h modula ed (PWM) signal [
29
],
which was p e iously passed h ough an ac i e second o de 1 Hz low-pass il e . Thus,
256 di e en cu en alues could be gene a ed o he ull scale selec ed. The pola i y o
he cu en could be con olled digi ally by ac i a ing o deac i a ing i s passage h ough a
ol age in e e . The schema ic design o he o iginal gal anos a p o o ype can be ound
in he Supplemen a y Ma e ials in [23].
Thus, he adap a ion o he gal anos a desc ibed abo e mainly consis ed o he
subs i u ion o he BAT connec ions o he elec odes o he elec olysis cell. The o he
digi al con ols we e kep o adjus he cu en passing h ough he elec olysis cell. Wi h
his con igu a ion, he selec ed cu en , which is passed h ough he elec oly ic cell, was
kep cons an by con inuously adjus ing he ol age applied o he ba e y unde s udy.
Thus, i he elec ic esis ance in he elec oly ic cell inc eases du ing he p ocess, o main ain
he cu en alue, he applied ol age will also inc ease. Mo eo e , in he case ha du ing
he se up o he elec oly ic cell, he esis ance be ween he elec odes is abno mally high,
o example, due o a connec ion ailu e, he ol age measu ed by he di e en ial OA will
inc ease o i s maximum alue o 15 V ( he alue o he powe sou ce). The e o e, by
moni o ing his ol age du ing he elec olysis p ocess, i is possible o de ec changes in he
e iciency o he elec olysis p ocess and/o a ailu e in he elec ical connec ion be ween
he elec odes. In any case, his ol age should be a posi i e alue and i has o be scaled
o he adequa e ope a ion o he mic ocon olle . The co ec pola i y is imposed by using
he digi al con ol o cha ging/discha ging, ensu ing ha a posi i e ol age is ob ained. To
scale his elec ical signal o he s anda d alue o 5 V ( he maximum alue suppo ed by
he mic ocon olle ), a simple ol age di ide was used, gua an eeing a maximum ou pu
ol age o 4.7 V o a 15 V powe sou ce. The ol age di ide ou pu was ead by one
analog inpu in he mic ocon olle .
(b) Elec olysis cell: Elec odes we e placed in a s anda d 150 mL beake . A pla inum
coil wi h an a ea o 1.25 cm
2
was employed as he indica o elec ode. A sil e squa e
shee wi h an a ea o 3 cm
2
was employed as a coun e elec ode o single compa men
cell expe imen s. A cylind ical ube (15 mm diame e ) wi h a i ed glass bo om co e ed
wi h aga –aga gel sa u a ed wi h KCl was used as a sal b idge when wo king wi h
a wo-compa men elec olysis cell. In he la e case, ano he P coil was used as he
coun e elec ode.
(c) Endpoin de ec o sys em: A simple colo ime e was de eloped based on a RGB
LED ligh sou ce [
30
], coupled wi h wo kinds o ligh de ec o s: a pho o esis o LDR [
31
]
and a pho odiode OPT101 [
32
]. The endpoin de ec o sys em allows o a limi ed selec ion
o wa eleng hs, cen e ed close o he h ee maxima o he RGB ligh (460 nm o blue,
520 nm o g een, and 630 nm o ed) o ob ain he maximum sensi i i y depending on
he colo de eloped in he solu ion. Be e sensi i i y is achie ed when he colo o he
RGB illumina ion and he indica o colo a e nea ly complemen a y. The in ensi y o he
ligh emi e colo is con olled by using h ee mic ocon olle PWM digi al pins. In ou
case, a single colo o he RGB LED was u ned on, aking in o accoun he colo emi ed by
he chemical indica o used. The elec ical signal ac i a ing he emi e can be se easily,
and he change in he ligh in ensi y eaching he de ec o has o be scaled and adjus ed o
maximize he ol age change.
Senso s 2022,22, 7541 5 o 15
(d) LDR de ec o con igu a ion: A LDR changes he esis ance om ens o k
Ω
unde
b igh ligh (1000 Lux) o app oxima ely 1 M
Ω
in he da k (<0.1 Lux). They a e eadily
a ailable a low cos bu ha e a d awback because i s elec ical esponse is slow and
no linea . Ne e heless, cheap LDRs possess su icien sensi i i y and can be easily
implemen ed. The pho o conduc i e cell is elec onically a anged in a ol age di ide
con igu a ion in which he ini ial ou pu alue can be p ecisely selec ed by using a couple
o mul i u n po en iome e s: one o ough selec ion and ano he o ine- uning he ou pu
ol age adjus men . The ou pu ol age dec eases as ligh in ensi y inc eases nonlinea ly.
Figu e 1schema ically shows he connec ions acco ding o [33].
Senso s 2022, 22, x FOR PEER REVIEW 5 o 16
he change in he ligh in ensi y eaching he de ec o has o be scaled and adjus ed o
maximize he ol age change.
(d) LDR de ec o con igu a ion: A LDR changes he esis ance om ens o kΩ unde
b igh ligh (1000 Lux) o app oxima ely 1 MΩ in he da k (<0.1 Lux). They a e eadily
a ailable a low cos bu ha e a d awback because i s elec ical esponse is slow and no
linea . Ne e heless, cheap LDRs possess su icien sensi i i y and can be easily imple-
men ed. The pho o conduc i e cell is elec onically a anged in a ol age di ide con ig-
u a ion in which he ini ial ou pu alue can be p ecisely selec ed by using a couple o
mul i u n po en iome e s: one o ough selec ion and ano he o ine- uning he ou pu
ol age adjus men . The ou pu ol age dec eases as ligh in ensi y inc eases nonlinea ly.
Figu e 1 schema ically shows he connec ions acco ding o [33].
Figu e 1. A schema ic o he ol age di ide LDR connec ion. R1: LDR (pho o- esis o ); R2: 100 kΩ,
R3: 1 MΩ, R4 and R5: 10 kΩ mul i- u n o he ough and ine- uned ol age ou pu adjus men ,
espec i ely.
By using his ci cui and employing wo en- u n po en iome e s, he ou pu ol age
can be adjus ed be ween 0 V and +5 V wi h a esolu ion (R4, R5) o abou one milli ol .
This p o ides g ea e sa ili y o he selec ion o he esponse ange o he pho o esis o
elemen .
(e) Pho odiode (OPT101) de ec o con igu a ion: Ligh de ec o de ices and elec-
onic ensembles o g ea e sensi i i y, which exhibi linea esponses, can be employed,
o example, hose based on pho odiodes, pho o ansis o s, and e en a simple LED [20].
Based on he adequacy o i s cha ac e is ics o his applica ion, a pho odiode ( ype
OPT101) was selec ed [32]. Basic connec ions o his de ice we e pe o med ollowing
he schema ics depic ed in igu e 21 in [32] minimizing he numbe o elec onic compo-
nen s equi ed. Minimizing he high- equency noise ha no mally occu s in da a acqui-
si ion when in e acing he digi al mic ocon olle o an analog ligh de ec o o senso is
impe a i e. Fo bo h de ec o s, a simple RC low pass il e wi h a cu o equency o 15Hz
was inse ed be ween he de ec o ou pu and he analog inpu o he A duino [28].
( ) Colo ime ic de ec o a angemen : The emi e and de ec o we e placed a he
bo om o wo s anda d plas ic spec opho ome ic cu e es (1 cm o pa h ligh ). Bo h elec-
onic de ices we e kep in place using a non-conduc ing black oam. The emi e and he
Figu e 1.
A schema ic o he ol age di ide LDR connec ion. R1: LDR (pho o- esis o ); R2: 100 k
Ω
, R3:
1 M
Ω
, R4 and R5: 10 k
Ω
mul i- u n o he ough and ine- uned ol age ou pu adjus men , espec i ely.
By using his ci cui and employing wo en- u n po en iome e s, he ou pu ol age
can be adjus ed be ween 0 V and +5 V wi h a esolu ion (R4, R5) o abou one milli ol . This
p o ides g ea e sa ili y o he selec ion o he esponse ange o he pho o esis o elemen .
(e) Pho odiode (OPT101) de ec o con igu a ion: Ligh de ec o de ices and elec onic
ensembles o g ea e sensi i i y, which exhibi linea esponses, can be employed, o exam-
ple, hose based on pho odiodes, pho o ansis o s, and e en a simple LED [
20
]. Based on
he adequacy o i s cha ac e is ics o his applica ion, a pho odiode ( ype OPT101) was
selec ed [
32
]. Basic connec ions o his de ice we e pe o med ollowing he schema ics
depic ed in Figu e 2in [
32
] minimizing he numbe o elec onic componen s equi ed.
Minimizing he high- equency noise ha no mally occu s in da a acquisi ion when in e -
acing he digi al mic ocon olle o an analog ligh de ec o o senso is impe a i e. Fo
bo h de ec o s, a simple RC low pass il e wi h a cu o equency o 15Hz was inse ed
be ween he de ec o ou pu and he analog inpu o he A duino [28].
( ) Colo ime ic de ec o a angemen : The emi e and de ec o we e placed a he
bo om o wo s anda d plas ic spec opho ome ic cu e es (1 cm o pa h ligh ). Bo h
elec onic de ices we e kep in place using a non-conduc ing black oam. The emi e and
he de ec o cu e es we e ixed acing one ano he and sepa a ed by a hi d one, aised
wi h espec o he o he wo, and kep in place wi h glue. The expe imen al assembly is
depic ed in Figu e 2.

Senso s 2022,22, 7541 6 o 15
Senso s 2022, 22, x FOR PEER REVIEW 6 o 16
de ec o cu e es we e ixed acing one ano he and sepa a ed by a hi d one, aised wi h
espec o he o he wo, and kep in place wi h glue. The expe imen al assembly is de-
pic ed in Figu e 2.
Figu e 2. Colo ime ic endpoin de ec o ensemble (LDR/OPT101_RGB_LED) showing he emi e ,
he ligh de ec o s, and he spa ial a angemen .
In his way, he colo ime e de ec o assembly can be easily imme sed in he cell so-
lu ion, keeping he dis ance and posi ion o bo h he de ec o and emi e ixed. When he
solu ion unde analysis s a ed o de elop a change in colo due o he excess elec ochem-
ically-gene a ed eac an in e ac ing wi h he chemical indica o , he in ensi y o ligh
eaching he ligh senso dec eased and he ou pu ol age changed. When he LDR de-
ec o was employed, he ou pu ol age inc eased, while i dec eased when he pho odi-
ode (OPT101) was used. The elec olysis p ocess con inued un il he ou pu ol age
eached a p ese alue. The colo o he solu ion was kep uni o m by using a magne ic
s i e (S ua CB162).
2.2. So wa e
The gal anos a was con olled by an A duino UNO mic ocon olle , whose desc ip-
ion and calib a ion is desc ibed in he li e a u e [23,27,34]. Howe e , he de elopmen o
he ch ono-coulome y expe imen s no only equi ed he a o emen ioned ha dwa e ad-
ap a ion, bu also he implemen a ion o so wa e adap ed o his ype o expe imen . In
o de o pe o m a p ope coulome e ope a ion, he so wa e has o be uploaded in o he
mic ocon olle and, in addi ion, an Excel shee in which he add-in is embedded has o
be used [26,27]. A simpli ied desc ip ion o he ope a ing so wa e o he coulome e will
be gi en in he ollowing de elopmen , while u he de ails a e gi en in he Supplemen-
a y Ma e ials.
Th ee undamen al s ages can be dis inguished wi hin he p og am, acco ding o he
gene al scheme o he mic ocon olle : he ini ializa ion, se up, and loop p ocedu es. The
i s s age co esponds o he ini ializa ion and assignmen o he di e en pins o he mi-
c ocon olle . In his same s age, he ypes o a iables ha will be used in he expe imen
a e de ined: accumula ed ime, po en ial, and cu en in ensi y. The co esponding alues
o he o dina e and slope co esponding o he linea calib a ion we e also se .
The nex s ep, which is also pe o med only once, ini ially con igu es he de aul al-
ues o he a iables and es ablishes he se ial po communica ion. The esis ance be ween
he elec odes is checked o e i y ha i s alue is low. Thus, he gal anos a is momen-
a ily swi ched ON using a e y low cu en alue. Simul aneously, he ol age de eloped
be ween he elec odes is ead, and i he alue is highe han he one p e iously speci ied,
he p og am ends, indica ing ha he e is an e o in he elec ical connec ion. The expe -
imen al a iables a e hen ead om he Excel shee using he PLX_DAQ se ial commu-
nica ion add-on and a e checked o compliance wi h he app op ia e anges se p e i-
ously. Thus, he cu en inpu alue en e ed in he sp eadshee has o be a alue wi hin
he selec ed sensi i i y ange manually selec ed using he PCB p o o ype gal anos a [23].
Figu e 2.
Colo ime ic endpoin de ec o ensemble (LDR/OPT101_RGB_LED) showing he emi e ,
he ligh de ec o s, and he spa ial a angemen .
In his way, he colo ime e de ec o assembly can be easily imme sed in he cell solu-
ion, keeping he dis ance and posi ion o bo h he de ec o and emi e ixed. When he solu-
ion unde analysis s a ed o de elop a change in colo due o he excess elec ochemically-
gene a ed eac an in e ac ing wi h he chemical indica o , he in ensi y o ligh eaching
he ligh senso dec eased and he ou pu ol age changed. When he LDR de ec o was
employed, he ou pu ol age inc eased, while i dec eased when he pho odiode (OPT101)
was used. The elec olysis p ocess con inued un il he ou pu ol age eached a p ese alue.
The colo o he solu ion was kep uni o m by using a magne ic s i e (S ua CB162).
2.2. So wa e
The gal anos a was con olled by an A duino UNO mic ocon olle , whose desc ip-
ion and calib a ion is desc ibed in he li e a u e [
23
,
27
,
34
]. Howe e , he de elopmen
o he ch ono-coulome y expe imen s no only equi ed he a o emen ioned ha dwa e
adap a ion, bu also he implemen a ion o so wa e adap ed o his ype o expe imen .
In o de o pe o m a p ope coulome e ope a ion, he so wa e has o be uploaded in o
he mic ocon olle and, in addi ion, an Excel shee in which he add-in is embedded has
o be used [
26
,
27
]. A simpli ied desc ip ion o he ope a ing so wa e o he coulome-
e will be gi en in he ollowing de elopmen , while u he de ails a e gi en in he
Supplemen a y Ma e ials.
Th ee undamen al s ages can be dis inguished wi hin he p og am, acco ding o
he gene al scheme o he mic ocon olle : he ini ializa ion, se up, and loop p ocedu es.
The i s s age co esponds o he ini ializa ion and assignmen o he di e en pins o he
mic ocon olle . In his same s age, he ypes o a iables ha will be used in he expe imen
a e de ined: accumula ed ime, po en ial, and cu en in ensi y. The co esponding alues
o he o dina e and slope co esponding o he linea calib a ion we e also se .
The nex s ep, which is also pe o med only once, ini ially con igu es he de aul
alues o he a iables and es ablishes he se ial po communica ion. The esis ance
be ween he elec odes is checked o e i y ha i s alue is low. Thus, he gal anos a is
momen a ily swi ched ON using a e y low cu en alue. Simul aneously, he ol age
de eloped be ween he elec odes is ead, and i he alue is highe han he one p e iously
speci ied, he p og am ends, indica ing ha he e is an e o in he elec ical connec ion.
The expe imen al a iables a e hen ead om he Excel shee using he PLX_DAQ se ial
communica ion add-on and a e checked o compliance wi h he app op ia e anges se
p e iously. Thus, he cu en inpu alue en e ed in he sp eadshee has o be a alue wi hin
he selec ed sensi i i y ange manually selec ed using he PCB p o o ype gal anos a [
23
].
The p ocess o e i ica ion ends when all equi emen s o he a iables a e sa is ied:
sensi i i y, ac ual cu en alue, RGB LED b igh ness, and iming pa ame e s.
In coulome y, i is common o pe o m a p e- i a ion o selec he endpoin alue.
Thus, a small amoun o he sample is added o he elec oly e solu ion and i a ed o
a ce ain de ec o alue. Subsequen ly, when he sample is added, he i a ion will end
Senso s 2022,22, 7541 7 o 15
a he p e iously selec ed endpoin [
35
]. Thus, his p ese alue o he op ical de ec o
co esponding o he endpoin is hen se .
P e iously, he colo o he ligh emi e and he mos app op ia e in ensi y would
ha e been selec ed. The ac ual de ec o le el alue, gi en by he de ec o , is e i ied be o e
he i a ion can be pe o med. Thus, o he LDR de ec o , he ac ual alue should be lowe
han he endpoin selec ed, and he opposi e o he pho odiode OPT101. The ligh de ec o
ype has o be p e iously selec ed, in o de o choose he adequa e p og am code lines
(see Supplemen a y Ma e ials o de ails). Se ing he expe imen al pa ame e s concludes
when he e a e no e o messages on he de ec o le el and equi ed pa ame e s. Two check
ma ks o hese possible e o s ha e o be e ased on he con olle . Finally, he s a us o he
s a /s op swi ch is checked and i s posi ion is held un il he s a o he i a ion.
As he communica ion be ween he A duino and he sp eadshee is pe o med ia
RS232 se ial, some pa ame e synch oniza ion e o s may occu , bu his can be easily
sol ed by ese ing he Excel con olle wi hou losing any da a.
In he hi d pa o he p og am, he p ocess o measu emen , endpoin con ol, and
he es ima ion o esul s akes place. Figu e 3shows a low diag am o hese p ocesses.
Senso s 2022, 22, x FOR PEER REVIEW 7 o 16
The p ocess o e i ica ion ends when all equi emen s o he a iables a e sa is ied: sen-
si i i y, ac ual cu en alue, RGB LED b igh ness, and iming pa ame e s.
In coulome y, i is common o pe o m a p e- i a ion o selec he endpoin alue.
Thus, a small amoun o he sample is added o he elec oly e solu ion and i a ed o a
ce ain de ec o alue. Subsequen ly, when he sample is added, he i a ion will end a
he p e iously selec ed endpoin [35]. Thus, his p ese alue o he op ical de ec o co -
esponding o he endpoin is hen se .
P e iously, he colo o he ligh emi e and he mos app op ia e in ensi y would
ha e been selec ed. The ac ual de ec o le el alue, gi en by he de ec o , is e i ied be o e
he i a ion can be pe o med. Thus, o he LDR de ec o , he ac ual alue should be
lowe han he endpoin selec ed, and he opposi e o he pho odiode OPT101. The ligh
de ec o ype has o be p e iously selec ed, in o de o choose he adequa e p og am code
lines (see Supplemen a y Ma e ials o de ails). Se ing he expe imen al pa ame e s con-
cludes when he e a e no e o messages on he de ec o le el and equi ed pa ame e s.
Two check ma ks o hese possible e o s ha e o be e ased on he con olle . Finally, he
s a us o he s a /s op swi ch is checked and i s posi ion is held un il he s a o he i a-
ion.
As he communica ion be ween he A duino and he sp eadshee is pe o med ia
RS232 se ial, some pa ame e synch oniza ion e o s may occu , bu his can be easily
sol ed by ese ing he Excel con olle wi hou losing any da a.
In he hi d pa o he p og am, he p ocess o measu emen , endpoin con ol, and
he es ima ion o esul s akes place. Figu e 3 shows a low diag am o hese p ocesses.
Figu e 3. Flow diag am o he measu emen and endpoin con ol. The e o condi ions a e also
e i ied: high esis ance and eme gency s op.
The sample olume is hen added o he solu ion and he p ocess is ini ia ed by ac i-
a ing he s a /s op swi ch. Elec olysis a a gi en in ensi y akes place un il he colo o
he solu ion eaches he p e iously selec ed de ec o alue. Du ing he i s elec olysis
p ocess, he de ec o alue is plo ed on an Excel g aph e sus ime, allowing i o be mon-
i o ed. When he selec ed de ec o alue is eached o he i s ime, he elec olysis is
s opped o a ce ain pe iod, ixed as one o he ini ial condi ions. This p ocedu e allows
he colo in he solu ion o become uni o m. Nex , he elec olysis is esumed and i his
nex elec olysis ime is sho e han he alue speci ied unde he ini ial condi ions, he
i a ion is conside ed comple ed and he o al elec olysis ime is de e mined. Comple ion
Figu e 3.
Flow diag am o he measu emen and endpoin con ol. The e o condi ions a e also
e i ied: high esis ance and eme gency s op.
The sample olume is hen added o he solu ion and he p ocess is ini ia ed by
ac i a ing he s a /s op swi ch. Elec olysis a a gi en in ensi y akes place un il he colo
o he solu ion eaches he p e iously selec ed de ec o alue. Du ing he i s elec olysis
p ocess, he de ec o alue is plo ed on an Excel g aph e sus ime, allowing i o be
moni o ed. When he selec ed de ec o alue is eached o he i s ime, he elec olysis is
s opped o a ce ain pe iod, ixed as one o he ini ial condi ions. This p ocedu e allows
he colo in he solu ion o become uni o m. Nex , he elec olysis is esumed and i his
nex elec olysis ime is sho e han he alue speci ied unde he ini ial condi ions, he
i a ion is conside ed comple ed and he o al elec olysis ime is de e mined. Comple ion
o he i a ion in s ages minimizes he e o due o ins an aneous colo a ion exceeding he
p ese alue o he de ec o . The numbe o s ages is limi ed o en cycles.
In he second and subsequen elec olysis eac i a ion p ocesses (s ages), communica-
ion h ough he in e ace wi h Excel is inhibi ed, s opping he simul aneous plo ing o he
g aph o a p ecise con ol o he elapsed ime. Th oughou he elec olysis p ocess, he
p og am moni o s he alue o he applied po en ial o de ec possible inc eases in elec ical
esis ance be ween he elec odes. I his si ua ion occu s, he p ocess s ops, showing an
Senso s 2022,22, 7541 8 o 15
e o message. The manual s op swi ch is also con inuously moni o ed, e mina ing he
en i e p ocess when ac i a ed.
Figu e 4shows a sc eensho o he sp eadshee in e ace o a simula ed measu emen
p ocess using LDR as he ligh de ec o and a esis o as he dummy elec ochemical cell.
Ini ially, he pa ame e s o be used in he expe imen a e se in sec ion B, as indica ed in
Figu e 4. These a e checked agains he accep ed ange (sec ion A). I he e a e no e o s,
he senso le el o he i a ion endpoin is se (sec ion C). Fo he LDR ligh senso case,
du ing he i a ion p ocess, he senso alue inc eases. Thus, he endpoin selec ed mus be
a g ea e alue han ha o he s a ing poin ( his equi emen is checked by he so wa e).
Fo his pu pose, wo po en iome e s (R4 and R5 shown in Figu e 1) we e ope a ed. The
plo in he sc eensho (Figu e 4) shows he e olu ion o he de ec o signal du ing he
p og ess o he simula ed i a ion. Fo hese dummy es s, se ial communica ion was no
inhibi ed, allowing o a ious s ages o be plo ed, which a e labeled by he numbe s 1, 2,
and 3 in sec ion D. The ho izon al ed line indica es he limi alue o he endpoin senso .
Fu he de ails on he measu ing ope a ion can be ound in he Supplemen a y Ma e ials.
Senso s 2022, 22, x FOR PEER REVIEW 8 o 16
o he i a ion in s ages minimizes he e o due o ins an aneous colo a ion exceeding
he p ese alue o he de ec o . The numbe o s ages is limi ed o en cycles.
In he second and subsequen elec olysis eac i a ion p ocesses (s ages), communi-
ca ion h ough he in e ace wi h Excel is inhibi ed, s opping he simul aneous plo ing o
he g aph o a p ecise con ol o he elapsed ime. Th oughou he elec olysis p ocess,
he p og am moni o s he alue o he applied po en ial o de ec possible inc eases in
elec ical esis ance be ween he elec odes. I his si ua ion occu s, he p ocess s ops,
showing an e o message. The manual s op swi ch is also con inuously moni o ed, e -
mina ing he en i e p ocess when ac i a ed.
Figu e 4 shows a sc eensho o he sp eadshee in e ace o a simula ed measu e-
men p ocess using LDR as he ligh de ec o and a esis o as he dummy elec ochemical
cell. Ini ially, he pa ame e s o be used in he expe imen a e se in sec ion B, as indica ed
in Figu e 4. These a e checked agains he accep ed ange (sec ion A). I he e a e no e o s,
he senso le el o he i a ion endpoin is se (sec ion C). Fo he LDR ligh senso case,
du ing he i a ion p ocess, he senso alue inc eases. Thus, he endpoin selec ed mus
be a g ea e alue han ha o he s a ing poin ( his equi emen is checked by he so -
wa e). Fo his pu pose, wo po en iome e s (R4 and R5 shown in Figu e 1) we e ope a ed.
The plo in he sc eensho (Figu e 4) shows he e olu ion o he de ec o signal du ing he
p og ess o he simula ed i a ion. Fo hese dummy es s, se ial communica ion was no
inhibi ed, allowing o a ious s ages o be plo ed, which a e labeled by he numbe s 1,
2, and 3 in sec ion D. The ho izon al ed line indica es he limi alue o he endpoin
senso . Fu he de ails on he measu ing ope a ion can be ound in he Supplemen a y
Ma e ials.
Figu e 4. A sc eensho o he sp eadshee in e ace o es ing he en i e coulome ic i a ion p o-
g am wi h a dummy cell and using he LDR as a ligh senso . (A) Inpu pa ame e s and e i ica ion
anges. (B) Ac ual alue se . (C) De ec o le els se . (D) Measu emen esul s: numbe s 1, 2, 3 e e s
o di e en expe imen al s ages. (E) Add-on module o se ial communica ion. (F) Plo scales.
The comple e p og am code, wi h ema ks, can be supplied by he au ho s upon e-
ques .
Figu e 4.
A sc eensho o he sp eadshee in e ace o es ing he en i e coulome ic i a ion p og am
wi h a dummy cell and using he LDR as a ligh senso . (
A
) Inpu pa ame e s and e i ica ion anges.
(
B
) Ac ual alue se . (
C
) De ec o le els se . (
D
) Measu emen esul s: numbe s 1, 2, 3 e e s o
di e en expe imen al s ages. (E) Add-on module o se ial communica ion. (F) Plo scales.
The comple e p og am code, wi h ema ks, can be supplied by he au ho s upon eques .
3. Resul s and Discussion
In his sec ion, he main esul s ob ained when using ue elec ochemical cell es s
will be p esen ed. The issues encoun e ed and possible imp o emen s a e also analyzed
and discussed. Two common coulome ic eagen s we e elec o-gene a ed in a ious com-
bina ions, assemblies, and di e en con igu a ions [
36
]. All measu emen s we e pe o med
a a con olled empe a u e o 25 ±2◦C.
Be o e using he gal anos a , i is impo an o ha e he cu en calib a ions o he
di e en cu en scales a ailable. P ecision esis o s a e connec ed a he elec ode e minals
o he di e en scales. The sys em exhibi s a mono onic and pe ec ly linea ela ionship
Senso s 2022,22, 7541 9 o 15
be ween he measu ed cu en and he PWM digi al alue, wi h less han 1% e o , inde-
penden ly o he selec ed scale. Conside ing he cu en scale used (sensi i i y), a single
linea eg ession (slope, in e cep ) can also be used o all scales (see he Supplemen a y
Ma e ials o a de ailed desc ip ion o he calib a ion p ocess).
3.1. Iodome y Analysis
The main edox eac ion in iodome ic i a ions is he educ ion o iodine o iodide
and, con e sely, he oxida ion o iodide o iodine:
I2(aq)+2e−2I−(aq)
Thus, iodine is gene a ed a he anode by oxida ion o he iodide, bu he iodine
p oduced can be educed again a he ca hode. The e o e, he coulome ic elec oly ic cell
needs o ha e sepa a e compa men s, as can be seen in Figu e 5. A glass ube con aining
i ed glass a he bo om wi h a laye o aga –aga illed wi h a sa u a ed KCl solu ion,
was used as a sal b idge. In his way, con ec i e eac an anspo om one compa men
o ano he was hinde ed, keeping he elec ical esis i i y o he memb ane low.
Senso s 2022, 22, x FOR PEER REVIEW 9 o 16
3. Resul s and Discussion
In his sec ion, he main esul s ob ained when using ue elec ochemical cell es s
will be p esen ed. The issues encoun e ed and possible imp o emen s a e also analyzed
and discussed. Two common coulome ic eagen s we e elec o-gene a ed in a ious com-
bina ions, assemblies, and di e en con igu a ions [36]. All measu emen s we e pe -
o med a a con olled empe a u e o 25 ± 2 °C.
Be o e using he gal anos a , i is impo an o ha e he cu en calib a ions o he
di e en cu en scales a ailable. P ecision esis o s a e connec ed a he elec ode e mi-
nals o he di e en scales. The sys em exhibi s a mono onic and pe ec ly linea ela ion-
ship be ween he measu ed cu en and he PWM digi al alue, wi h less han 1% e o ,
independen ly o he selec ed scale. Conside ing he cu en scale used (sensi i i y), a sin-
gle linea eg ession (slope, in e cep ) can also be used o all scales (see he Supplemen-
a y Ma e ials o a de ailed desc ip ion o he calib a ion p ocess).
3.1. Iodome y Analysis
The main edox eac ion in iodome ic i a ions is he educ ion o iodine o iodide
and, con e sely, he oxida ion o iodide o iodine:
𝐼(aq) + 2𝑒⇄2𝐼
(aq)
Thus, iodine is gene a ed a he anode by oxida ion o he iodide, bu he iodine p o-
duced can be educed again a he ca hode. The e o e, he coulome ic elec oly ic cell
needs o ha e sepa a e compa men s, as can be seen in Figu e 5. A glass ube con aining
i ed glass a he bo om wi h a laye o aga –aga illed wi h a sa u a ed KCl solu ion,
was used as a sal b idge. In his way, con ec i e eac an anspo om one compa -
men o ano he was hinde ed, keeping he elec ical esis i i y o he memb ane low.
Figu e 5. Schema ics o he wo-compa men coulome ic cell employed o iodome ic i a ions.
Molecula iodine is no soluble in wa e , bu is solubilized in he p esence o iodide,
o ming iodine–iodide complexes such as 𝐼
 o 𝐼
. These anions in e cala e in o he
s a ch s uc u e, o ming a cha ge ans e complex wi h an in ense blue colo , wi h a
b oad abso p ion band in he isible ange be ween 600 nm and 650 nm [37]. A ew d ops
Figu e 5. Schema ics o he wo-compa men coulome ic cell employed o iodome ic i a ions.
Molecula iodine is no soluble in wa e , bu is solubilized in he p esence o iodide,
o ming iodine–iodide complexes such as
I−
3
o
I−
5
. These anions in e cala e in o he s a ch
s uc u e, o ming a cha ge ans e complex wi h an in ense blue colo , wi h a b oad
abso p ion band in he isible ange be ween 600 nm and 650 nm [
37
]. A ew d ops o
colloidal s a ch solu ion added o an iodide solu ion will cause he solu ion o u n a blue
colo in he p esence o iodine. The ligh emi e endpoin de ec o is illumina ed by he
ed ligh o he RGB LED, hus ob aining op imal sensi i i y.
Two ypes o iodome ic i a ions we e pe o med using I
2
as he oxidizing agen : he
classical hiosul a e i a ion is acco ding o he eac ion:
2S2O2−
3(aq)+I2→S4O2−
6(aq)+2I−(aq)
Subsequen ly, he i a ion o asco bic acid is acco ding o he eac ion:
C6H8O6(aq)+I2→C6H6O6(aq)+2I−(aq)+2H+(aq)