# Analytical Applications of Reactions of  Iron(III) and Hexacyanoferrate(III) with 2,10-Disubstituted Phenothiazines

Authors: Helena Puzanowska-Tarasiewicz; Joanna Karpińska; Ludmiła Kuźmicka
Journal: International Journal of Analytical Chemistry
 (2009)

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## Abstract

The presented review is devoted to analytical applications of reactions of Fe(III) andK3[Fe(CN)6] with 2,10-disubstituted phenothiazines (PT). It was found that iron(III) and hexacyanoferrate(III) ions in acidic media easily oxidized PT with the formation of colored oxidation products. This property has been exploited for spectrophotometric determination of iron(III) ions and phenothiazines. Some flow-injection procedures of the determination of PT based on the oxidation reaction by means of  the above-mentioned oxidants have been proposed. In the presented review, the application of 2,10-disubstituted phenothiazines as indicators in complexometric titration of iron(III) as well as procedures of PT determination based on generation of ternary compound in the system Fe(III)-SCN−- PT was also described.


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## Body

## 1. Introduction

Phenothiazines
constitute one of the largest chemical classes of organic compounds in official
compendia. Over 4 thousands compounds have been synthesized and about 100 have
been used in clinical practice [1]. 2,10-Disubstituted phenothiazines are very
important drugs which are widely used in psychiatric treatment as
tranquillizers. Invention and introduction of phenothiazine derivatives into
treatment of mental disease has changed the modern psychiatry. This fact has
improved the life style of patients and allowed quick development of ambulatory
system of treatment for such sickness. The common use of phenothiazines has
generated the need for
fast and reliable methods for quality control of phenothiazine pharmaceuticals
and monitoring them in clinical samples. Over fifty years of the medical use of
phenothiazines have resulted in countless number of analytical procedures
devoted to resolve this problem [2, 3].Phenothiazine derivatives are interesting from analytical point of view due
to their characteristic structure—the presence of
chemically active sulfur and nitrogen atoms in positions 5 and 10 and
substituents in position 2 and alkylamine side chain atN10 atom. Phenothiazine
and its derivatives are characterized by low ionization potentials [1, 4]. They
are easily oxidized by different chemical, electrochemical, photochemical, and
enzymatic agents with the formation of colored oxidation product—intermediate
cation radical [1]. Colors of formed intermediate depend on a presence and a structure
of substituents in positions 2 and 10 (Table 1).Table 1
The
structures of 2,10-disubstituted phenothiazines studied with Fe(III) andK3[Fe(CN)6]
and colors of their cation radicals.
No.Trivial nameColor of oxidation products (radicals)–R1–R21Promazine–HCl–HOrange2Chlorpromazine–HCl–ClRed3Levomepromazine–C4H4O4–OCH3Violet4Methopromazine–C4H4O4–OCH3Violet5Perazine–HOrange6Trifluperazine–2HCl–CF3Orange7Propericiazine–CNRed8Perphenazine–ClRed9Fluphenazine–2 HCl–CF3Orange10Thioridazine–HCl–SCF3BlueThe run of
reactions with the oxidants (e.g., Fe(III),[Fe(CN)6]3−, Cr2O72−,
IO3−, IO4−, BrO3−,
H2O2, chloramine T) has been studied and employed for determination of
used phenothiazine or oxidant. The stability of oxidation products depends on
acidity, concentration of oxidizing agents, time, temperature, and the presence
of some salts [5]. Recently the redox properties of 2,10-disubstituted
phenothiazines radicals have been studied by Madej and Wardman [6]. They have established
the reduction potentials of phenothiazine radicals and equilibrium constants
using a pulse radiolysis and a cyclic voltammetry. The further oxidation leads
to a generation of colorless sulphoxide. A lot of published works have been based
on the oxidation behavior of 2,10-disubstituted phenothiazines [1, 7, 8]. The
oxidation involves a series of one-electron steps providing free radicals and
cations [1]. The distribution of π-electrons
in the 2,10-disubstituted phenothiazines, according to theoretical
considerations, may lead to the formation of some resonance forms of free
cation radical [9].Phenothiazines have exhibited complexing properties due to the presence
of the condensed three-ring aromatic system and amine nitrogen atom in a side
chain in position 10. They have reacted with some metal ions or thiocyanate
complexes of metals forming colored, hard soluble in water but easy soluble in organic solvents compounds [1, 10]. Some
organic substances (e.g., picric, flavianic acid, alizarin S, brilliant blue,
pyrocatechol violet)
have formed with 2,10-disubstituted phenothiazines colored ion-association
compounds sparingly soluble in water, but quantitatively extracted into organic
phase [10, 11]. The conducted spectroscopic studies have confirmed ion-association
nature of these conjunctions [10, 11]. Phenothiazines also have created charge-transfer
complexes with nitroso-R-salt [12] and chloranilic acid [13].In our earlier works, it has been found that 2,10-disubstituted
phenothiazines are useful as redox indicators [8, 19] and spectrophotometric
reagents [10, 11, 20]. Some elements have a catalytic influence on a run of
oxidation of phenothiazines [21, 22]. The catalytic effect of presence of
iodide, nitrite, vanadium, and iron ions on the reactions of phenothiazines
with KBrO3, H2O2 has been described by Mohamed
[23] in his dissertation.After careful
analysis of articles concerned with phenothiazines determination, it could be stated that iron
ions and its anionic complexes are reagents most often used for this purpose.
The mild oxidation potential of Fe(III)/Fe(II) couple and stability of its
complexes make them very convenient reagents for phenothiazine derivatives
assay. The oxidation reaction conducted in the system Fe(III)-NO3−-CIO4− (Forrest’s reagent) is still used for quick examination of presence of
phenothiazines in studied sample [24]. Taking the above-mentioned facts into account, we have
decided to get together the most important information focused on analytical applications of the
reactions of 2,10-disubstituted phenothiazines with Fe(III) and [Fe(CN)6]3− ions.


## 2. Reactions of Phenothiazines with Iron(iii) and Hexacyanoferrate(iii) Ions

As mentioned above, the most important property of phenothiazines is
their susceptibility to oxidation by many oxidizing agents, for example,
Fe(III),[Fe(CN)6]3− with the formation of colored
oxidation products (free radicals) in acidic media [8, 21]. The stability of the
oxidation products depends on the nature of the substituents at positions 2 and 10 (Table 1)
[25]. The stability of 57 radicals has been studied by Levy et al. [26] using ESR
method and the Hammett metasubstituent constant. The kinetics [27] of oxidation reactions of
eight phenothiazines with [Fe(H2O)]3+ and [Fe(CN)6]3−
and an influence of micellar system [28] on the run of studied processes have
been investigated by Pelizzetti and Mentasti [27]
and  Pelizzetti et al. [28].Basavaiah and Swamy [29] have studied oxidation reaction of five
phenothiazine derivatives with hexacyanoferrate(III). The reduced
hexacyanoferrate(II) reacted further with ferrin forming ferroin. The
measurements of absorbance of final product allowed to determine studied
compounds in the concentration range 1–12 μg/mL with the
molar absorption coefficient ranged from 2.08×104 for chlorpromazine
to 3.49×104 for prochlorpromazine.In our previous works [14, 15], we have
described the optimal conditions for the formation of colored products of
2,10-disubstituted phenothiazines with FeCl3 and K3[Fe(CN)6]
in acidic media. The absorption spectra of these products in aqueous solutions have
been recorded. The spectra of the nonoxidized and colored oxidation product, for
example, promazine
hydrochloride (PM) are given in Figure 1. We did not obtain the spectrum of
sulphoxides using FeCl3 [14] and K3[Fe(CN)6] [15]
as oxidants.Figure 1
UV-VIS spectra of 1-colored product
of reaction PM-Fe(III) (CPM=3⋅10−4M, CFe(III)=3⋅10−4M; 2-nonoxidized
form of PM (CPM=4⋅10−4M).The investigations in the UV-region have testified that the reaction of
PT withFeCl3
or K3[Fe(CN)6] proceeds only to the first
steep, for example, promazine
(Scheme 1).Scheme 1
The oxidation of PM by means ofFeCl3.The presence of iron(II)
after the oxidation of phenothiazines studied has been confirmed by means of
2,2′-bipirydyl [15, 17]. A reversible nature of oxidation process of
phenothiazines has been reentered by the electrochemical method—cyclic
voltammetry [14]. Figure 2 shows a cyclic voltammogramm for promazine. As
can be seen, promazine exhibits an oxidative peak at 0.55 V.
Acorresponding reductive peak (0.47 V) appears when the polarization
of electrode is
reversed. This suggests that the reaction can be regarded as reversible (Scheme
1).Figure 2
Cyclic voltammogramm of promazine
(PM) at Pt electrodeCPM=5⋅10−5M,
in a mixture of 5⋅10−1MK2SO4 and 5⋅10−1MKHSO4 (pH=1.3), scan
rate 50 mV/s; in potential window 0.0–1.05 V.From already used various oxidants, iron(III) ion has been chosen as a
mild oxidation agent for spectrophotometric determination of 2,10-disubstituted
phenothiazines. The formal redox potential of the Fe(III)-Fe(II) couple (E°=0.77 V, v SME) does not allow further oxidation of the colored free radicals to the uncolored
sulphoxides [9]. The formed oxidation products are stable. These properties have
been exploited in chemical analysis [1, 8, 19, 20]. The oxidation properties of iron(III)-phenothiazines
system have been employed for the spectrophotometric determination of iron(III)
ions [16] and diethazine [18].


## 3. Application of The Reactions in Fia Systems

Some flow injection procedures have been described for the determination
of PT. They have been
based on the oxidation reaction of 2,10-disubstituted phenothiazines with
iron(III) or hexacyanoferrate(III) (Table2).Table 2
Determination of some PF with Fe(III) andK3[Fe(CN)6] using flow injection methods.
OxidantCompoundDetermination range    (μg mL−1)Pharmaceutical formulationRef.FeCl3Promazine10–130Thioridazine10–130Promazin
(injection)[14]Fe(CIO4)3Promazine6–128Chlorpromazine6–124Largactil
(tablets)Levomepromazine7–133Promethazine6–117[15]Fluphenazine5–312Phenergan
(tablets)Thioridazine11–230Majeptil (tablets)Thioproperazine12–248[15]Trifluoperazine12–230Promethazine0,5–8,0Syrup, cream
and tablets[16]Trifluoperazine0,5–10K3[Fe(CN)6]Promazine2,5–25Promazine (injection)[17]Chlorpromazine1–2UrineFluphenazineThioridazine[18]PromethazineMethotrimeprazinePhenothiazines solutions have been injected into a stream of distilled
water, which has been merged with the stream of iron(III) chloride in
hydrochloric acid [30] or with the
stream of iron(III) perchlorate in perchlorate acid medium [31]. One of the
used FIA manifolds
is presented in Figure 3.Figure 3
Schematic diagram of the FIA manifold used for the assay of promazine; R:
iron(III) in hydrochloric acid solution; CS: water; P: peristaltic pump; S: samples
injector; C: reactor; D: spectrophotometer adjusted to the corresponding
wavelength of the oxidized form of phenothiazines; X: recorder; W: waste.Another method for determination of promazine [32] has been proposed by
Kojło et al. This assay has
been based on promazine oxidation by K3[Fe(CN)6]
previously retained on anion exchange column. The oxidation has been carried
out at room temperature in aqueous acidic medium.The flow injection (FIA) methods are preferable to other conventional
methods because they are fast (from 50 to 200 samples studied per h) and
precise (RSD values ranging from 0.6 to 2.5%). Additional advantage of
flow methods is a possibility of combination with preconcentration of assayed phenothiazine
on sorption column mounted on line [33]. Another applied approach is the use a detection
cell filled with suspension of an appropriate sorbent (so-called a solid spectrophotometry mode) [34].


## 4. Application of Phenothiazines As Visual Indicators in Complexometric Titration

2,10-Disubstituted phenothiazines are useful
redox indicators. The radical cations, which are stable enough under acidic
conditions, exhibit quite intense color [1, 4, 6]. This property allows employing
phenothiazines as redox indicators in many redoxometric determinations. The
values of reduction potentials of some PT established by Madej and Wardman  [6] and Gowda and Ahmed [35]
are given in Table 3.Table 3
Reduction potentials of PT.
PhenothiazinesmVPhenothiazinesmVChlorpromazine*860Propericiazine*966Promethazine*925Trifluoperazine880Diethazine845Prochlorperazine799Thioridazine*789Butaperazine865D*etermined by Madej and Wardman at pH ~ 5–7 [6]; others
values established by Kojło et al. in 0,5 MH2SO4 [32].Phenothiazines have been used as indicators
for complexometric determination of iron(III) with dissodium versenate [36].
They form with Fe(III) ions colored oxidation products (red, orange, or blue).
The addition of dissodium versenate to the titrated solution containing
iron(III) solution and phenothiazines as indicator has caused a change of the
test solution in end point of titration as shown in Table 4.Table 4Colorless→redColorless→orangeColorless→blueChlorpromazinePropericiazineThioridazineDiethazineTrifluoperazinePromethazineProchlorperazineButaperazineThe usefulness of PT (chlorpromazine, promazine, perphenazine,
methopromazine) as redox indicators in chromatometric determination ofK4[Fe(CN)6]
has been described by Puzanowska-Tarasiewicz et al. [38].Phenothiazines indicators are superior to conventional indicators (e.g.,
ferroin, variamin blue). They give sharper end point and act over a wider range
of acidity than other
conventional indicators.


## 5. Application of Complexation Reactions in Assay of Phenothiazines

As it was  mentioned in introduction
section, phenothiazines show the ability to create stable compounds in reaction
with anionic complexes of metal ions, some organic anions or withπ-electron
acceptors.According to Ozutsumi et al. [39], the formation of the thiocyanate-iron(III)
complexes in aqueous solution and the development of the red color are related to [Fe(SCN)]2+, [Fe(SCN)2]+, [Fe(SCN)3], [Fe(SCN)4]−,
[Fe(SCN)5]2−, [Fe(SCN)6]3−.
Tarasiewicz [37] has found that one of these complexes reacts with
2,10-disubstituted phenothiazines forming red-brown compounds. The optimal conditions
for the formation of the compounds have been established and the composition
determined. The absorption spectra have been recorded in UV-VIS and IR regions [37].
On the basis of obtained data, the following reaction course in PT-Fe(III)-SCN− system has been suggested:
(1)Fe(III)+mSCN−→[Fe(SCN)m]n−,PT+H+→(PT•H)+,(PT•H)++[Fe(SCN)m]n−→(PT•H)n[Fe(SCN)m], wheren=2,
m=5.The spectral properties of these compounds suggest that the formation
of color compound occurred due to interaction between opposite charged ions
(large phenothiazines cation and an anionic thiocyanate complex of a metal).
The obtained color precipitate is quantitatively extracted with chloroform or
dissolved in acetone with formation colored and stable solution. This property has
been the basis of sensitive extractive-spectrophotometric or spectrophotometric
method of determination of phenothiazines [37]
(Table 5).Table 5
Extractive-spectrophotometric
and spectrophotometric methods of determination of some of 2,10-disubstituted
phenothiazines.
Organic phasePhenothiazinesDetermination range(μg mL−1)Ref.ChloroformChlorpromazine120–300Levomepromazine140–400[37]Promethazine160–550AcetoneChlorpromazine20–150Levomepromazine20–300[37]Promethazine60–400Chlorpromazine and some phenothiazines react with ferro- and ferricyanate
ions [40] and nitroso-ferricyanate
[41] to form ion association-compounds sparingly soluble in water. The
composition of these compounds has been established and physicochemical
properties have been investigated.Valero [42] has stated, using UV-VIS
spectrophotometry, that iron(III) forms ternary complexes with pyrocatechol
violet (PCV) and chlorpromazine (CPZ) which compositions have been established
as: Fe:PCV:CPZ = 1:2:3
and Fe:PCV:CPZ = 1:3:4.
The last complex can be used for spectrophotometric determination up to 1.6 ppm
of iron(III).


## 6. Other Applications

Some ions ofd-electron elements exhibit a catalytic effect on the oxidation of
2,10-disubstituted phenothiazines [21]. Fukasawa et al. [22] have described a
spectrophotometric determination of trace amounts of iron by its catalytic
effect on the thioridazine-H2O2 reaction. It was found
that others d-electron ions of metals
have the catalytic effect on phenothiazine reactions with H2O2 [21].It is known that stability of color
cation radical depends mainly on oxidation agent used. In the case of strong
oxidant, the color of radical disappears quickly due to the second step of
reaction which leads to the formation of a colorless sulphoxide. This effect resulted in
decrease of sensitivity of assay and reproducibility. In purpose to improve
these analytical properties indirect methods of phenothiazines determination have
been proposed. One of them has been described by Basavaiah and Swamy [43]. They
have applied potassium dichromate and iron-thiocyanate for spectrophotometric
investigations of phenothiazines (chlorpromazine, promethazine,
triflupromazine, trifluoperazine, fluphenazine, prochlorperazine). They have used
a combination of dichromate and iron(III)-thiocyanate system for the
determination of phenothiazines (Scheme 2).Scheme 2
Reaction scheme showing the formation of iron(III)-thiocyanate complex and
correlation of the latter’s concentration with phenothiazine drugs
concentration.Potassium dichromate as a
strong oxidizing agent (coupleCr2O72−/Cr3+E0=1.33 V, versus standard hydrogen electrode) oxidizes 2,10-disubstituted phenothiazines via colored
radical cation to a colorless sulphoxide [43]. The excess of used oxidant has
been further reduced by iron(II) ions. Next, the thiocyanate ions have been
used for quantification of produced iron(III) ions. It has been stated that the
absorbance of iron(III) thiocyanate solution is proportional to amount of the determined
phenothiazines. The sensitivity of the proposed method has been the best at the
molar ratio K2Cr2O7:PT equal to 1:6 at room temperature [43].


## 7. Conclusions

Based on information gathered in the presented review, it can be
concluded that iron(III) ion and its anionic complexes are valuable reagents
useful in an analysis of phenothiazines (PT). The mild oxidation potential of
iron(III) andK3[Fe(CN)6] allows quantification of
phenothiazines in batch and flow systems.
The proposed methods are characterized by simplicity, sensitivity, and
good precision. The determination of PT by flow injection (FIA) methods is
preferable to other conventional methods because they are fast (from 50 to 200
samples studied per hour) and precise (RSD values ranging from 0.6 to 2.5%).The ability to crate ion-pair
compounds can be employed for selective and sensitive determination of iron ions(III)
and phenothiazines as well.



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