Indian Journal of ChemistryVol. 14A, February 1976, pp.
115-117
Interhalogens as Oxidimetric Titrants: Part VI* -
OxidimetricDeterminations with Bromine Monochloride
C. G. R. NAIR & (Miss) R. LALITHA KUMARIDepartment of
Chemistry, University of Kerala, Trivandrum 695001
Received 17 February 1975; accepted 16 April 1975
New analytical methods, employing bromine monochloride as an
oxidimetrfc titrant, aredescribed for the determination of
isonicotinic acid hydrazide, triphenylphosphine, phosphore-thioate,
trithionate, xanthate, dithiocarbamate, reineckate, oxine and
oxlnates of Mg, AI, Mn,Fe, Co, Ni, Cu and Zn.
IN our previous communicationsi-s we haddescribed some
convenient excess-backtitration oxidimetric methods for the
deter-mination of several technologically important com-pounds
using iodine monochloride in 5N HCImedium as the oxidant. In the
present paper, amore reactive inter halogen , viz. bromine
mono-chloride, is suggested as an oxidimetric reagent. Ithas been
shown that, in general, bromine mono-chloride effects a more
extensive oxidation thaniodine monochloride, giving larger
equivalent con-sumption of the oxidant and therefore leading toan
improved accuracy. Analytical methods usingbromine monochloride are
described for the deter-mination of the following: isonicotinic
acid hydrazidetriphenylphosphine, phosphorothioate,
trithionates,xanthates, dithiocarbamates, reineckates, oxine
andmetallic oxinates.
Pioneer work on the use of bromine monochlo-ride as an
analytical reagent has been done bySchulek and coworkers". Schulek
and Burger usedbromine monochloride as a halogenating agent inthe
determination of antipyrine, phenol, p-nitro-phenol and
fluorescein. Other compounds whichhave been determined with bromine
mono chlorideare hypophosphite", hydroxylamine", maleic andfumaric
acids", formic acid!", hydrazine!', hexo-barbital= and thiocyanate,
urea and thiourea-e. Itis hoped that the present studies would
extend thescope of BrCI as a versatile and convenient oxidi-metric
titrant.
Materials and MethodsPreparation of stock solutions of bromine
mono-
chloride - Preliminary studies showed that thepresence of even
small amounts of bromate inBrCl solution causes complications.
Therefore, themethod described in literature was slightly
modifiedas follows, in order to ensure absence of any freebromate.
Potassium bromide (3·976 g) and potassiumbromate (2·7835 g) (molar
ratio bromide/bromate= slightly more than 2) were dissolved
together inwater (125 ml). To this 100 ml of cone. hydrochloric
*For Parts I to V, see references 1 to 5.
acid was added. The solution was shaken well withcarbon
tetrachloride (2 X 10 ml), in order to removefree bromine. Finally
cone. hydrochloric acid (400ml) was added and the solution diluted
to one litre.The solution was stored in contact with a layer
ofcarbon tetrachloride which served to extr e.ct anytrace of
bromine produced in the aqueous phase.Before use, the solution was
shaken well and allowedto settle and the aqueous layer standardized
byadding, to measured aliquots (20 ml), 10 mi 10%aqueous KI,
diluting to 200 ml and titrating theliberated iodine with standard
thiosulphate. It hasbeen noted that under these conditions
brominemonochloride exists as a complex-".
The formation of BrCI in the above preparationmay be represented
as:
2Br-+BrO:i+6HCI-l>-3BrCl+3Cl-+3H20The excess HCI taken serves
to stabilize the BrCI,possibly by complex formation-s. It may be
notedthat stock solutions of bromine mono chloride arereasonably
stable and undergo no change in titre evenfor periods up to two
weeks.
Stock solutions of various compounds to be estimated- Solutions
of potassium trithionate (prepared bythe method of Goehringw),
potassium ethyl xanthate(USSR), sodium die thy I dithiocarbamate
(BDH),sodium phosphorothioate (prepared by the methodof
Kubierschky-P}, isonicotinic acid hydrazide (Koch-Light) and
Reinecke salt (E. Merck) were preparedin distilled water while that
of triphenylphosphine(Koch-Light) was prepared in carbon
tetrachloride.All these solutions were standardized by
proceduresinvolving oxidation with chloramine-T17-20 or
iodinemonochloridet+. Cadmium thiourea reineckate wasprepared by
the procedure of Rulf et aI.21 and wasused as such; the purity of
the sample was checkedby the chloramine-T procedure". Oxine
(Riedel)was dissolved in 2N HCI and the solution stan-dardized by
the bromate-bromide method. Theoxinates of magnesium, aluminium,
manganese,iron, cobalt, nickel, copper and zinc were preparedby
standard methods involving the addition of asolution of oxine in 2N
acetic acid to the hot aqueoussolutions of the respective metal
sulphates underspecified conditions of pH24. The precipitates
were
115
INDIAN J. CHEM., VOL. 14A, FEBRUARY 1976
dried as recommended. Solutions of these oxinateswere prepared
in 2N HCI and were standardized usingthe bromate-bromide
method.
General procedure - All the experiments werecarried out at room
temperature (28 ± 2°C). (Itmay be noted that, unlike solutions of
iodinemonochloride- in 5M hydrochloric acid, solutionsof bromine
mono chloride in 5M hydrochloric aciddecompose on heating). To
measured volumes(20-40 ml) of the bromine monochloride
solutiontaken in stoppered conical flasks, measured aliquots(5 to
12 ml) of the reductant solutions or a knownweight of the reductant
(in the case of cadmiumthiourea reineckate) were introduced. The
contentswere shaken and were allowed to stand, in
separateexperiments, for different time intervals (5 min to24 hr)
at the end of which the unconsumed brominemonochloride was
determined by adding 20 ml 10%aqueous potassium iodide and
titrating the liberatediodine with standard thiosulphate. A
standing timeof 10 min is recommended for the oxidation of allthe
reductants studied, except for oxinates (15 min)and cadmium
thiourea reineckate (30 min). Alonger standing time (up to 12 hr)
did not lead toany increased consumption of the oxidant in anycase.
Blanks were done concurrently for eachexperiment. Blank corrections
were negligible (lessthan 0·1 ml) even on keeping for 24 hr.
Results and DiscussionTypical results are presented in Tables 1
and 2.
For convenience, these results could be evaluatedunder two
categories:
(a) Oxidations - It may be seen from the datain Tables 1 and 2
that the number of equivalentsof bromine monochloride consumed per
mole of thereductant is 4 for isonicotinic acid hydrazide, 2
fortriphenylphosphine, 8 for trithionate and phosphoro-thioate, 16
for xanthate and dithiocarbamate, 32for reinecke salt and 80 for
cadmium thioureareineckate. These values agree with the
followingoxidation reactions.
BrCI+2e---+Bc+CI-CsH,NCONH - NH2 + HzO--+C.H(NCOOH +
NzH,N2H,--+Nz+4Hf. +4e-Ph3P+H20--+Ph3PO+2H+ +2e-SlIOi-+6H:P
--+3S0:'+ 12H++8e-POSS3- +5HzO--+PO~-+SOi" + tOH+ +8e"ROCSS-+
10HzO--+2S0i-+ROH+COz+ 19H+ + 16e-R2NCSS-+ tOHzO--+2S0i-+RzNH+COz+
19H+ + 16e-CNS-+5H20--+CNO-+ lOH+ +SO!-+8e-[Cd (NHz-CS- NHz)z]
2+[Cr(CNS)( (NH3) Z]2 + 52HzO--+Cd2.,. +2Crl > +8NH~ +92H+
+2C02+8CNO-+
lOSO~- + 80e-
On comparing these results with the results ofiodine
monochloride oxidation it may be noted thatin the case of four of
the compounds in this study,i.e, triphenylphosphine,
phosphorothioate, trithio-nate and isonicotinic acid hydrazide, the
numberof equivalents of oxidant consumed per mole ofreductant is
the same for both the reagents. In these
116
cases the bromine mono chloride method has nospecific advantage
over the iodine monochloridemethod and it is simply a useful
alternative method.
But, with the remaining compounds, viz.
xanthate,dithiocarbamate, reinecke salt and cadmium
thioureareineckate, bromine monochloride effects a moreextensive
oxidation than iodine monochloride. Thenumbers of equivalents of
iodine monochloride con-sumed for the above compounds are
respectively14, 14, 24 and 64 whereas, with bromine monochlo-ride,
the values are 16, 16, 32 and 80 respectively.This increased
consumption of BrCI here may betraced to the ability of this
oxidant to oxidize theformate ion to carbonate (in the case of
oxidationsof xanthate and dithiocarbamate) and the cyanideion to
cyanate (in the case of reinecke salt and cad-mium thiourea
reineckate). For the above-mentionedcompounds the bromine
monochloride methodis therefore intrinsically more accurate than
theiodine monochloride method.
I t is noteworthy in this context that in manyother oxidimetric
methods for the determinationof xanthates and dithiocarbarnates,
the number (n)of equivalents of oxidant consumed per mole of
thereductant is
KAIR & LALITHA KUMARI.: OXIDIMETRIC DETERMINATIONS WiTH
BrCI
oxidants for the determination of xanthates
anddithiocarbamates.
(b) Halogenations (brominations) - It was foundthat in dil.
hydrochloric acid medium oxine reactswith b~omine monochloride to
give dibromooxine,consummg 2 moles (4 equivalents) of BrCI per
moleof oxine.
C9H70N +2 BrCI-i>-CgH50NBr2+2HClIt may be seen from Table 2
that the number of
eq~ivalents of BrCI consumed per mole of the?xmates of Mg(II),
Mn(II), Co(II), Ni(II) Zn(II) etc.IS 8 and that for AI(III) n = 12;
for copper oxinaten = 7 and for iron oxinate n = 11. The
apparentdeviations in the case of Cu(II) and Fe (III) are dueto the
fact that during subsequent addition of KI(for estimating the
unconsumed BrCl) , there is aconcomitant reduction of Cu(II) to
Cu(I) and Fe(III)to Fe(II).
Oxine and metallic oxinates have been determinedby Cihaliks''
using iodine monochloride as theoxidant, in a medium whose acidity
is carefullycontrolled by the addition of ammonia in
suitableamounts. If excess ammonia is added, oxine isthrown out.
Further, increase of acidity was reportedto slow down the rate of
this reaction considerably.In the present investigation, it is
found that thereaction of oxine with bromine monochloride
takesplace very quickly in the acidic media employed.Increase of
acidity does not decrease the rate of thereaction. The present
method is also superior tothe conventional bromate-bromide method,
in whichspecial precautions have to be taken to preclude lossof
bromine.
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