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_iII;IES Ill!=. TFi :-li_.-,~~_~-~._'-.
<br />•
<br />60
<br />a~
<br />]o
<br />9
<br />~ 20
<br />10
<br />hn,.~.~, I_~1 _~_ _.cl-,= I,~~..--
<br />
<br />
<br />• Felll ®7aSnm
<br />Fen ~ 9a5~m a
<br />+ Fenl ®2a~nm
<br />+• Fell ®2aanm
<br />
<br />Irradlatlon ilma (cola.)
<br />Figure 3. Photoeonverabn of Iron eyanltle Complexes ualnq a,a GPST6
<br />uaravl0let light source.
<br />be measured ss a function of the stability of the competing
<br />metal complex and of the contact time of the complex with
<br />rho silver. TaUle III shows the results from thex exporimente.
<br />Two uentls are evident in these data. Firstly, as the formation
<br />constant u[ the metal cvanido complex increases, the ability
<br />of the silver to succesefttlly compete for the cyanide ligand
<br />decreases. Secondly, as the flow race dacroaaee end the so-
<br />lotion contact time with the titter increases, the percentage
<br />of bound ligwids acquired by the silver also increases. Theca
<br />results indicate that selective kinetic equilibrium can be
<br />utilized to aid in the categorical differentiation of rite various
<br />cyanide species. By choosing a 2.5 mL/min flow rate, the free,
<br />molecular, and zinc cyanides can be selectively detected by
<br />Cho silver Cilter, while leaving the remaining metal cyanide
<br />complexes essentially undetected. Since there three species
<br />ere dissociated by conditions that era prevalent in natural
<br />aquatic systems, they typically comprise the bulk of the free
<br />cyanide category.
<br />Determination of Total Cyanide, Currently moat
<br />Countries with Cyanide regulatory standards set limitntioas
<br />only on total cyanide concentrations. Whether auclt a limi-
<br />tation is well founded or not, it dues make the total cyanide
<br />determination of foremost importance W most invastigatinna.
<br />Since Gouldan (13) showed in 1972 that complex metal
<br />cyanides can be phutodecomposod b}' ultrsviolot light, many
<br />others (7, 8, 14, 15) have attempted W utilize photodecom-
<br />position in place of rho standard strung acid distillation in
<br />total cyenido dotermittation procedures.
<br />Pltotodecrompasiliuneffers several passible advantages over
<br />traditional strong acid distillation methods, e.g„ reduced
<br />analysis times, improved accuracy and precision, sofa attd
<br />simplistic operation, and adaptability inward automation. The
<br />proUletn with phutodecompnsitinn, ae pointed ottc by Pohlandt
<br />(8), has been rho varied results reported by different ittvee•
<br />tigotars. Tire inconsistencies found in the literature (8, 10,
<br />16) concerning the effectiveness of various photodecomposition
<br />systems are likely a result of rite greatly differing experimental
<br />parameters used far these studies. Through testing, we have
<br />[otind that several parameters play important colas in the
<br />photoconversion of instal cyanide complexaa to free cyanide.
<br />The wavelength of sample irradiation is extremely Important.
<br />Nut only must the light source be Considered, but also the
<br />experimental gemuetry and tunstruction of Utt phntocelh In
<br />constructing our ^uw-through system, we had the choice to
<br />cnntairt the sample in narrow tubing made of either quartz
<br />~~r eery "I'e!lon. The FEP Teflon end quartz proved equally
<br />,rer.,rz: e:u :i,r:~ughuut the spectral regions of interest. The
<br />-.-... _, ~,.vuver. more ecunumical and easiip faLrii lied
<br />solo varies ~~cil ~enrnetrieo, making it the material of ehi~ice
<br />• - •~~ ri:otocell. R bile using FEP CL'ou!q. "vri••t::.
<br />., ~ ~.~cre le:de~i. The light w.•r<<•• .... r~i in'
<br />..... .. rid spectral tart put. Lamps v.it :~:;~:ea~
<br />.~:
<br />__ ..... ...... .., .e ;eneral L• iectnc G~ST3 ;?.m„ _ '9.e•~b
<br />100
<br />90
<br />70
<br />60
<br />aD
<br />AO
<br />90
<br />20
<br />tD
<br />0
<br />-70
<br />Yc~.
<br />+- Fell CND
<br />'~ Grt.N
<br />~ ('p~J
<br />F r•tp~CN
<br />a Yw.N
<br />G+
<br />... SCtI
<br />Irfetllatlon Tlma (coin.)
<br />Flqure A. Total cyanltls detection using eh$ H3gK8-775 Ilgnt sowea.
<br />nm) provided rapid photoconversion with a peek recovery after
<br />only a Cew minutes of exposure. This was fnilowod by a slow
<br />decrease in the free cyanide concontratinn, until eventually
<br />all liberated free cyanide was eliminated (see Figure 3). A
<br />borosilicata glass filter added to this same ]amp shifted the
<br />emission output to a lower energy (Jl,,,, ~ 365 nm). This
<br />produced an otttirely different phutureaction in which free
<br />cyanide was slowly produced end, every after extended expo•
<br />sores, continued to produce an incre9se in the free cyanide
<br />concentration, 13y using another lower energy emission lamp
<br />(am„ ~ 966 nm), but with a much more pawetCu] output
<br />(Philips H39-Kf3, 175 W, eve Wtal output 31 W), ateodily
<br />increasing photoconvartcd free cyanicje concentrations were
<br />again obtained. Sy use of the H39+KB lamp, essentially
<br />comploto convoreion of a variety of metal cyanide complexes
<br />was achieved efts[ 33 min of exposure as illuvtrated in Figure
<br />4.
<br />Such behavior, at least fur the ferrid cyanide complex, has
<br />been well explained and similar aituatinns exist far other metal
<br />cyanides (17). Tha high-energy irradiation has Veen associated
<br />with photooxidation reactions that produce cyanate. This
<br />agrees with our results, showing a dakrcose in cyenido con-
<br />centratiun with continued irradiation gat lower wavelengths.
<br />Alternatively, irradiating at 366 nm ha9 been shown to induce
<br />ligand diaaoc;ations that quantitatively ¢¢reduce simple cyanide
<br />ions. Tho rate uC phuWcunvenitln for tote different mmpleaes
<br />is a function of both the stability constant and the absorption
<br />mamma of the raepoctive compounds,
<br />Ttte roles these faCWnl play in inftuepcing recovery rata are
<br />exemplified by two extreme exampleb, cobalt cyanide end
<br />ferric cyanide. As expected, the ex remely stable cobalt
<br />complex requires extended eapaattro t~mea to ac}tieve a ]sigh
<br />percent of photoconversion (Figure A). Ferric cyanide is also
<br />a very stable complex, but unlike cobalt, it has a strong ab-
<br />eorption band over the wavelengths of irradiation, allowing
<br />it to decrompuse rapidly. Ferrous and chromium cyenido
<br />represent more typical complexes that, even with their high
<br />stability constants, are photncanvetted al tate9 intermediate
<br />W the extremes.
<br />In order to obtain phutuconversioniecoveries as high as
<br />those prnentad shove, rho electrochetaical characteristics of
<br />the phntnlysis environment must be chntrollad. if the pha•
<br />Wlysis environment contains n sigttificapl amount of oxidizing
<br />agent, irradiation even at higher endrgy wavelengths wilt
<br />predominantly produce undesirahlo cypnata ions rather than
<br />cyanide inns. A O.dS \1 concent.ratlun of sodium hypu-
<br />phoephito waa Counts to effectively sµppresss any cyanide
<br />oxidation. Suclt a 9ulution did not reduce cyanate already
<br />present in the sample. This is importaht since cyanate is mgt
<br />to be included In the defined total cyanide determination.
<br />Through our experimenta[iun, we have Cuund that the ~[~
<br />Cectiveness of a photodecumposition system depends un the
<br />0 to 20 90 as 50 60
<br />0
<br />e le ee ee
<br />
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