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F1DG TECHhJOLOGIE~ INC. TEL~30~-.92-5633
<br />,BBB • ANALYTICAL CHEMISTRY,~L. 83, N0. 7, APRIL 1, 1991
<br />' ~~ Table IV. lloloctlon o[ Cyanide omploxoe by the CNATC
<br />rreC6d pro
<br />~~
<br />I complex ton % dekction rot to total cyanide proc
<br />CN- 0
<br />Zn(CN)~} 0
<br />~~ AgICN), 12
<br />CrtCN)e"- 10
<br />Fe(CN) r- 100
<br />Fe(f.N)•~ 96
<br />Cp(CN) ~ 100
<br />wavelength of irradiation, duration of irradiation, power and
<br />geometry of rho irradiation cell, and the environment in which
<br />i the sample is irradiated. When these factors are all optimized,
<br />photomnvereion can effectively replace strong paid distillation
<br />es art analytical techniytte for disaocipting motel cyanide
<br />complexes.
<br />Weak Acid sad Cyanides Not Amenable to Chlorina-
<br />tion. The datormirtetion of free and tote! cyanides has been
<br />discussed; this leaves only two categorise in the total cyanide
<br />~ equation feq 1) to be ovaltteted. The determination of either
<br />the weak acid dissoeiable cyanides or the cyanides nut
<br />amenable to chlorination will provide the value of the other
<br />through subtraction. Determinetione of both categories were
<br />investigated; however, much more success was found with
<br />CNATC dewrminationa.
<br />By utilizing procedures developed in the total cyanide
<br />determination, a simple procedure for determining CNATC
<br />wee developed. In chlorination determinations, a strong ox-
<br />idizingagent, such u calcium hypochlorite, ie added to Cyanide
<br />samples. The hypochlorite selectively oxidiue cyanide com-
<br />pounds from the free and weak acid diaeocieble cyanide
<br />categories to produce Cyanate ions in the process. YeL the
<br />strong metal cypnide compounds that make up the CNATC
<br />cakgory are unaffected by the oxidizing agent. A 1-h reaction
<br />time is allowed for this process. Thp oxidizing agent ie then
<br />neutralized with ascorbic acid. The precipitate formed In this
<br />neutralisation proceu is filtered out of solution; and the intact
<br />metal cyanides ere determined by using the total cyanide
<br />procedure. Table IV shows the cyanide detection of varloue
<br />motel cyanide complexes subjoctad to the silver AAS CNATC
<br />procedure. Ae the results indicate, near complete recovery
<br />is achieved for hlgh•atability compounds, and about 40%
<br />borderline CNATC compounds, such as chromium, pre re•
<br />covered. Such a division is very similar to that seen with the
<br />currently accepted ASTM Cyanides Amenable to Chlorination
<br />Procedure B (3).
<br />Chemical Iatorferencee. Some of the most severe limi-
<br />Cations with the standard methods of cypnide analysis ere
<br />caused by chemical intorferencea. By far the most uoubleeoma
<br />interference for tenth distillation end many photodecompo•
<br />sition methods (8, f~ of cyenido anplyaia tees bean that reused
<br />by thiocyanaw; it has produced large, up to 1;1, positive in•
<br />terference affects. To Further complicate this problem,
<br />thiocyanate is generally present in rnnoentretions greater than
<br />the tow! cypnide concentrations. The silver filter is, however,
<br />highly inert in reference to thiocyanate ions; ea a rasttlt,
<br />thiocyanate produces no detecwble interference. Moreover,
<br />ae a benefit of irradiating the sample et lower energy weve-
<br />lengthe, the potentially !ergo positive thiocypnak interference
<br />hoe been reduced to a signal equivalent to a factor of 0.0002
<br />times that of the thiocyanate concentration after 96 min of
<br />irradiation. Such a vest reduction in the observed interference
<br />effect allows the thiucyenato to be ignored in all but the most
<br />severe cases end easily corrected for in those cases. Resides
<br />thiocyanate, the ihfluenca of other anions wee exomined at
<br />the 700 mg/L level for both positive and negative effects;
<br />carbnrtak, chloride, bromide, sulfate, nitrite, and nitrate were
<br />May 04 92 5~ 6 Na.u04 F'.06
<br />all examined. Fur all categorical procedures, no dateetable
<br />atomic absorbance silver signal w s found to result from any
<br />of those ions. The cyanata ion al u did not show a positive
<br />ar negative interference. The eul de ion showed nu positive
<br />interference when introduced to he silver filter. Howavar,
<br />when mixed with free cyanide sal lions, no free cyanide was
<br />detected. This fa a result of the sulfide reacting with the
<br />cyenido iou w produce thiocyan te, which is not detected.
<br />This reaction hoe been shown to ,ogress rapidly at elevated
<br />pH (]8). Sulfide should, thareforo, bo removed as soon as the
<br />sample is collected, before it is a bilized. Lead acetate lost
<br />paper may ba used to indicate the rosonca of sulfides in the
<br />sample. Lead carbonate can the be added in small incro•
<br />manta to remove rho sulfides. Sin the reaction of the sulfide
<br />is with the cyanide, samples pretr sled as above produce no
<br />sulfide interference with the situ AAS defection method.
<br />Fatty acids, which distill over i distillation methods, un•
<br />dergo eaponiGcatiott reactions in t e alkaline absorber. Tho
<br />seep produced interferes wi1.h the election skp- Fatty acids
<br />posed nn problem with rho silver AAS method other then
<br />occasional filler clogging. This c n be easily corrected by
<br />stacking membrane prefiltera ate the silver filter. The in•
<br />corporation of prefiltera was also a own to extend rho useful
<br />life of the silver filter. This ind cotes that regular use of
<br />prafilters mokos economical Sens .
<br />It was observed that the silver filter reaction efficiency
<br />degrades with e:[ended use. W le new•, well•conditionetl
<br />silver filters provide 100% of the theoretical reaction effi•
<br />cioncy. After extended use, that ti ro can drop w ea low as
<br />40%. Two factors were identifie as important in rho lose
<br />of filter [section efficiency. Th first is simple physical
<br />clogging. As more pores become cl gged, the contact limo in
<br />the rempining pores ie reduced, r suiting in lower tearlion
<br />yields. A second factor is filter po coning. This occurs only
<br />when metal cyanide complexes nr introduced to the filter.
<br />boring the redox reaction of the cy aide to the silver cyenido
<br />complex, sufficient potential may a provided w deposit the
<br />original complex metal onto t}te situ r filter. By utilizing X-ray
<br />fluorescence mewode, metals ouch iron attd zinc, introduced
<br />as [natal cyanides, were identified rt the filler surface. This
<br />metal depoaitiort may 6e the result f direct deposition or dtte
<br />to the filtration of Insoluble metal droxides from the highly
<br />alkaline aolutione. Such clogged r poisoned filters can be
<br />regenerated by beck rinsing the fi [era with a 0.01 M nitric
<br />acid wash solution. This can imp ve the filter reaction ef-
<br />ficiency without damaging the Gl er. Through weak acid
<br />rinsing, reaction efficiencies can bo raised back to near 90%.
<br />although complete tegeneratlon has ever beet[ achieved. Due
<br />to the above two feelers, as well as others, it is important to
<br />always rttn standard cyanide solutior s for calibration purposes.
<br />Methods for dealing with same a containing silver have
<br />also been examined. While silver i not commonly found in
<br />appreciable concenuetions in Hato 1 waters, it may occur in
<br />industrial applications. Soluble ail er will generally exist in
<br />either cationic or chelated forms. hen any CNATC mewls,
<br />such pe iron, are prosottt, simple eq tilibria cnlculationa show
<br />the cyanide will preferentially co plex rho CNATC metal,
<br />leaving the silver ion in solution. Th addition of a base carless
<br />all but the most tightly cbelated si ver trr precipiate out of
<br />solution as the hydroxide and ox de. Conversely, the di-
<br />cyanosilvor complex is extremely eta to and soluble in alkaline
<br />solutions. AB a result, the silver co tent of a sam Pte that has
<br />beet[ pH adjusted to 12 or above a d filtered at 0.4b µm or
<br />below provides a measure of the sit r cyanide concentration.
<br />'Phis allows the free cyanide date ntinetion to be used on
<br />aolutione containing silver. Simpl subtraction of the silver
<br />cyanide concenuation provides stn •curow measurement for
<br />the actual free Cyanide content. o ouch subtraction Is re•
<br />
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