niination of boron as directed above. but use concentrated hydrochloric acid in the neutralization of the solution previous t o the methanol distillation. The results obtaiiied by t'his method are comparable to those obtained by' the hydrothermal refining met'liocl. It is probable that hydrothermal refining tecliiiiques \vi11 be applicable in other problenis in analytical chemist'ry. Accuracy. To test t8he accuracy of the method, several synthetic saniple mixtures of known boron content were prepared and analyzed. Seven niilliliters of 1YGsodium hydroxirlc solution
plus a measured amount of standard boric acid solution (0.1 y of boron per ml.) and enough deionizcd water to make a total volume of 14 nil. were added to t8he cavity of an autoclave. One gram of highest purity BO-mesh D u Pont silicon was added. The results are sho~vnin Table I. The values listed iii tlie last column w r e obtaiiied by Yubtracting the weight of l~oronpresent in tlie reagents phis silicon-Le., 0.04 y-from the total \vciglit of boron found in tlie iiiist'urc. Little or no borate is h t hy ocaclusioii or :iclsorptioii; hence the li~-drotliernia1q x i r a t i o n of boron is quantitative.
LITERATURE CITED
(1) Ducret, L., A n a l . Chini. ilcta 17, 213
(1957).
(2) Ducret, L., Seguin, P., Zbid., 17, 207
(1957).
(3) Hannny, S . B., Ahearn, 1.J., -1s.1~. CHEM.2 6 , 1056 (1954). (4) Luke, C. L., Ibid., 27, 1150 (1955). (5) llorrison, G. H., Rupp, R. L., Ihid., 29, 892 (1957). ( 6 ) Parker, C. A,, Barnes, \I. J., .A j t d g s t 82, 606 (1957). ( 7 ) Pohl, F. .I., Z. anal. C'hctri~ 157, 0
(1957).
RECEIVEDfor review AIay 11, 1957. .Iccepted March 21, 1058. Ilivir?ion of Alrialyticnl Chemistry, 1321id lIeeting, ACS, S e w l-ork, S. T.,September 1057.
Colorimetric Determination of Cyanide Tris(1 ,I0-phe na nthroline)-l ron(II) Ion as a Selective and Sensitive Reagent ALFRED A. SCHILT Department of Chemistry, University o f Michigan, Ann Arbor, Mich.
b A new method for the spectrophotometric determination o f cyanide i s based upon the formation and extraction of the neutral dicyano-bis( 1 ,1 0-phenanthro1ine)-iron(l1) complex produced through exchange reaction between tris( 1 , l 0-phenanthro1ine)-iron(11) and cyanide ions. The method is convenient, relatively free of troublesome interferences, and applicable to microgram quantities of cyanide ion and concentrations o f the order o f parts per million. A sensitive and highly selective test for the identification and detection o f cyanides, based upon the same reaction, i s also described.
D
a recent investigation of the iron(I1) complexes of cyanide and 1.lO-phenanthroline ( 3 ) it n-as observed that the neutral dicyano-bis( 1,lO-phenanthro1ine)-iron(I1) complev might afford a useful analytical form in nhich trace quantities of cyanide could be isolated for purposeq of identification and determination. This neutral complex can be conveniently prepared by reaction between tris(1,lO-phenanthro1ine)-iron(11) (commonly referred to as ferroin) and cyanide ions. It is only slightly soluble in aqueous solutions, it yields readily to extraction by certain inimiscible solvents which do not a t the bailie time extract other colored iron(I1) species from the aqueous system, and its intense 1-iolet colored solutions obey Beer's law. A consideration of these properVRIXG
ties led to the preberit stud! n hich concerns the application of ferroiii as a colorimetric reagent for cyanide. APPARATUS AND REAGENTS
Absorbancc nicasurenients were made Tvith a Beckman Slodel D t - spectrophotometer and 1.000-cni. cells. A Cary Model 11 recording spectrophotometer \vas used to obtain spectral curves, and pH was nieaeured n 3 h a Beckman Model G pH meter. The 1,10-phenaritliroliiie monoliydrate was obtained from the G. Frederick Sniit'h Chrniical Co. K i t h the exception of the aniines used in interference st'udies, all chemicals were of reagent grade. Ferroin Reagent Solution. Dissolve together 1.96 grams of ferrous ainmoilium sulfate hexahydrate and 3.17 grains of 1,lO-phenanthroline nionohydrate to a volunie of 1 liter using distilled rvater. The resulting solution is 0.005X in ferroin sulfate and 0.001dl with respect to 1,lO-phenanthroline. Standard Cyanide Solution. Dissolve a sample of reagent grade potassium cyanide. previously analyzed for cyanide content ( 1 ) and found equivalent to 1.00 gram of cyanide ion, in 1 liter of distilled n-ater. This solution may be used to prepare appropriately diluted standard solutions.
5 nil. of 1JI disodium hydrogen pliosphate solution, 1 ml. of 1Oyohydroxylamine hydrochloride solution, and 2 drops of 0.1% thymol blue indicator to the saniple in the flask. At this point if the indicator does not impart a yrllow color to the solution: add 1JI acrt'ic acid dropnise until it does. Add 0.5J1 sodium hydroxide dropnise until tlic indicator changes to blue; an internictliate light green color \vi11 be observed just before this point is reached. Tlirn add 5 nil. of the ferroin reagent and heat in a boiling water bath for 10 to 15 minutes with the glass stopper in p l a c ~ . Cool the contents of the flask to rooin temperature or lower, transfer to a 60ml. eeparatory funnel, and extract four times with 5-ml. portions of chloroforni. Combine the chloroforni extracts in R 25-m1. volumetric flask and dilutc to volume with chloroforni. SIca.urc, the absorbance a t 597 nip in a 1 - ( 3 1 1 1 . wll using chloroforni as the rc>fc.rence soliition. The measured absorbance docs not' require a reagent blank corrrction if reagent grade chemicals arc eniploycd. Solutions of the complex in cliloroforni should be protected from dirrct sunlight and measured within 2 to 3 hours after preparation. Determine the cyanide ion mii[>(wtration of the saniple by referring to a calibration curve. Prepare the curve using standard solutions of potassium cyanide analyzed as described above.
PROCEDURE
Add a measured volume of 25 ml. or less of the saniple containing not more than 200 y of cyanide ion to a 50-ml. glass-stoppered Erlenmeyer flask. Add
VARIABLES
The influences of pH, buffer coiiilxi'ition, temperature, reaction time, ~ i i reagent concentrations on the extent of VOL. 30, NO. 8, AUGUST 1958
1409
d
the reaction between ferroin and cyanide ions were investigated. Also various extraction solvents were tested. Experimental conditions favorable for maximum formation and extraction of the neutral dicyano-bis( 1,lO-phenanthroline)-iron(I1) complex were established. Three different buffer systems were studied over the respective p H ranges of their effective buffering capacities: HPOd-- - PO4---. KH4- - NH3,and HC03- - COS--. For each the attainment of maximum reaction in a reasonable length of time necessitated heating the solutions in a boiling water bath. At room temperature the reaction between ferroin and cyanide ions requires approximately 20 hours to a p proach completion. This period is shortened to less than 10 minutes when the reaction mixture is held a t 100" C. in a boiling water bath. Only slight differences rrere observed between the three buffer systems n-ith regard to minimum heating time for attainment of maximum reaction. However, the ammonia and the carbonate buffered solutions gave increasingly lower results when heated longer than 15 or 20 minutes. The phosphate system can be heated for 30 minutes without adverse effect, but slightly low recovery of cyanide results after 40 minutes or longer a t 100" c. The extent of formation of the desired complex under optimum heating conditions and for a given p H appeared to be essentially the same in either the ammonia or the phosphate, but noticeably less in the carbonate-buffered solutions. For this reason carbonate buffering was not employed. The phosphate n as selected in preference to the ammonia buffer system because the latter requires extra experimental precautions to avoid loss of ammonia and subsequent decrease in p H during the requisite heating period. Another advantage afforded by the phosphate system is the wider latitude of heating time which can be employed without adverse effect on the cyanide recovery. The influence of p H on the extent of formation and extraction of the desired complex from phosphate-buffered solutions is shown in Figure 1. Recovery of cyanide as a function of p H depends primarily upon the extent of the reaction between ferroin and cyanide ions. LOR-recoveries below p H 8.5 are a result of decreasing cyanide ion concentration accompanying hydrocyanic acid formation. Above p H 10 the decreasing solution stability of ferroin a t the temperature used appears to be one factor contributing to low recovery. A second and perhaps more significant factor is the atmospheric oxidation of cyanide to cyanate which becomes more favorable with increasing pH. The rather narrow p H range for optimum recovery requires that this vari1410
ANALYTICAL CHEMISTRY
' en
Figure 1. cyanide solutions
Effect of pH on recovery of from phosphate-buffered
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4 z
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