Anal. Chem. 1985, 57, 621-624
621
Determination of Chloride, Bromide, and Iodide in Silver Halides by Ion Exchange and Ion Interaction Chromatography Frederick J. Gustafson* and Craig G. Markell 3M, Central Research Laboratories, St. Paul, Minnesota 55144 Sharon M. Simpson 3M, Photo Color Systems, St. Paul, Minnesota 55144
An ion chromatographic method is described by which halide ratios in silver halide matrlces can be determined over the range of 0.01 to 100 mol %. Halides are recovered from sliver halides dissolved in a cyanide-free system by reduction of silver in basic aqueous hydrazine solution. Preparative cation exchange Subsequently eliminates hydrazine and hydroxlde from the sample which is analyzed by ion exchange chromatography for chloride and bromide and by ion Interaction chromatography for iodide. Chromatographic conditions have been established for halide elution In less than 10 min with complete separation of the individual halides at ail concentration (mol % ) ratios and complete separation from anlons of insoluble silver salts. Recoveries for mixed halide standards were 98% for chloride, 94% for bromkle, and 92% for iodide with a reproducibillly better than 3% relative standard deviation.
The primary light sensitive properties of a photographic emulsion are determined by the molar ratios of silver chloride, silver bromide, and silver iodide in the emulsion ( I ) . A number of procedures have been developed in response to the need for an analytical method to determine silver halide ratios. Przybylowicz et al. (2),Gerard et al. (I),and Russel (3) have reviewed these techniques which include potentiometric titration with silver nitrate, automatic coulometric titration, polarography, X-ray spectrometry, neutron activation analysis, and activation analysis using a 252Cfneutron multiplier. Difficulties inherent in the polarographic and potentiometric methods include inadequate differentiation between responses of one halide ion from another particularly when one halide is present at a low mole percent value. The neutron activation analysis method developed by Gerard et al. (1)is fast and accurate for the determination of halides over a wide mole percent range. The limitations of neutron activation stem from the relative thermal neutron absorption cross sections of silver and the halides. Because silver has a cross section 10 times greater than any of the halides, the sample cannot be accurately analyzed without first complexing the silver with aqueous cyanide. The chloride detection limit is greater than the bromide and iodide detection limits because the chloride cross section is 5 times less than the cross sections for the other two halides. This paper describes a cyanide-freeprocedure for dissolving silver halides by reduction of silver ion in basic aqueous hydrazine solution. Hydrazine was initially described by Andresen (4,5) as a weak photographic developing agent in strongly alkaline solution. Reduction of silver in silver halides by hydrazine is thermodynamically favorable according to the standard potentials of the reactions. Previous two-step halide isolation procedures have used cyanide (6) or ammonia (7) to form silver complexes followed by zinc reduction of silver. 0003-2700/85/0357-0621$01.50/0
In the one-step method to be described, the only anions present in the system are hydroxide and the halides. Hydroxide and hydrazine are subsequently eliminated by passage of the sample through a hydrogen-form cation exchange resin column. Ion chromatography is capable of determining sub-partper-million levels of halides in the presence of other anions. In the analysis of real silver halide samples, the chromatography must be capable of separating the halides from essentially all anions of insoluble silver salts. These constraints made it necessary to utilize ion exchange chromatography (IEC) for the determination of chloride and bromide and ion interaction chromatography (IIC) for the determination of iodide. The original IEC method of Small et al. (8)cannot be used for determining all three halides in a single injection due to severe tailing of iodide on the polymeric resin column. The postsuppression gradient elution method of Sunden et al. (9) is characterized by poor iodide peak shape and an iodide elution time of greater than 20 min. Other single injection procedures on polymeric anion exchange columns include the column-switching dual detector method of Wang et al. (10) and the aromatic eluant method in the Wescan Instrument (Santa Clara, CA) applications literature. Both techniques compromise resolution of chloride and bromide for the ability to elute iodide with minimal tailing. Silica based anion exchange columns do not cause iodide to tail. Silica slowly dissolves in aqueous mobile phases causing a decrease in ion exchange capacity and analyte retention times. IEC on polymeric anion exchange columns is useful for the separation of chloride and bromide from virtually all other anions. The determination of chloride, bromide, and iodide by IIC has been demonstrated by Iskandarani and Pietrzyk (11)using a mobile phase containing tetrapentylammonium salts as the ion interaction agent. The column was packed with macroreticular styrenedivinylbenzene resin. Due to the long equilibration time of the tetrapentylammonium ion with the column, tetrabutylammonium salts were employed in this study. Carbonate was used as the eluant counterion because of its compatibility with eluant suppression and conductivity detection (12).Carbonate also controls the selectivity of the analysis. All of the tested divalent anions eluted before iodide, and iodide was cleanly separated from monovalent anions such as formate, chloride, bromide, thiocyanate, nitrate, cyanide, and perchlorate. Because of this selectivity, IIC was the method of choice for determining iodide. EXPERIMENTAL SECTION Apparatus. Liquid chromatography was performed on a Dionex Model 2120i LC (Dionex, Inc., Sunnyvale, CA) and an IBM Model 9533 LC (IBM Instruments, Inc., Danbury, CT) equipped with a Wescan Model 213A conductivity detector (Wescan Instruments, Santa Clara, CA) and a Perkin-Elmer Model LC-75 ultraviolet absorption detector (Perkin-ElmerCorp., Norwalk, CT). Data were recorded and integrated on an H-P 0 1985 American Chemical Society
622
ANALYTICAL CHEMISTRY, VOL. 57, NO. 3, MARCH 1985
Table I. Conditions for Ion Exchange and Ion Interaction Chromatography
ion exchange for C1- and Br-
analytical column
250 mm Dionex AS4 4 X 50 mm guard column Dionex AG4 suppressor column Dionex AFS fiber suppressor eluant 7 x 10-4 M NaHC03 4
X
ion interaction for I4 X 250 mm Dionex NS1 4 X 50 mm Dionex NG1
Table 11. Recovery of Halides from Silver Halides
[NzH~I W O W , (v/v %) M 5.0 0.5 5.0 0.5
0.04 0.04 0.01
% halide recovered
C1-
T,O C
Br-
I-
98 f 3.0" 94.5 f 2.4n 92.0 f 2.1" 97 i 4.5b 88 f 3.2b 96 f 7.0b c 90 f l.Ob 8.5 f 8.0b 0.01 97 f 1.5b 98 f 6.5 f 2.8b Average recovery of five separate samples with relative standard deviation given. bAverage recovery of two separate samples with average deviation given. CChloridewas not present in this samde. 70 70 22 22
(I
10 X 100 mm Dionex
packed anion suppressor 10-3 M
tetrabutylammonium hydroxide 5.6 X M 15% (v/v) acetonitrile, 3 X Na2C03 M NazC03 flow rate 2.0 mL/min 1-1.5 mL/min injection volume 15-50 WL 15-50 r L detector sensitivity 1 pS full scale 1 WSfull scale (Wescan (Wescan detector) detector) Model 3390 integrator (Hewlett-Packard Co., Avondale, PA), or an IBM Model 9000 computer (IBM Instruments, Inc., Danbury, CT). Ion chromatography columns were purchased from Dionex and are described in Table I. Reagents. Water used in this study was purified with a Millipore RO-4 reverse osmosis unit followed by Milli-Q mixed bed ion exchange and organic removal cartridges (Millipore Corp., Bedford, MA). Tetrabutylammonium hydroxide (0.4 M from Eastman Kodak Co., Rochester, NY) was purified by passage through a strong base cation exchanger, Amberlite IRA-400 (Mallinkrodt Chemical Works, St. Louis, MO). This resin had been previously converted to the hydroxide form with aqueous NaOH. Amberlite IR-120 H C.P. (Mallinkrodt) was used in sample preparation. This hydrogen form strong acid resin was extensively prerinsed in the filter section of a solvent filtration flask to remove residual chloride. Halide free aqueous hydrazine solutions were prepared from anhydrous hydrazine (99%, MCB Manufacturing Chemists, Norwood, OH). Due to its toxicity, concentrated hydrazine was handled in a hood. Silver halide (99% purity) standards were obtained from Aldrich Chemical Co. (Milwaukee, WI). UV grade acetonitrile was obtained from Burdick and Jackson Laboratories (Muskegon, MI). All other inorganic salts used in this study were analytical reagent grade and used without further purification. Sample Preparation. Up to 30 mg of silver halide was weighed into a glass vial with a polyethylene-lined cap. Five milliliters of 6% (w/v) aqueous hydrazine and 1 mL of 0.25 M NaOH were pipetted into the vial. The vial was capped and placed in a heating block for 24 h at 60-70 "C. Reduction of silver was apparent by the appearance of metallic silver. The sample was cooled and filtered through a 0.45 pm pore size Nylon 66 membrane filter (Rainin Instrument Co., Inc., Woburn, MA) placed on the end of a plastic syringe. To analyze major constituents of the sample, a 1-mL aliquot was diluted to 100 mL and analyzed without removing base and hydrazine. Repeated injections of 0.06% hydrazine did result in some deterioration of the performance of the anion exchange column evidenced by peak broadening. In order to determine halides at trace levels, hydrazine must be removed from the samples. Four milliliters of the sample was passed through a 10-mL column of a hydrogen form strong acid cation exchange resin and collected in a 25-mL volumetric flask. The resin converts sodium hydroxide and hydrazine into water. A few drops of 0.25 M NaOH were added to the volumetric flask before collecting the sample to retard acid-catalyzed air oxidation of iodide. Rinsings of the resin column with small portions of deionized water were added to the volumetric flask until 25 mL was collected. RESULTS A N D DISCUSSION
Reduction of Silver. Halides can be liberated from silver halides by reduction (3),complexation (7), or ligand exchange
with anions such as sulfide (13). In this study, silver ion was reduced by hydrazine according to the following reaction (14) 4AgX
+ 40H-+ N2H4(aq)
-
4Ag(s)
+ 4X- + 4H20 + N2(g)
Hydrazine reduction is thermodynamically favorable for all three silver halides and does not introduce anionic interferences into the sample. Recoveries of halides in mixed silver halide standards (10 mg of AgC1,lO mg of AgBr, and 10 mg of AgI) were greater than 90% as shown in Table 11. The low solubility product of silver iodide (Kgp= 8 X results in low iodide recoveries at room temperature. The rate of silver reduction was greater at 70 "C,and the average iodide recovery increased to 92%. The relative standard deviation of the halide recoveries for five mixed halide standards was less than 3% for each halide ion. When the samples were stirred during the silver reduction, halide recoveries were low. Samples were turbid after 24 h at 70 "C. Filtration through 0.45 pm pore size nylon filters did not always eliminate the turbidity. Turbid solutions were probably due to silver halide particles dispersed in the sample. The cause of the turbidity was not investigated. Chromatography. Since an unknown silver halide sample can contain halides at various ratios as well as other anionic impurities, the chromatographic methods used in this study were developed to separate chloride, bromide, and iodide without interferences from other anions. IEC was used to determine chloride and bromide. The eluant composition in Table I was chosen to allow a chloride peak from a 30 mg sample of silver chloride to decay to base line before a small bromide peak eluted (Figure 1A). Trace chloride can be determined in a similar fashion in 30 mg of silver bromide (Figure 1B). Trace level quantitation of one halide was not affected by injections of up to 50 pg of another halide. Under conditions suitable for separating chloride and bromide, iodide eluted with severe tailing (k' = 40). High concentrations of iodide did not directly interfere with the chloride and bromide separation or quantitation. The chromatographic behavior of other possible interfering anions was also investigated. Capacity factors for these anions are presented in Table 111. All of these anions were completely resolved from chloride and bromide. Figure 1C shows the separation of iodate, chloride, bromide, sulfite, and phosphate. The IIC mode was used to separate and determine iodide in this study because IIC can separate iodide from all of the anions listed in Table 111. This was not the case with the anion exchange columns evaluated for iodide determination. For example, sulfate and iodide were incompletely resolved by the Dionex AS5 column under standard eluant conditions. IIC is also advantageous in that capacity factors and selectivities of anions can be independently controlled by changing mobile phase conditions. Pietrzyk and co-workers (11,15,16) have published a series of papers on the IIC of anions on resin columns using mobile phases consisting of tetraalkylammonium (R,N+) ions, water, acetonitrile, and inorganic
ANALYTICAL CHEMISTRY, VOL. 57, NO. 3, MARCH A
T a b l e 111. Capacity Factors of
0
1
2
3
4
5
6
7
0
formate iodate chloride bromide iodide phosphate sulfite sulfate oxalate nitrate chromate thiosulfate thiocyanate perchlorate carbonate cyanide sulfide
9
Time (min) B
n
I
623
Anions
R (1EC)O
ion
1985
K' (IIC)"
0.1
20 >20 >20
0.85 0.89 2.3
>20 13
>10
