input of the column a t subatmoq~heric or vacuum pressures. This technique might introduce the pos4bility of extending the application of chroma-
to
tographic compounds.
LITERATURE CITED
c.,
(1) Giddings, J. A ~ cHEhf. ~ 34, ~ ;311~ (1962). (2) Schalr, G., "Theoretische Grund1:Lyen
der Gaschromatographie," Deutsches Verlag der Wissenschaften, Berlin
iristable
(1960).
(3) V&radi, P. F., Ettre, X., A N ~ L . ,
CHEV.
34, 1117 (1962).
PETER F. Y ~ R A D I KITTY ETTRE
The Rlachlett Laboratories, Inc. Springdale, Conn.
An lodometric Method for the Macro- and Microdetermination of Peroxyd isulfate SIR: Methods reported in the literature for the determination of persulfates (peroxydisulfates) are based on the oyidation and titration of ferrous ion ( I ) , oxalate ( I ) , or iodide in strong acid ( 2 , 5 ) . Each of these, however, requires lengthy heating or reaction times, catalysts, and, for the ferrous or iodide methods, the exclusion of air. However, a t neutrality and with very high molar ratios of iodide to persulfate, iodide is oxidized quantitatively to triiodide without heat, catalysts, or interference by atmospheric oyygen. This reaction has been made the basis of a simple, rapid, and precise method which is applicable both to macro titrations, and to microspectrophotometry. Addition of 6Jf KI to a solution of persulfate, buffered a t p H 6.85, liberates 13- stoichiometrically. The liberated 13-is then titrated with SLO~-' or read spectrophotometrically a t 355 mp (3).
indicator may be used if deiiretl, but we have found it unnecessary. Micro Procedure. To 1.5 ml. of solution containing from 0.01 t o 0.15 pmole of persulfate add 1.O ml. of buffer and 0.5 ml. of 6 J I KI. Read abscxbance a t 355 mp from 5 to 30 minutes after addition of the K I . The color developed follon-s the BeerLambert law ( E = 28,600 liters mole-' em.-') in the Cary 11 (1-cm. cuvettes, 0.07-mm. slit width) and the Beckman DU (1-cm. cuvettes, 0.30-mm. slit width). With the Bausch and Lomb Spectronic 20-340 spectrophotometer (1.15-cm. cuvettes, 20-mp band width), deviation from strict linearity was noted above an absorbance of 0.4. hlso, probably as a result of its greater band width, the apparent absorptivity over the linear portion was somewhat, lower, 635, = 25,200 liters mole-' cm.-1 RESULTS
RELIABILITY O F h h C R 0 l \ I E r H O U . *\lthough the customary volumetric procedures were routinely employed with good precision (=k0.27, relati1 e standReagents. Potassium iodide soluard deviation), a gravimetric-spectrotion (iodate-free), 6 X . Sodium thiophotometric procedure was employed sulfate, standard solution, O.liYJ standfor increased precision in analyses comardized against re.;ublimed I,. Phosphate huffer, p H 6.85, O.lX, 0.05 moles paring the oxalate method. Solutions KHd'O, plus 0.05 moles K,HPO, per as well as solid reagents were weighed to liter. 4.; or seven significant figures on a Macro Procedure. Weigh accu$ingle-pan analytical balance. In this rately 1 to 2 minoles of (SI14i2S208, r a y the mas5 of thiosulfate solution KA08 or other persulfate into a 100-ml. equivalent to a known mass of resubflask and dissolve in 20 ml. of buffer. limed iodine was determined. -4dd 10 nil. of 6Jl KI and titrate xvith In titration, the unknown solution standard thiosulfate until the yellon rvas m i g h r d before and after titration color just disappear?. Soluble qtarrh EXPERIMENTAL
Table I.
Percentage Purity of Persulfates"
Snit
a
Oxalate method
96 48 i 0 97 27 rt 0 99 53 i 0 99 90 f 0 95 21 + 0 95 95 f 0 98 01 i 0 98 90 f 0 Standard deviations are given for the series
412
ANALYTICAL CHEMISTRY
217 23 15 13 22 17 20 11 of five runs made
Iodometric macro method 96 41 f 0 0 l C , 96 35 i 0 02 99 64 i 0 01 DO 97 i 0 01 95.19 + 0 02 95 99 f 0 01 98 05 =t0.02 98 97 rt 0 01
on each sample.
and the mass, rather than the volume, of titrant added was obtained by difference. Titrations w r e carried only to the point of a very faint yellow color, rather than to complete loss of color to ensure that a ion- concentration of 1 8 remained. The concentration of the 13remaining was then determined spectrophotometrically a t 355 mp by comparison with known dilutions of standardized 11-solutions. I n this way it was possible to obtain replicate runs Ivith a precision of 10.02% or better. Because of 1o.s by evaporation with the hot oxalate solutions required, this same method was not applicable to the oxalate determinations. Results of five replicate runs on each of four samples of K2d208and four of (SHJLS20s done in this way show excellent agreement (Table I), The samples were commercial mnples from four different sources. RELIABILITYOF XICHOMETHOD. Fresh persulfate solutioris, made from samples analyzed by the macro method, were determined by the micro method. The results obtained were both precise and accurate (relative error) to =tl% (relative standard deviation) n ith either the Cary or Beckrnan spectrophotometers and to i~ta7~ n-ith the Bausch and Lonib spectrophotometer. The color de1 clops fully in leq5 than 2 minutes and is itablc for a t least 30 minutes. After that tinir a slow air oxidation of I- begins to liecome e l ident. PrmrIrsrnLE Y ~ R I ~ T I O SKeither ~. the macro nor micro methods were appreciably affccttd by changes of = t O 3 u1iit.i in the p H of the buffcr, k507c in its roncrntration or ilOyGin the concmtration of the K I . Thui. saturated solution< of KI (5,921a t 15" C., 6.111 a t 20" C., 6.2111 a t 25" C.) could he uwd rather than the 6Jf solution actually employed. SOLUTIOX STmILIrT. Solutions of K I autoxidize slon-ly on standing, cren if well stoppered in dark brown bottles. T o obviate high blanks we discarded solutions when their absorbance a t 355 nip exceeded 0.1. This level of autoxidation, however, normally took many
weeks $0 that the solution could be ronsidered stable for at least a month. Solutions of S20s-*are not stable and decompobe to d04-2 and O2 with a half life of the order of 1 day. Hence, fresh solutions must be made up just before use. IKTLRFERENCES. The method is, of course, interferred with by species capable of ovidizing I- a t neutrality, for e\amplr peroxides, chromates, arid permanganates. DISCUSSION
Iletcrmination~ of t h r triiodide arid sulfate produced s h o w d the production of 1mole of 13-and two of S04-2per mole of Thus, the over-all reaction is S208-3 31P-t 2S04-? 13-. The rate of the reaction i, c-ontrolled not only by concentratioiii of the I - and S20s-2 but
+
+
also by the pH,although H + does not appear in the rate expression. Thus, a t low pH values the reaction b e b e e n 0.03X SzOs-2 and 0.3M KI is very slow at room temperature, even with Fe+3 added as a catalyst ( 5 ) . At neutrality, the reaction a t high concentrations is nearly instantaneous. Even a t lO-5,lf &Os-*, the reaction is complete in less than 2 minutes. This p H dependence may be due to protonation of either S208-2 or I- to yield appreciable equilibrium concentrations of HS20s- or HI. Below p H 6, also autoxidation of Ibeconies appreciable, while at a pH higher than 8, IB- is destroyed b y the reaction 3 Is60H-+ lo3- 8 I3 H 2 0 (4). Thus, the optimum p H for the reaction is near neutrality. Phosphate buffer mas chosen because phosphate, unlike organic acids, i i not ovidized by Also, a simple 1: 1
+
+
+
solution of HPO4+ and H?P04- is a strong buffer poised exactly at pH 6.85. LITERATURE CITED
(1) Furman, N. H., “Scott’s Standard Methods of Chemical Analysis,” p.
1015, Van Nostrand, Yew York, 1962. (2) Indelli, A., Prue, J. E., J . Chem. SOC., 1959, .107. (3) Lavine. T. F., J . Bid. Chem., 151, 281 (1943). ( 4 ) Moeller, T., “Inorganic Chemistry,” p. 441, Wiley, Xew York, 1952. (5) Wahba, N., El Asmar, M.F., El Sadr, M.M., ANAL.CHEM.,31, 1870 (1959). N. A. FRIGERIO Division of Biological and Medical Research Argonne National Laboratory Argonne, Ill. This work was performed under the auspices of the U. S. Atomic Energy Commission.
The Effect of Detector Block Fouling on Observed Separations in Gas-Liqu id Chromatog ra p hy SIR: Attention has recently been drawn to the effect of rarbon deposits in the injection port of gas liquid chromatography apparatus on the separation of strongly absorbed polar compounds ( 5 ) . Similar troublc can a l ~ ooccur in the detector block. and n ith nonpolar compounds. A drastic tailing effect developtd rather sutlclciily in aiialyqes of methj 1 esters of marine oils on polyester . This nas aicolumns (Figure s u m d to be clur to column failure, and considerable time n BS \\ abted in preparing and testing QCT era1 new columns 11 ith different upp poi th, lots of polyester, etc., with no im:irovcment. The injection port \vaj also cleaned without any changc i n rwultF Finally the de tector block i t a- t1iqm:intled and examination rm c~altda .oft tarry film in all parts of the detcction catherometer block pascage-. ‘I’his n a i reqistant to all co11rntc and had t o be removed nicclianical1~- Tlir improvement in the peak h l ) c a i d rlnrific~ation of minor componeiit~,usiriq th(>same colunin as in Figure 1.1, i i 4ionii in Figure 1B. IJndoitbtedly 311soi ption of the peaks from the column in thiq film n i t h subsequent 4 x 5 tl(wrl)tion was responsible for t h r tailing. This trouble lint1 not previously developed iii a 2 - y p~r ~~i o dof operation of the apparatu. (\TTilhins Aerograph A-SO), arid occuired despite regular, routine flushing of the detector block n ith both polar and nonpolar solvents.
I t is thought possible that this deposit arose from a series of analyses of methyl esters of air-blon-n marine oils, which contain appreciable amounts of conjugated unsaturation. F a t t y acid esters of this type have very long retention times (2) and would be retained on the column when the apparatus was shut down. On resuming operation n-ith this particular type of apparatus, v - h e r ~
the catherometer block is in the column oven space, the column Lvould heat up more rapidly than the block, allowing the conjugated esters to pass into the block, condense, and polymerize. It is also possible that a change in the polyester employed t o ethylene ylj-col succinate, R hich has a high bleed rate, nould also contribute to this deposit . However, continued usage of 811
18.1
n
Ii;; // iI
I
I ,
‘IO
d
I
l
20
30 ‘IO 20 30 TIME (MINUTES) Figure 1. Gas liquid chromatograms (in part) of 0.005 ml. of seal oil methyl esters on the same column (EGS) before ( A ) and after (6)cleaning of the catherometer block. Helium flow, 90 cc. per minute; oven temp., 200° C.; bridge current, 240 ma. Numbers a r e shorthand notation of fatty acid identification ( 4 ) VOL. 35, NO. 3, MARCH 1963
413
