nace and bubbled through 20 ml of tetrachloromercurate solution. The SO2 absorption method appears to be less sensitive than the pararosaniline colorimetric procedure and than the several other instrumental finishes, but much more sensitive than the titration methods. The method is, also, direct and specilic for SO2, quite simple to set up, and
should have wide applicability. It is being tested a t the present time for the analysis of flue gases of coal-burning furnaces and for the analysis of solutions encountered in the paper manufacturing process. Received for review December 15, 1972. Accepted March 5 , 1973.
Mercuric Iodate as an Analytical Reagent Determination of Chloride by Spectrophotometric Measurement of Mercuric Chloride with Phenolphthalein Complexone or Xylenol Orange Ray E. Humphrey’ and Willie L. Hinze2 Department of Chemistry. Sam Houston State University. Huntsville. Texas 77340
Most spectrophotometric methods for chloride are indirect and depend on the reaction of the ion with a slightly dissociated or relatively insoluble mercury compound to form mercuric chloride and release a n absorbing species or an ion which can react with another substance to develop a color. One widely used method is that involving the use of mercuric chloranilate in which the visible ( I ) or ultraviolet ( 2 ) absorption of the released chloranilate ion is measured. Probably the most common procedure is that in which mercuric thiocyanate reacts with chloride to release thiocyanate ion which then interacts with ferric ion to form the red F e S C V - complex ( 3 ) . This method has the advantage of being a homogeneous system with no separation step involved. Although soluble mercuric chloride is formed in both these reactions, little attention has been paid to this product as a means of determining chloride spectrophotometrically. Apparently the first report of the measurement of the mercuric chloride for chloride analysis was a radiochemical technique in which the radioactive isotope 203Hg was present in the mercuric iodate and the activity of the 203HgC12 in the reaction solution was determined ( 4 ) . An ultraviolet procedure was developed in which an excess of either Br-, Cl- , I - , or SCN- ions was added to the reaction solution containing the HgClz to form the corresponding complex ion HgX42-, all of which absorb in the ultraviolet region ( 5 ) so that chloride ion can be measured in the low parts per million range. Another ultraviolet method has been reported in which phenylmercuric nitrate reacts with chloride ion to form phenylmercuric Author to whom correspondence should be addressed. 2Present address, Department of Chemistry, Texas A and M University, College Station, Texas 78-43, ( 1 ) J. E. Barney II and R. J. Bertolacini,Anal. Chem.. 29, 1187 (1957). (2) R. J. Bertolacini and J. E. Barney 1 1 , Anal. Chem.. 30, 202 (1958). (3) D. M. Zall. D. Fisher. and M . 0. Garner, Anal. Chem.. 28, 1665 (1956). ( 4 ) F. Szabadvary, E. Banyai, and L. Erdey. Chim. Anal. (Paris). 45, 289 (1963). (5) R . E. Humphrey, R . R. Clark. L. Houston. and D. J. Kippenberger. Anal. Chem., 44. 1299 ( 1972)
chloride which then is interacted with sodium diethyldithiocarbamate. The phenylmercuric diethyldithiocarbamate is extracted into chloroform and the ultraviolet absorption measured (6). No literature reports were found describing the determination of chloride ion by reaction of the soluble mercuric chloride with a complexing agent to form a species absorbing in the visible region which is soluble in the aqueous system. We have found that phenolphthalein complexone, PPC, and Xylenol Orange, XO, can be used for the spectrophotometric determination of chloride by formation of the corresponding mercury complexes. The complexes formed between mercuric ion and PPC (7) and between mercuric ion and XO (8) have been described. The use of these reagents may have some advantages in that the complexes formed absorb in the visible region and no extraction step is necessary since they are both water soluble.
EXPERIMENTAL Apparatus. Absorption measurements were made with Beckman ACTA I11 and DK-2A spectrophotometers. A Lab-Line J u nior Orbit Shaker was used to agitate the reaction solutions. Reagents. Phenolphthalein complexone (PPC) and the tetrasodium salt of Xylenol Orange (XO) were Eastman reagent chemicals. Mercuric iodate was obtained from City Chemical Corp., New York, N.Y. The potassium chloride used was a Baker and Adamson reagent chemical while the hexamethylenetetramine ( H M T ) was a Fisher reagent chemical. The stock solution of PPC was 0.001M and was made up in 1: 1 ethanol-water. The Xylenol Orange solution was 0.0012M and was also prepared in the ethanol-water solvent. The standard solution o f KC1 was made up in distilled water for the work with PPC and in ethanol-water for the XO procedure. The buffer solution of pH 11 was made by mixing 500 ml of a 0.05M NaHC03 solution with 227 ml of a 0.10M NaOH solution. Procedure. Reactions of Chloride Solutions with Mercuric lodate. Measured volumes of a stock solution of KC1 were diluted to 20 ml with distilled water for the PPC work and to 10 ml with (6) R . Belcher. J. A . Rodriguez-Vazquez, and W . I . Stephen, Anal. Chim. Acta, 61, 223 (1972). (7) T . Nomura. K. Watanabe, and S. Komatsu. Bull. Chem. 45.640 (1972). (8) H.’H. Walker and J. A . Poole, Talanta. 16, 739 (1969).
Soc. Jap.,
ANALYTICAL CHEMISTRY, VOL. 45, NO. 9. AUGUST 1973
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~
Table I. Beer’s Law Data for Chloride with Phenolphthalein Complexone Low dilutiona
High dilution* CI-,
GI-,
ppm 1.o 2.0 4.0 6.0 a
A, 568nmc
e
0.21 0.40
7500 7100 7300 7300
0.80 1.20
ppm 3.5 7.0 10.5 17.5
A , 568 nmc
c
0.53 0.90 1.32 2.05
4300 4000 4000 3900
Final volume 15 ml. Final volume 25 ml. C A = 0.40 for t h e blank.
Table II. Recovery Data for Chloride with Phenolphthalein Complexone Low dilution
High dilution CI-
present, ppm 1.2 2.4 4.4
5.2
CI -
found. ppm 1.3 2.4 4.2 5.1
Error, YO +8.5
... -4.5 -1.9
CI- present,
ppm
5.3 8.8 12.3 15.8
CI found, ppm
5.4 8.6 11.9 15.2
Error, % +1.9 -2.3 -4.9 -4.0
1:l ethanol-water for the XO procedure. Solid Hg(IO&, 20-30 mg, was then added and the solutions were shaken for 20 min. The excess solid was then removed by filtration through filter paper and 5.0-ml aliquots were withdrawn for development of the color. Development of the Xylenol Orange Complex. This procedure is essentially that of Walker and Poole (8). First 3.5 ml of a saturated solution of HMT were placed in a flask. The pH was adjusted to 7.0 by adding 0.5 ml of a 0.6M HNOJ solution and 0.5 ml of a pH 6.9 phosphate buffer. The 5.0-ml aliquot of the reaction solution was then added followed by an additional 1 g of HMT. Finally 0.5 ml of the Xylenol Orange solution was added and the absorbance measured at 588 nm. The complex forms instantly and is stable for at least 30 min. Deuelopment of the Phenolphthalein Complexone Species. Five-milliliter aliquots of the reaction solutions containing mercuric chloride were placed in flasks. The appropriate amounts of the PPC and pH 11 buffer solutions were then added. For the lower concentration range 1.5 ml of PPC solution and 8.5 ml of the buffer were used while 2.5 ml of PPC and 17.5 ml of the buffer solution were used for the higher concentrations. The final volumes were 15 and 25 ml, respectively. Absorbance values were measured at 568 nm after 5-10 min. The color developed immediately and was stable up to 1hr.
RESULTS AND DISCUSSION Reaction of Mercuric Chloride with Phenolphthalein Complexone. The mercury(I1) ion has been reported to react with PPC in an aqueous buffer of pH 11 to form a complex which has an absorption maximum at 568 nm with a molar absorptivity of 35,000 (7). The complex, which is reported to have the stoichiometry Hg(PPC)2 and an apparent stability constant of 1 x loll, forms immediately on mixing the reactants and is reasonably stable. The absorbance is constant for 20 min and decreased about l%/hr after that time. It was also stated that the absorbance of the Hg(PPC)2 complex decreased on adding cyanide, iodide, or thiosulfate ions but did not change with bromide, chloride, or thiocyanate ions. In this work, it was found that HgC12 reacted with PPC in pH 11 buffer to form a complex with a maximum absorption at 568 nm and an apparent molar absorptivity of approximately 40,000. Beer’s law is followed over the range of 1-4 X 10-5M HgCl2. The maximum absorption for a fixed amount of HgCl2 was obtained when the ratio of PPC to mercury(I1) was slightly greater than 2:l. A 1748
small excess of PPC was used for the chloride measurements. The color was found to be reasonably stable with no significant change in absorbance after 1hr. Determination of Chloride Ion with PPC. The absorbing species is formed by simply adding the PPC stock solution and pH 11 buffer to an aliquot of the Hg(IO&-Clreaction solution after removal of the excess solid by filtering. Maximum sensitivity was obtained using a final volume of 15 ml. Chloride ion can be measured in the range of 1-6 ppm with an effective molar absorptivity of 7300. Higher concentrations can be determined by diluting to a volume of 25 ml. The effective molar absorptivity at this dilution is 4000 for chloride levels of 3-18 ppm. Beer’s law data are summarized in Table I. Reproducibility of the method is acceptable, as shown by the recovery data for known levels of chloride in Table II. The precision would appear to be as good as for most spectrophotometric methods. Study of Anion Interferences. Neither cyanide nor iodide ions were found to yield a color in the same concentration range as chloride. Both anions do react with Hg(I03)2 (9) but the Hg(CN)2 and HgI2 formed do not react with PPC to produce a complex in agreement with the earlier report (7). Sulfite ion also reacts with mercuric iodate (9) but no color is produced when PPC is added. Bromide ion can be measured with PPC in the same manner as chloride, Beer’s law being followed, with an effective molar absorptivity of 3800 using a 25-ml final volume. Bromide ion can be measured in the range of 5-35 ppm using the dilution. Sulfide ion also reacts with Hg(I03)2 (9) but should not interfere in this method since the HgS formed is very insoluble. Study of Metal Ion Interferences. The possible effect of the presence of certain metal ions on this method was evaluated using millimolar solutions of Ca2+, Cu2+, Fe3+, Mg2+, Ni*+, Pb2+, and Zn2+ ions. At pH 11 only Ca2+ yielded a color with PPC. However, with any of the other metal ions present, no color developed on adding HgCl2 to the solutions containing PPC and the metal ion. Apparently the metal ions react with PPC but do not form colored complexes. It is obvious that metal ions would have to be removed before measuring chloride ion by this method. This could probably be done effectively by use of a cation exchange column. Comparison with Other Methods. The Hg(IO&-PPC procedure is considerably more sensitive than the mercuric chloranilate method when measuring visible absorption ( I ) . It has been our experience that mercuric iodate is preferable to the chloranilate in regard to reagent purity, stability, and ease of removal of the excess solid (IO). Of course, the chloranilate procedure is more sensitive when the ultraviolet absorption is measured (2). The Hg(I03)2PPC method also results in a highe; effective molar absorptivity than the use of the mercuric thiocyanate-ferric ion procedure ( 3 ) . However, the thiocyanate method does have the advantage in that no separation step is required. It is likely that more anion interferences would be encountered with the thiocyanate procedure (11). The only method similar to that described here in which a complex involving mercury(I1) is measured is that of Belcher, Rodriguez-Vazquez, and Stephen (6). The reagent used is phenylmercuric nitrate and the ultraviolet absorption of the mercuric diethyldithiocarbamate complex is measured in chloroform solution. The procedure is (9) W. L. Hinze and R. E. H u m p h r e y , Anal. Chem., 45, 385 (1973). (10) R. E. Humphreyand W. Hinze,AnaL Chem., 43,1100 (1971). (11) W. L. Hinze, J. Elliott, and R. E. Humphrey, Anal. Chem., 44, 1511 (1972).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 9, AUGUST 1973
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Table I V . Recovery Data for Chloride with Xylenol Orange CI- present, pprn CI- found, pprn Error, %
Table Ill. Beer’s Law for Chloride with Xylenol Orange CI -, ppm
A , 588 n m a
8
2.0 4.9 7.4 9.8 12.3
0.45 0.74 0.95 1.35 1.72
5300 4500 4800 4900
...
2.9 5.8 7.9 9.8 11.6
2.7 5.3
8.0 10.2 12.3
-6.9 -8.6 +1.3 4-4.1 +6.0
a A = 0.30to 0.40for the blank for the lowest concentrations. Values are somewhat uncertain because of a blank correction problem.
lenhhy, involving two extractions and a concentration step. Molar absorptivities are reported to be 21,300 a t 257 nm and 6,500 at 297 nm. These values are based apparently on the concentration of the mercury complex in the final chloroform solution. It is uncertain as to how the effective molar absorptivity for chloride would compare with the use of PPC as a complexing agent. Reaction of Mercuric Chloride with Xylenol Orange. Measurement of chloride using this reagent is somewhat more involved than with PPC. The molar absorptivity of the HgC12-X0 complex was found to be about 24,000 at 588 nm, somewhat lower than that reported for the complex with mercuric nitrate (8). The stoichiometry is reported to be 1:l which was also found to be the case in this work using HgClz. The color develops rapidly and is reasonably stable. The p H is critical and must be very close to 7.0. It is also necessary to have a high concentration of hexamethylenetetramine for maximum color. Chloride ion can be measured in the range of 2-14 ppm with Xylenol Orange. Beer’s law plots from data obtained in this procedure were not quite linear over the entire range measured. A straight line was obtained from 2-7
ppm C1- and a second straight line with somewhat greater slope resulted from 7-15 ppm. Beer’s law data are shown in Table I11 and recovery data presented in Table 1V. Precision is acceptable if reasonable care is used in obtaining the Beer’s law plot. The effective molar absorptivity for chloride at the higher concentrations is approximately 4800. A 1:l ethanol-water solvent is necessary in order for the blank to be acceptably low. Since Xylenol Orange forms colored complexes with a large number of metal ions (12), it would also be necessary to remove most metal ions prior to the development of color. Possible interferences of other anions were not evaluated for Xylenol Orange but would be expected to be about the same as with the use of phenolphthalein complexone. Received for review January 8, 1973. Accepted March 19, 1973. The authors express their appreciation to the Robert A. Welch Foundation of Houston, Texas for partial support of this research. This work was also supported in part by the Faculty Research Fund of Sam Houston State University. (12) D. F. Bok and M . G. Mellon,Anal. Chem., 42 ( 5 ) , 152R (1970)
Analog Cross Correlation Readout System for a Spectrometer Using a Silicon Protodiode Detector Gary Horlick and Edward G. Codding Department of Chemistry, University of Alberta, Edmonton, Alberta, T6G 2G2 Canada
Smoothing, resolution enhancement, and differentiation of spectra can be carried out using cross correlation techniques (1-4). The application of a self scanning linear silicon photodiode array detector to a spectrometer (5) has facilitated the development of a real time analog cross correlation readout system. By utilizing the system presented here, all of the above operations can be performed on spectra right a t the spectrometer by cross correlation with an electronic waveform that replaces the conventional mechanical exit slit.
(1) A. Savitzkyand M . J . E. Golay, Anal. Chem., 36, 1627 (1964). (2) Jean Steinier, Yves Termonia, and Jules Deltown, Anal. Chem., 44, 1906 (1972). (3) Gary Horlick, Anal. Chem., 44,943 (1972). (4)W. Snelleman, T. C. Rains, K. W. Yee, H. D. Cook, and 0. Menis, Anal. Chem., 42,394 (1970). (5) Gary Horlick and Edward G. Codding, Anal. Chem., 45,1490 (1973).
EXPERIMENTAL Apparatus A block diagram of the cross correlation system is shown in Figure L4.The monochromator, photodiode array detector, clock, and measurement system were as described in reference ( 5 ) . A slit width of 100 fim was used for all measurements. A spectral region of about 110 8, was detected by the array which is 0.256 inch long and is made up of 256 photodiodes. The array was scanned a t a clock rate of 30.0 KHz so that a complete spectrum (110 8, region) was read out in 8.5 msec with a repetition time (time between start pulses) of 60 msec. The electronic gate waveforms which were cross correlated with the spectra (Figure 1B) were synthesized by the gate function generator which consisted of three delay monostable-gate monostable combinations and a system of operational amplifiers. In order to synchronize the gate waveforms with the spectral signal, the delay monostables were triggered by the photodiode array start pulse. Depending on the application, widths of the gate waveforms varied from 100 to 400 fisec which is equivalent to approximately 1.3 to 5 8, on the spectral scale. Procedure. To perform the cross correlation, the spectral signal from the photodiode array and a gate waveform were multiplied together (-17 multiplications per sec) using an analog multiplier
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