ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
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Spectrophotometric Determination of Trace Sulfate in Water Purnendu K. Dasgupta, Lionel G. Hanley Jr., and Philip W. West" Environmental Sciences Institute, Chemistry Department, Louisiana State University, Baton Rouge, Louisiana 70803
Determination of trace levels of sulfate in water samples is described. The method relies upon the formation of insoluble 2-perimidinylammonium sulfate, (PDA),SO,, by reaction of the sulfate ion with 2-perimidinylammonium bromide on a glass fiber substrate. The excess bromlde is removed by frontal elution chromatography with methanol and the remaining organic amine, bound as the sulfate salt, is converted to highly colored 2-amino-4,6,9-trlnitroperimldine by treatment with concentrated nitric acid. The colored compound may be monitored in acidic solution at 420 nm or in basic solutlon at 550 nm. No pretreatment is requlred and for a 100-yL aliquot the sensitivity is 1 ppm and the working range is 2-500 ppm sulfate. There are no known cationic interferences and, among anions likely to be present, only oxalate and phosphate interfere.
T h e determination of sulfate ion a t trace levels is one of the most widely studied problems in analytical chemistry. A detailed account of the two centuries of scientific investigation devoted to this end is now available (1). Stephen ( 2 ) introduced 2-aminoperimidine (in the form of the hydrochloride salt) as a sulfate precipitant. 2-Perimidinylammoniumsulfate, (PDA)*S04,is second only t o barium sulfate in insolubility (20 mg/L a t 18 "C). An automated version of Stephen's nephelometric method has since been develclped ( 3 )and, in an alternative approach, after filtering off the organic sulfate, remaining excess of accurately added precipitant has been determined by UV absorptiometry ( 4 , 5 ) . Work done in this laboratory ( 6 ) ,however, revealed that t h e data obtained by this method for most field samples are substantially higher than those obtained by the gravimetric barium sulfate procedure which is t h e accepted reference method (7) or a standard turbidimetric barium sulfate method (8). The high results are caused by simultaneous precipitation of other anions, notably bicarbonate. Maddalone e t al. (9) discovered t h a t pyrolysis of the precipitated (PDA),S04 results in the quantitative formation of SO2 which could then be determined by t h e West-Gaeke method or by means of the flame photometric detector. This principle was adopted and optimized by Thoma e t al. (6) for aqueous sulfate determinations. Based on a new reaction, we report here a new spectrophotometric procedure that is fast, sensitive, requires no sample pretreatment, and does not involve pyrolysis. EXPERIMENTAL Reagents. 2-Perimidinylammonium bromide (1%). The reagent was synthesized and purified according to Dasgupta et al. (10). One hundred milligrams of this reagent were dissolved in 10 mL of warm methanol and stored in a dark bottle. Addition of a small amount (- 1 mg) of tert-butyl hydroxy toluene (BHT) lengthened the shelf life of the reagent to several months. All other reagents used were of analytical reagent grade. The preparation of standard sulfate solutions and standard solutions of other anions has been described (6). Apparatus. A Beckman DB spectrophotometer with a 1-cm path length cell was used. Gelman spectrograde A glass fiber fdters were cut in 18 mm x 72 mm wide strips. Fifty such strips were 0003-2700/78/0350-1793$01,00/0
cleaned in a batch by boiling them for 4 h in a mixture of 50% ethanol, 20% HC1, and 30% water (v/v). The solvent was then decanted and the strips washed thrice with 50-mL portions of ethanol and twice with 50-mL portions of acetone and then oven-dried at 80 "C. Teflon-tipped stainless steel forceps were used throughout to handle these strips. Procedure. A precleaned glass fiber strip was placed on a Teflon block having a 13-mm diameter annular opening. The bottom of the strip was -20 mm from the center of the aperture (a guide mark was inscribed on the block to facilitate positioning, see Figure 1) and covered with another identical block to hold it in place. Fifty FL (one drop) of the PDA-Br solution was added from a Pasteur pipet to the strip at the center of the aperture followed by the sample aliquot. Another 50 pL of PDA-Br solution was next added to the same spot. The strip was then removed and dried (-3 min) in a stream of filtered cold air. Methanol was placed in a 250-mL tall form Griffin beaker, to a depth not exceeding 5 mm. Up to five strips were then lowered in the solvent and allowed to remain i n the beaker for a period of 10 to 15 min during which time the methanol moved up the strip thus removing the unreacted PDA-Br. The strips were removed from the beaker, placed on the Teflon block, and the injection spot was cut away with a stainless steel filter cutter. The sample circle was then placed in a funnel resting on a 10-mL volumetric flask. Methanol was allowed to evaporate from the circle until it was just moist. Six drops (-300 KL)of concentrated nitric acid were added from a Pasteur pipet to the circle, distributing it as evenly as possible. The material on the filter was then allowed to react for approximately 1 min. Measurement was carried out according to either one of the following procedures. Measurement in Acidic Solution. The colored product on the circle was washed into the flask with 2 mL concd "OB from an automatic pipet. One mL was added at a time, slowly, to facilitate the removal of all the colored material. The solution was then made up to the mark with distilled deionized water, mixed thoroughly, and filtered through glass wool into the measuring cuvette. The filtration step was necessary to remove fibrous material that unavoidably resulted from the substrate. Absorbance was measured a t 420 nm against water as reference and the amount of sulfate was calculated from the calibration plot. Measurement in Basic Solution. The colored product was washed into the flask with 1 mL of acetone using the technique described above. Two mL of 4 N NaOH was next added and the solution made up to the mark with distilled deionized water. The solution was filtered into the cuvette and the absorbance was measured a t 550 nm. RESULTS AND DISCUSSIONS Sulfate ion was liberated and 2-amino-4,6,9-trinitroperimidine was formed when nitric acid reacted with (PDA)*SO, under the conditions described (Figure 2). Details of structural determination of this and related compounds will be published elsewhere (11). Calibration plots for both acidic and basic solutions were perfectly linear within t h e range studied (0-50 pg sulfate) and therefore are not shown. The absorbance obtained in acidic solution is 0.021 absorbance unit/Fg sulfate put on the filter, while measurement in basic solution increased the sensitivity by 30% to 0.028 absorbance unitlyg sulfate put on the filter. All data were obtained in sets of five and representative standard deviations are shown in Table I. The limit of detection, normally defined to be 2.5 times the standard deviation of the blank, is hard to define in this case because the absorbance of the blank was uniformly 1978 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
Table I. Precision of the Aqueous Sulfate Determination Method measurement in amount of - acid sulfate o n mean % standard mean absorbance filter, ii.g absorbance range deviation 0.56 0.011 0.010 14.8 0.015 1.39 0.0 29 0.038 0.01 3 8.3 3.46 0.097 0.019 4.6 0.070 8.67 0.243 0.048 4.3 0.182 10.0 0.211 0.280 0.030 2.4 21.7 0.457 0.104 3.9 0.610 1.14 54.2 0.210 4.1 1.53 Cover block
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Figure 1. Positioning the substrate for sample injection
and reproducibly 0.000 (Procedure carried out with distilled water instead of sample solution). One-tenth of a microgram sulfate corresponding to -0.002 absorbance unit may be taken as the detection limit, this being the smallest scale increment in the spectrophotometer used. Glass fiber filters are water repellent and aqueous solutions tend to bead on it. T o transfer an aqueous solution effectively to such a substrate, the substrate must first be moistened with a water-miscible solvent that has a surface tension low enough to wet the substrate. Methanol was a suitable solvent for this purpose. If t h e original spot of methanolic PDA-Br put on the filter became dry by the time transfer of the sample solution was attempted, the spot was remoistened w t h another 50-pL drop of methanol. However, this was done only if necessary, because it was advantageous to keep the diameter of the injection spot as small as possible. Two separate sets of calibration plots were constructed to study the effect of the sample volume. One set was constructed by varying the amount (1-100 pL) of a 200 ppm sulfate solution placed on the substrate and in the other set 100 pL of sulfate solutions of varying concentration was used. There was no discernible difference in the data obtained. Transfer of solutions up to 25 pL in volume was rapid and convenient. Aliquots exceeding 25 pL here slow in transfer but the process could be facilitated somewhat by adding another drop of PDA-Br a t some halfway point during the transfer. Sample aliquots should be kept within 25 pL to reduce analysis time, unless higher sensitivity is necessary. Reproducibility was improved, especially with amounts below 1 p g , by allowing the spot to dry as completely as possible before chromatographing, thus apparently allowing t h e (PDA12S04to form and nucleate completely. With very small amounts of sulfate, proper precleaning of the substrate (and keeping it free from contamination) is an absolute necessity
% standard
range 0.011 0.018 0.027 0.044 0.052 0.141 0.350
deviation 11.8
8.4 4.4 2.8 3.1 4.0 4.1 _
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Table 11. Extent of Interference on Aqueous Sulfate Determination (lOO-c(L 100 ppm sulfate injection, methanol elution) concentration (times that of sulfate) interfering ion 1:l 1:lO 1:lOO 1:1@@0 Fnone none a none CI none none Brnone none Inone HCOOnone none 50-100%C C20,'133% v.h.b v.h. CH,COOnone none 16.5% H,PO, 33% v.h. v.h. HCO; 11% v.h. v.h. NO; none none v.h. a Not sufficiently soluble. v.h. = very high, > 500% Irreproducible. -
Table 111. Threshold Concentrations at Which Interference First Appears (100-pLinjection, methanol elution) interfering ion threshold concentration Cl-, Br', I - , F - saturated sodium/potassium salt solutions do not interfere HCOO > 1000 ppm c,o, 2 interferes at all levels studied, down to 10 ppm CH,COO > 5000 ppm H,PO; > 2 5 ppm HCO > 100 ppm NO; > 1000 ppm since, according to manufacturer's specifications, the sulfate content of this substrate may range u p to 2 pg/cm2 (12). Ethanol cannot be substituted as a solvent for methanol because it undergoes a rapid explosive reaction with concentrated nitric acid via the formation of ethyl nitrate. Methanol was not found to undergo any visible reaction under the experimental conditions. Acetone is the best solvent to remove the colored product from the filter. While acetone is known to undergo an acid or base catalyzed aldol condensation leading to yellowish products, no appreciable change in color was noted after keeping 1:1 mixtures of acetone with concd HN03 and NaOH, respectively, for several hours. Similar t o water, nitric acid does not penetrate the glass fiber substrate. The substrate must be moist with a suitable
0
N'
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NO2
Figure 2. 2-Perimidinylammonium sulfate-nitric acid reaction
base
NH
ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
Table IV. Comparison of Analytical Results Obtained by Different Methods (ppm sulfate) (PDA),SO, BaSO, pyrolytic graviBaSO, barium (PDA),SO, sample PH method metric turbidimetric chloranilate nephelometric 43.2 t 7.3 Baton Rouge, 8.67 9.8 10.4 t 0.3 6.96 i- 0.19 10.4 t 0.5 La. Garnavillo, 7.86 71.0 i- 2.4 11.3 i. 1.9 12.6 13.6 z 0.2 13.24 i 1.04 Ia . Kenner, La. 9.61 61.2 i 0.3 54.9 i- 1.9 52.8 54.4 I 0.8 46.8 i- 0.84 Natchitoches. 7.32 48.4 t 0.5 43.6 45.2 i 0.4 41.20 + 0.99 42.0 i 1.0 La. Diamondhead, 8.67 8.2 * 0.3 9.1 9.5 i 0.1 7.60 i- 0.40 19.8 i 0.5 Miss. solvent to ensure proper distribution of solutions into the filter. If the circle became dry before acid treatment, it could be remoistened with methanol/acetone. Excessive amounts of solvent should be avoided because overdilution of the acid results in a longer reaction time and precision also decreases, presumably because of introduction of undesired products through side reactions (11). With sulfate levels over 10 pg, it was not possible to remove every trace of color developed from the substrate. However, the amount remaining was not sufficient to significantly affect the results. In regard to measurement in acidic or alkaline medium, there is no significant superiority of one approach over the other. While measurement in basic solution enhances the sensitivity slightly, measurement in acidic solution is slightly faster. If the latter method is chosen, the nitric acid used must be free from oxides of nitrogen, which absorb strongly in the same region. The colored product is yellow in strong acid and purple in strong base and does not exhibit a great extent of pH-dependence at either extreme of the p H scale and therefore no adjustment of p H is necessary. Interferences. Interference studies were carried out in detail only with selected anions, initial experiments indicated no cationic interference, as may be expected from the chemical basis of the method. Interference studies were carried out to determine (a) the percentage interference (in terms of increased absorbance from calibration value without interferant) imposed by the interfering ion on 10 pg sulfate (100 pL, 100 ppm) where the former was present in amounts 1000, 100. 10, and 1times the amount of sulfate; (b) the interfering threshold, Le., the smallest concentration of an interferant ion which produces the first detectable tinge of color in the substrate when the procedure is carried out with a 100-pL aliquot of the interferant ion solution in the absence of sulfate. The results of the study (all determinations were carried out in sets of five) are shown in Tables I1 and 111. If the mean of the results were within 5% of the mean calibration value without interferant, extent of interference is reported as none. From the tables, it can readily be ascertained that the method can be regarded as relatively interference free, a t least as far as application to actual water samples is concerned. The only significant interferences were due to oxalate, phosphate, and bicarbonate. (Studies with carbonate could not be carried out because highly alkaline solutions such as sodium carbonate react with PDA salts liberating the free amine, the free amine is rapidly air oxidized to tarry products. For the same reason, the method was inapplicable to samples with pH >9 without pretreatment.) Studies were carried out with an eluting solvent of 1% monochloracetic acid in methanol to investigate if the extent
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this work 10.8
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of interference could be reduced. T:he precision of the data was poorer than obtained with pure methanol. The standard deviation was acceptable (
