Determination of Sulfate and Phosphate in Water by Ion Exchange-Titrimetric Method George W. Dollman
Litton Systems Inc., Woodland Hills, Calif. 91364 -
_-
Sulfate in natural water or other aqueous media can be accurately determined as sulfuric acid after passage of the sample through a hydrogen-form cation exchange resin. The method, which is especially suited to routine laboratory determinations o n large numbers of samples. involves evaporation of the column effluent (a mixture of acids) under conditions which drive o f all ordinary acids while quantitatively retaining the sulfuric acid, which is then determined by titration with standard base. With prior sample dilution or concentration, sulfate in parts-per-billion to whole per cent amounts may be determined. Interfering substances are discussed, and data demonstrating high accuracq and reliability are presented. The procedure may also be used to determine phosphate, or both phosphate and sulfate. Methods for determining organically bound sulfur or phosphorus, following ashing of sample, are presented. H
F
ishman, Robinson, et al. (1967) have reviewed methods for the determination of sulfate in water. Many of these involve the use of ion exchange resin to remove interfering cations, followed by precipitation of sulfate with standard barium or lead solution, and a complexometric titration t o determine excess barium or lead. For low concentrations of sulfate, turbidimetric or colorimetric methods are generally used. Many recent articles on sulfate determination report techniques for improving accuracy or sensitivity of an existing method. The classical gravimetric determination as barium sulfate is still the referee method, though little used these days because it is time-consuming and subject to a number of interferences. An analytical text (Scott, 1939) describes the determination of sulfuric acid as a contaminant in more volatile acids by evaporating the sample on a steam bath to eliminate the volatiles, then titrating the residue with standard base. No data are given regarding losses of sulfuric acid during the evaporation, and it is known that sulfuric acid is slowly evaporated at steam bath temperatures when volatiles have been driven off. No mention is made of other possible nonvolatile acids which, if present, would interfere in the determination of sulfuric acid. The method described is rapid and more accurate than any method known to this author. It involves the conversion of
sulfate and other anions to the corresponding acids by passage through a strong acid-type ion exchange resin in the hydrogen form. The effluent is evaporated under controlled conditions which completely volatilize ordinary acids while quantitatively retaining the sulfuric acid. Titration with standard base completes the determination. Any phosphate present will remain as phosphoric acid, and a procedure for the stepwise determination of phosphate and sulfate is described. Interference from organic matter can be eliminated by solvent extraction, filtration, or oxidation with bromine water. Equiprent ctnd Reagents
Equipment includes a small resin column packed with about 20 ml. of strong acid resin, 20- to 50-mesh. hydrogen form; a surface temperature thermometer, 0" to 400' C., or other suitable means for determining the surface teniperature of the hot plate-e.g., a thermocouple in an aluminum block heat sink; bromocresol green indicator, or a pH meter equipped with a miniature combination electrode (the small titration volumes demand a miniature electrode system); electrical conductivity apparatus for estimating total ionic concentration of liquids; 25-ml. platinum crucibles or dishes; and a 10-ml. buret. The platinum crucibles and surface temperature thermometer are required only if phosphate is to be determined. The conductivity apparatus is required only for the analysis of brines, brackish water, or other samples of high ionic strength (specific conductance > 2000 inicromhos per cm.).
Procedure Determination of Sulfate. If organic matter in excess of about 100 mg. per liter is present, extract it with an appropriate water-immiscible solvent, or, if nonextractable, use bromine water and boiling t o destroy the major portion. For natural water samples, this step is unnecessary. Take an aliquot containing about 5 mg. of sulfate, and of total ionic strength not greater than 0.02N (dilute if necessary). Pass through the resin column at a rate of 3 to 5 ml. per minute, followed by one column-volume of rinse water. Collect the effluent in a 100-nil. beaker and evaporate t o 5 ml. on a hot plate. Finish the evaporation in a n electric oven with thermoregulator set for 75" C. About 2 hours' oven time is required-overnight does no harm. The residue should appear oily, with no trace of solid matter. If any solids are observed, this indicates incomplete ion exchange as the result of one of the following
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causes: Resin is partially or completely exhausted from previous samples; passage rate is too fast; ionic strength is too high; o r there is much dissolved organic residue. I n any event, a solid residue will result in a low value for sulfate o r phosphate, except that small amounts of organic residue, 10 mg. or less? are usually tolerable. The oven time can be reduced to about hour if samples are taken down to 1 ml. on the hot plate, but sample should not be allowed t o go t o dryness o n the hot plate. Conduct the evaporation in an area of the laboratory free from ammonia and other vapors which might react with the acid mixture. When all volatile acids have been driven off (odor test), remove from the oven and add 1 ml. of water plus a drop of bromocresol green indicator. Titrate with 0.02N sodium hydroxide to a blue end point (pH 5). Record the volume to the nearest 0.01 ml. Determination of Phosphate. Follow the procedure for sulfate down to the low volume evaporation. Then transfer the sample to a small platinum dish or crucible and, avoiding spattering, evaporate to dryness on the hot plate. Then set the temperature control to yield a surface temperature of hour to drive off all sulfuric acid. 275" C. and heat for Remove from the hot plate, add 10 ml. of water, and heat to near boiling. Maintain this temperature for 15 minutes t o hydrolyze polyphosphates t o HsP04. When the volume is 1 to 2 ml. add a drop of bromocresol green indicator and titrate with 0.02N sodium hydroxide to a blue-green end point (pH 4.6), preferably in the platinum crucible. Confirm the complete removal of sulfuric acid by acidifying with hydrochloric acid and adding barium chloride solution. If a turbidity appears, repeat the analysis and verify the 275" C. hot plate temperature. Calculation
Both hydrogens in sulfuric acid, and one hydrogen in phosphoric acid, are titrated at the pH's specified. Hence, 1 meq. of base equals 48.0 mg. of sulfate or 31.66 mg. of phosphate. Discussion
Thin films of sulfuric o r phosphoric acids suffer no detectable evaporative loss when heated for 16 hours in an 80" C. oven, but a 903 C. temperature causes some loss of sulfuric acid. Hence, a 75" C. oven temperature was adopted for the margin of safety it affords. At 75" C . a thin film mixture of sulfuric, phosphoric, hydrochloric, nitric, and hydrofluoric acids was completely free of volatile acids within 15 minutes after evaporation to apparent dryness. Hence, the 2-hour oven exposure is adequate to eliminate the remaining water and volatile acids. This procedure was developed primarily for the analysis of natural water, more for sulfate than for phosphate. Ordinarily, the concentration of phosphate and organic matter in natural water is very low, usually less than 0.1 mg. per liter, and hence their effect on a sulfate determination is nil. However, samples such as sewage, industrial wastes, brines, and sea water can also be analyzed. Organics. Three categories of organics have been considered: filterable sediment (as in sewage), oils and greases (as in sewage, industrial wastes, oil field brines, etc.), and watersoluble organics, partially or wholly oxidizable by bromine. It was assumed without proof that oily or sedimentary con1028 Enikonmental Science and Technolog,
taminants would interfere with efficient ion exchange, either by coating the resin beads or by blocking the passages between them. Interference from these sources is eliminated by filtration o r benzene extraction. Water-soluble organics, if still present when the sulfuric acid is in the form of a thin film of concentrate, interfere either by reacting with titratable hydrogens or by charring, the resultant dark color then interfering with the visual end point determination. In most instances, boiling with bromine water will destroy many organics and thus eliminate interference from them. Phosphate. Phosphoric acid is much less volatile than sulfuric acid and hence will register as sulfuric unless corrected for-for example, whereas sulfuric acid is quickly and completely evaporated from a platinum crucible kept at 275" C.. loss of phosphoric acid is very trivial in 2 hours at this temperature. This is highly convenient, for it affords a simple: accurate method for determining and correcting for any phosphate present in the sample. While this method is not recommended for determining minute amounts of phosphate, a 1-ml. titration volume of 0.02" base is equivalent to 0.63 mg. of phosphate o r about 0.21 mg. of phosphorus. Hence relatively low levels of phosphorus may be determined. The 275" C. heating must be carried out in platinum ware, because glass is attacked by phosphoric acid at this temperature, and some of the acid is thus neutralized by basic constituents in the glass. When phosphoric acid is heated to 275" C. partial internal dehydration occurs (polyphosphate formation) with consequent loss of titratable hydrogens. However, a 15-minute heating with water fully hydrolyzes the polyphosphate and restores the acidity. Table I shows the excellent agreement between gravimetric and volumetric determinations of sulfate in natural water
Table I. Gravimetric and Volumetric Determinations of Sulfate in Various Samples Sulfate Found, Mg./L. Gravimetric Volumetric Sample" 1 427 426 419 42 1 2 390 391 3 188 4 188 165 5 167 162 161 6 7 101 104 95.7 96.5 8 87.0 87.5 9 59.7 60.2 10 40.1 39.4 11 26.4 27.0 12 Sea waterb (one sample) Sewage' (one sample) (I
2650 2640 2640 125 124+ 124
2630 2630 2630 124 124 124
Natural waters (surface streams, wells, reservoirs, etc.). Diluted 1 to 50 prior to ion exchange. Filtered and brominated prior to ion exchange.
Table 11. Determination of Sulfate in Standard K2SOaSolution Sample volumes, 50 ml. __________hlilligrams SO,*-
Present
Found
12 5
12.5 12 5 12.5 12.5
Std. Dev.
...
1.25
1.25 1.25 1.25 1.25
...
0 125
0.127 0.125 0 127 0 . I27
0.0014
‘I 3
I 4
5
M/L L / L KRS
6
Ma OH ~ O M) Z
Figure 1. Titration curve for first H+ in H3P04 Table 111. Determination of SO,*- and POa?-in Mixed Standards Sample volumes. 50 ml. Sulfate, Mg. Phosphate, M g . Run Present Found Present Found ~~~
1
2
10 0 10 0
10 0 10 0
1 00 1 00
0 95 0 98
3 1
5 00 5 00
5 00 5 00
5 00 5 00
4 95 4 95
5
1 00 1 00
I 05 1 03
10 00 10 00
IO 0 9 9 f
6
Procedure.
I . Determine S01*- plus P043-. 2. Determine on separate sample. 3. Determine S 0 4 2 - by difference
samples. In general, the gravimetric results are a trifle higher; this may be due t o slight impurities in the barium sulfate. Table I1 demonstrates the high accuracy of the volumetric method, even when dealing with widely varying quantities of sulfate. Table I11 shows the excellent results obtainable when determining sulfate and phosphate together in samples of widely varying sulfate-phosphate ratio. Figure 1 shows the curve obtained when phosphoric acid is titrated with sodium hydroxide. The inflection point in this curve. corresponding to the end point in the titration of the first hydrogen, occurs at about p H 4.6, which corresponds to the blue-green color of bromocresol green. For very low concentrations of phosphoric acid, the theoretical end point occurs at slightly higher pH, approximately that of the pure blue color of the indicator. By stopping the sulfuric acid titration at p H 5 , one largely overcomes the effect of atmospheric C 0 2 on this titration, and also that of moderately low carbonate in the standard base. Hence, there is no special need to exclude C 0 2 during the titration.
Solution 0.01 M
The method is admirably suited to determination of sulfate in large numbers of samples such as are usually encountered in public health, water resources, waste disposal, and oceanographic work. The oven evaporation can, and should, be allowed to proceed overnight. so that the time for this step does not count in analysis time. Miscellaneous materials which can be analyzed for sulfur and phosphorus by this method, include: insecticides, food and drug samples, organic materials in general, following ashing of the sample (oxygen bomb preferred), and certain metal alloys which contain 0.1 or more of sulfur or phosphorus. For example, phosphorus in electroless nickel films deposited from a sodium hypophosphite bath has been determined with high accuracy by solution of the film in 8N nitric acid, evaporation of excess nitric, ion exchange to remove the nietal ions, controlled evaporation t o eliminate remaining traces of nitric acid, then titration. If organic materials are being analyzed, the oxygen bomb contents are ion-exchanged to eliminate traces of metal ions from either the bomb or the sample itself, then evaporated and titrated. I n general, whatever sample preparation is necessary to render the sulfate and phosphate in ionic form in aqueous solution is done, and then the ion exchange-evaporation procedure is used for the determination. Some highly unusual samples may contain nonvolatile acidic anions such as arsenate, chromate, mol) bdate, and borate, but these were not investigated. Literature Cited Fishman, M. J., Robinson, B. P., Midgett, - M. R., A n d . Chein. 39,261R (1967). Scott, W. W., “Standard Methods of Chemical Analysis,” 5th ed., pp. 2204, 2212, Van Nostrand, New York, 1939. Receicedjor reciew April 17, 1968. Accepted August 26, 1968. Wuter, Air and Waste Dicision, 155th Meeting. ACS, San Francisco, Calif., April 5, 1968.
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