Determination of cyanuric acid levels in swimming pool waters by u.v. absorbance, HPLC and melamine cyanurate precipitation

Water Res Vol. 18, No. 3. pp. 277-280, 1984 Printed in Great Britain. All rights reserved

0043-1354 84 S300-.-0.00 Copyright (_" 1984 Pergamon Press Ltd

D E T E R M I N A T I O N OF C Y A N U R I C ACID LEVELS IN S W I M M I N G POOL W A T E R S BY u.v. A B S O R B A N C E , HPLC AND MELAMINE CYANURATE PRECIPITATION C. J. DOWSES t, J. W. MITCHELLz, E. S. VtOTTO~ and N. J. EGGERS" tChemistry Division, DSIR, Private Bag, Petone and -'Chemistry Division. DSIR, Auckland. New Zealand (Receit,ed May 1982) Abstract--A u.v. method, recommended for use in surveys of the operation of swimming pools, depends on measuring the difference in absorbance at 225 nm between basified and acidified aliquots of sample and comparing with values obtained for standard solutions ofcyanuric acid similarly treated. Also described is a High Performance Liquid Chromatography (HPLC) method which would be advantageous for large numbers of samples and for other analytical applications where cyanuric acid must be separated before quantitation by measuring the u.v. absorbance. Turbidimetric methods based on the precipitation of melamine cyanurate are less precise but still useful for pool-side monitoring of cyanuric acid levels. The performance of commercial test kits based on this principle should be checked by a controlling laboratory. Key words--cyanuric acid, swimming pools, u.v. absorbance, HPLC, turbidimetric, melamine cyanurate. test kit

INTRODUCTION

Chlorinated derivatives of cyanuric acid (1,3,5triazine-2,4,6(l H,3H,5H)-trione) are one class of disinfectants used to maintain sanitary conditions in swimming pools, their action being attributed largely to the presence in solution of"free" chlorine, as HOCI and OCI -, arising from the hydrolytic equilibria of the various chlorinated species (O'Brien et al., 1974). When used in this way there is a gradual accumulation of the residual cyanuric acid in the pool water. Although there has been a move away from using chlorinated cyanurates as a source of chlorine for sanitizing school swimming pools in New Zealand, the most c o m m o n source now being calcium hypochlorite powder, cyanuric acid is still often added to outdoor pools as an economy measure because much of the total chlorine available is then present as chlorinated cyanurates which are much less susceptible than "free'" chlorine to degradation by sunlight (O'Brien etal., 1974). In recent surveys of the operation of school swimming pools one of the parameters we wished to include was the level of cyanuric acid; the current New Zealand guideline set by the Department of Health is that the concentration should not exceed 100 g m -s. Proprietary kits are available for measuring cyanuric acid concentrations of pool waters, most being turbidimetric methods depending on the addition of a melamine reagent (Nelson, 1967) but, in our experience, they have not proved to be particularly reliable in the hands of pool operators. The Palin testkit (The Tintometer Limited, Salisbury, England), which we have not had the opportunity of using, is

stated to involve precipitation of cyanuric acid by a mercuric salt at an appropriate pH, followed by a colorimetric determination on the filtrate. Higher precision than that available from a test kit was desired for this work and, because a literature search failed to reveal any simple alternative method, a u.v. method was developed which is based on the strongly pH dependent absorbance at 225 nm of aqueous cyanuric acid solutions. Initially, there was some doubt as to the validity of this method due to the observation that acidified samples of some pool waters have an appreciable u.v. absorbance, so an additional high performance liquid chromatography (HPLC) method was used to provide confirmation. Subsequently, modifications of Nelson's (1967) melamine cyanurate turbidimetric method were investigated. Since completing the experimental work of the present paper a note describing an H P L C method has been published (Jessee et al., 198 I).

277

EXPERIMENTAL

u.v. Method A concentrated stock solution was prepared by dissolving l g of pure anhydrous cyanuric acid (Koch-Light Laboratories, England) in 20 ml of 1 M NaOH and then making up to 1 I. with distilled water. A dilute standard solution was made by diluting 10 ml of the concentrated stock to 100 ml with distilled water. Portions (0, I, 2.5 and 10 ml) of dilute standard cyanuric acid solution were pipetted into glass-stoppered 50 ml measuring cylinders, 1 ml of 1M NaOH added to each and then made up to 50 ml with distilled water to give concentrations of 0, 2, 4, 10 and 20 g m -3 of cyanuric acid. Further portions were similarly treated except that I ml of 0.5 M HzSO~. was added instead of the 1 ml of I NI NaOH. The absorbance (A 225) of these solutions at 225 nm. I cm cell, was measured against distilled water as reference.

C.J. Dow>/ts er al.

2-',

A standard cur',e ,:,as constructed from blank-corrected ~alues of A225 ~basicl minus A225 lacidic) Notted against concentration and for t?pical results ITable i! the average deviation from the least squares straight line was 0. l g m -3 -~bose a concentration of 20 g m - ~ there is significant curvature. After treating a suitable ~olume of each sample. 10 ml for all results reported here. in the manner described for the standards, the cyanuric acid concentration was read off the standard curve and then multiplied by the factor 50/ (~olume of sample taken in roll to give the final result for the sample in g m + ~.

Table 3. Turbidimetric results, absorbancc at 420 nm, ! cm cell (A420) and depth to obliteration (DI for c,,anuric acid standar&;

HPLC The apparatus consisted of an automatic sampler WISP 710A, a pump M-6000A and a variable wavelength detector M450 set at 215 nm, all from Waters Associates (Milford. MA,, U.S.A.}. together with a Hewlett-Packard (Avondale, PA, U.S.A.) 3380A integrator. The column used was PARTISIL-10/25 SAX, 25 c m x 4 . 5 mm i.d. supplied by Whatman Ltd (Maidstone, Kent, England), the mobile phase was 10 -3 M (Tris(hydroxymethyl)methylamine) sulphate buffer, adjusted to pH 7.8 with sulphuric acid, degassed ultrasonically under reduced pressure and run at 2 ml min - t. The sample injection volume was 15 pl. Lack of significant variation in the operation and response of the system was confirmed by running a 50 g m - s cyanuric acid standard after each sample. Peak areas obtained for the standard cyanuric acid solutions (Table 2) are linearly dependent on the concentration with the average deviation from the least squares line being 0.6 g m -a. Results for the swimming pool samples were evaluated from this line.

Melamine precipitation

Cyanuric acid concentration (gm -3 }

a.'2t)

0 I0 20 40 60 80 100

0001 O.ll)9 0244 0.441) 0.630~3! 107[

D qmm) > [8~ t5[ '4., 535 ~3 3,)

melamine reagent with the sample. Typical results ITable 3) for standard cyanuric acid solutions deviated from the least squares straight line plot ofabsorbance versus concentration by an average of 1.4 g m -3. Also given in Table 3 are representative results of the depth of solution required to obliterate a black object viewed through a column of the melamine cyanurate slurry in a 25 mm diameter Nessler tube (Nelson, 1967L RESULTS AND DISCUSSION C y a n u r i c acid levels o b t a i n e d by the u.v. a n d H P L C m e t h o d s for a series of o u t d o o r p o o l s are in g o o d a g r e e m e n t (Table 4). In view of the validity of the u.v. m e t h o d , w h i c h is established below, t h e H P L C m e t h o d can be r e g a r d e d as p r o m i s i n g (see also note by' Jessee et al., 1981). G o o d recoveries were o b t a i n e d by' the H P L C m e t h o d for spiked s a m p l e s p r e p a r e d using w a t e r f r o m the Riddiford o u t d o o r p o o l w h e r e c y a n u r i c

A stock solution of melamine was prepared by dissolving 2.5 g of laboratory reagent grade melamine tBritish Drug Houses Ltd, England) in 700 ml of warm water. After cooling to room temperature l0 g of sodium acetate were added and the pH adjusted to 5.8 before making up to I I. Equal volumes of the test solutions and the melamine reagent were added to glass-stoppered cylinders and shaken by hand for two minutes before determining the absorbance of the mixture at 420 nm with distilled water as reference. In all cases the cylinders were gently inverted once immediately before introducing the solutions to the sipper cell, which was generally about 20 rain after the initial mixing of the

over in the s a m p l e r , it is d e d u c e d that interfering s u b s t a n c e s were not p r e s e n t in significant c o n c e n -

Table I. u.v. Absorbance at 225 nm (A2251 of cyanuric acid standards

Table 4. Comparison of cyanuric acid levels of swimming pools determined by u.v. and HPLC methods

Cyanotic acid concentration (gm -31

A225 (basic) A225 (acid)

2 5 10 20

0.081 0.219 0.445 0.872

Tablc 2, HPLC peak areas of cyanuric acid standards Cyanuric acid concentration (gm-JI

Peak areas (arbitrary unitsl

5 10 25 50 75 100 200

62 107 265 508 741 993 t938

acid is n o t used. As only a single peak was o b s e r v e d in the H P L C traces for pool w a t e r s a n d only a very small peak (area 15 units, see T a b l e 2) at the retention time for c y a n u r i c acid was p r e s e n t for the u n s p i k e d Riddiford s a m p l e , w h i c h was p o s s i b l y due to carry-

Pool Maria College Strathmore Redwood Miramar Central Bellevue Paparangi Kelburn Normal Karori West Newtown Ngaio Makara Ohariu Valley Te Aro Ridgway Wadestown Brooklyn Riddiford* Riddiford spiked ( 10 g m - 3) Riddiford spiked (50 g m- 3)

Q, anuric acid concentration (gm - 3} u.~ HPLC < 5 2~ 3~ <5 30 7 36 <5 63 < 5 30 < 5 16 7 35 29 Not determined Not determined Not determined

*: 2 27 ~2 <2 32 S 38 3 68 ,,-, 2 ~5 5 18 4 38 30 <2 8 49

*Riddiford is a public outdoor pool, c?unuric acid is not used

Determination of cyanuric acid levels

279

Table 5. Analytical quality test of u.v. and turbidimetric methods. 3`lean and standard de',iations of cyanuric acid concentrations (g m "J) for duplicate analyses in six independent runs Synthetic sample 15 g m - 3

Synthetic sample 75 g m -3

Natural

Natural spiked v, ith 20 g m - J

15.0 0.6

75.0 2.4

31.5 1.5

51.7 0.5

202

Turbidimetric ~spectro) .',,lean 0.8 SD 0.3

20.0 2.8

83.3 7.5

28.0 37

48.6 4.:"

20.6

Turbidimetric (',isual) Mean SD

16.5 3.7

58.6 4.7

32.6 2.8

48.4 4.5

15.8

Method uA. Mean SD

Blank

-0.4 0.8

trations. Potentially interfering substances are possibly prevented from accumulating because of degradation by reaction with chlorine. Deterioration of the H PLC column in other unrelated work prevented further analytical quality checks. The H P L C method is probably the most convenient for large numbers ofsamples, and it will be more universally applicable in other analytical situations where cyanuric acid must be separated from interfering substances before quantiration by measuring the u.v. absorbance. The precision and recovery efficiency of the u.v. and turbidimetric methods were investigated by determining, in duplicate, for six independent runs of each method values for: blanks run as samples, synthetic samples at the 15 and 75 g m - 3 level, a swimming pool sample, and the same water spiked with 20 g m - 3 of cyanuric acid. Blanks could not be determined in the second turbidimetric method because the "'depth to obliteration" exceeded the length of the tube. Excellent results (Table 5) were obtained by the u.v. method for the synthetic samples and also for the recovery of the spiked sample. Our detection limit for this method is 5 g m - 3, on the basis of it being 5.5 times the standard deviation of the blank. Care is needed in adjusting the spectrophotometer to ensure the results are not spurious due to there being insufficient light passing through the sample. The sipper cell we use routinely for colorimetric determinations is deficient in this respect, but a conventional 1 cm cell in a Beckman Acta M 1V spectrophotometer with the bandwidth set at 2.3 nm performs satisfactorily. There is no interference, within the precision of this method, due to the presence in samples of 200 g m - 3 bicarbonate, 10 g m -3 of urea, 10 g m -3 of free chlorine or species resulting from the addition of 4.5 g m -a chlorine and I g m -3 ammonia over the pH range 5-8. These concentrations encompass the levels to be expected in most pool waters. The melamine cyanurate method with spectrophotometric determination of the turbidity is less precise (Table 5) than the u.v. method but is acceptable if the aim is to ensure the cyanuric acid level of a pool is, say, 30 + 10 g m - a. The detection limit of 1.5 g m - a allows samples in the lower range of interest to be determined. Above 50 g m -3 the absorbance tends to decrease

Recover',

during a measurement, presumably due to sedimentation of particles within the spectrophotometer cell. so samples in this range should be dilt, ted before repeating the determination. As with man', turbidimetric methods, it is important that all samples and standards be treated in an identical manner with respect to mixing conditions, temperature and elapsed period before measurement of the turbidity. Bicarbonate, chlorine and urea at the levels of 200, 2 and 10 g m -3, respectively, do not interfere in the determination. In principle, the "'depth of obliteration" method with very modest apparatus requirements is adequate (Table 5) for pool-side monitoring of the usual cyanuric acid concentration of about 30 g m -3" however, it is suggested that individual variants of the method available in test kit form should be checked by a controlling laboratory. Samples above 50 g m -3 should be diluted to bring them into the more sensitive range of the method (Table 3). For practicable depths of the slurry column, the detection limit will be between 5 and 10 g m -3. Despite the simplicity of the method some highly erroneous field values were reported in our surveys. Experiments over the 5-35:C range indicate that a correction needs to be applied ira test is conducted at a temperature which is significantly different from that used when preparing the calibration graph, to be used at the pool-side, of obliteration depth vs cyanuric acid concentration of the standards. The correction is slightly concentration dependent and, in relation to our calibration graph prepared at a room temperature of 23~C, 0.6 g m - 3 per degree should be subtracted for test temperatures below 23:C, and added for test temperatures above 23:C for concentrations near 25 g m - 3 At the 50 g m - a level the correction is 0.8 g m - 3 per degree. CONCLUSIONS

The u.v. method described in this paper is recommended for the laboratory determination of the cyanuric acid concentration of swimming pools. With additional validation the H P L C method should also be suitable and possibly preferred where there are large number of samples.

2sl)

C.J. D o ~ , ~ et al.

The melamine cyanurate precipitation method with spectrophotometric determination of the turbidity is a less precise but still useful method. The "'depth to obliteration" method for comparison of the turbidities of melamine cyanurate slurries possesses adequate sensitivity over the range 10-50 g m - 3 required for the poolside monitoring of the usual levels of cyanuric acid. However, the accuracy of commercial test kits based on this principle should be checked by a controlling laboratory. Periodic surveys with laboratory analysis of the samples are required if health authorities wish to ensure residual cyanuric acid concentrations of swimming pools do not exceed a predetermined limit. These checks also provide confirmatory results for those

pool operators endeavouring to maintain chlorinestabilizing concentrations of cyanuric acid. REFERENCES Jessee J. A., Valerias C., Benoit R. E.. Hendricks A. C. and McNair H. M. (1981) Determination of c?anuric acid by high-performance liquid chromatograph,, J. Chromat, 207, 454--456. Nelson G. D. (1967) Swimming pool disinfection v,ith chlorinated isocyanurates. Special Report No. 6862. Monsanto Company, Inorganic ChemicaEs Division. Research Department. St Louis. MO. O'Brien J. E.. Morris J. C. and Butler J. N. (1974} Equilibria in aqueous solutions of chlorinated isocyanuraie. In Chemistry of Water Supply; Treatment. and Distributi~m (Edited by Rubin A. J.), pp, 333-358, Ann Arbor Science, Ann Arbor, MI.