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Determination of ozone in water by the indigo method
Water Research Vol. 15, pp. 449 to 456, 1981 Printed in Great Britain
0043-1354/81/040449-08102.00/0 Pergamon Press Ltd
DETERMINATION OF OZONE IN WATER BY THE INDIGO METHOD H. BADER and J. HOIGNI~ Federal Institute of Water Resources and Water Pollution Control (EAWAG), Swiss Federal Institute of Technology, CH-8600 Diibendorf, Switzerland (Received 20 May 1980) Al~tract--The concentration of aqueous ozone can best be determined by the decolorization of indigo trisulfonate (600 nm, pH below 4) whenever the ozone cannot be measured directly by its u.v. absorption. The method is stoichiometric and extremely fast. The change of absorbance vs ozone added is -2.0 _ 0.1 x 104 M -1 cm -~ and is independent of the concentration of aqueous ozone in the range 0.005-30 mg 1- t. The precision of the analysis is 2% or 3/~g 1-1 for low concentrations if a spectrophotometer or a good filter instrument is used. Visual methods can be used to measure 0.01 mg 1-1 ozone. Secondary oxidants produced by ozone in natural water, including hydrogen peroxide or chlorite, do not interfere; chlorine can be masked. The reagent solution is stable for 3 months. The method is recommended for kinetic measurements, for studies of ozonation processes and for visual field methods.
1. INTRODUCTION Approximately a dozen analytical methods for the determination of aqueous ozone have been described in the literature and about half of them are still applied. However, most of these methods are not specific and often give incorrect results. Among the first reagents used to measure ozone in air or exhaust gases were indigo and its water soluble derivatives, the sulfonated indigo compounds such as indigo disulfonate, indigocarmine and indigo trisulfohate (Thenard & Thenard, 1872; Fonrobert, 1916; Dorta-Sehaeppi & Treadwell, 1949; Dr~iger & Driiger, 1966; Verein Deutscher Ingenieure, 1977). In contrast, the application of the indigo methodfor the analysis of ozone in drinking water has been neglected. We can cite 3 reasons for this: (1) In most waterworks ozone is measured by merely adapting analytical methods already accepted for determination of chlorine or other oxidants; (2) Most analytical chemists are sceptical about methods based on decolorizations; and (3) It was not recognized that erraneous results are often only due to a preliminary decomposition of ozone and that this can easily be avoided by maintaining pH values below 4 (Hoign6 & Bader, 1976). In the past few years we have accumulated experience on the application of the indigo method for analyzing aqueous ozone in many different types of waters (Hoign6 & Bader, 1979a, b, 1980). For these studies this method showed many advantages: it is very sensitive, precise, fast, specific, and easy to handle. Since other analytical methods have presented difficulties, many research and process chemists have approached us for more detailed information concerning this method. Indigo is a well known classical blue vat dye (example: Blue Jeans). Its water-soluble derivatives, indigo disulfonate and indigo trisulfonate, are corn449
merciaUy available reagents, listed as pH or redox indicators. Aqueous solutions absorb light at about 600 n m with a rather high molar absorptivity (Preisler et al., 1959, reports 2max = 605 rim, ~ = 23,800 M - 1 cm-1 for indigo trisulfonate). The indigo molecule contains only one C----C double bond which can be expected to react with ozone with a very high reaction-rate constant (see Fig. 1). At low pH the amino groups are protonated and therefore unreactive (Hoign6 & Bader, 1978, 1979a). Thus, it can be assumed that one molecule of sulfonated indigo reacts with one molecule of ozone under proper reaction conditions including high rate of mixing. Ozonolysis of the reactive ~ bond produces sulfonated isatin and similar products and eliminates the absorbance at 600 nm as shown in Fig. 2 (Dorta-Schaeppi & Treadwell, 1949). 2. EXPERIMENTAL
2.1 Chemicals Indigo recu3entfor determination of ozone. Generally, the stock solutions of Indigo Reagent were prepared by dissolving 0.6 g l- ~ (1 mM) potassium indigo trisulfonate in 20mM phosphoric acid. These solutions were replaced when their absorbance at 600 nm had decreased to below 80% of the initial value (typically after 3-4 months storage in transparent glass bottles). Potassium indigo trisulfonate was generally purchased from Riedel-de Hahn AG (Redox Indicator), but a product from Merck, which had been stored for 10-20 years, gave similar results. The microanalysis for C/N/O and titrations with permanganate indicated an indigo content of about 80% of the theoretical value. (An uncertainty of the concentration of the Indigo Reagent does not influence the accuracy of the analytical method which is based on standard curves for changes of absorbance achieved per added mole of ozone.) o-Tolidine hydrochloride and DPD (N,N-diethyl-l,4 phenylene-diamine sulfate (GR) were Merck p.A. products. (Comparable results were also achieved with the DPD test kit "AQUAMERCK Nr. 11100 for chlorine".) ACVK (Acid chrome violet K, Michrome Nr. 1047) was an Edward
450
H. BADER and J. HOIGNI~ so~
method; the u.v. method was applied only as a more convenient secondary standard.
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indigo trisuffomc acid
[, o.e2oooo .-' cm-' 1
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Fig. 1. Indigo trisulfonate and its ozonation product. n.Ri -I ozone o~ded
Ascff 0.~
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,, 500
. 600
700 nm
Fig. 2. Absorption spectra of Indigo Reagent after stepwise ozonation (Indigo trisulfonate added: 10/tM).
Gurr Ltd (London) product; Phosphate Buffer Reagent for pH 2: 28g N a H 2 P O 4 ' H 2 0 and 35g H s P O , (85V.) dissolved in t 1. dist. water. Other reagents were of commercial pro analysis grade. Aqueous ozone. Stock solutions of aqueous ozone were prepared by continuously bubbling ozonated oxygen (about 4%) through a gas-washing bottle into distilled water chilled to 2°C. This stock solution generally contained a steady-state concentration of ozone of 40 mg 1- ~. Lower concentrations were achieved by dilution with distilled water. Whenever not required for later use, this water was acidified with phosphoric acid to pH 2. When the bottle was kept closed the concentration of ozone decreased less than 0.2~ per min (Hoign6 & Bader, 1978). The aqueous ozone was transferred into the individual test bottles with glass pipettes or by plastic dispensers. The concentration of the diluted ozone water was determined before and after the samples were withdrawn. These calibrations of the stock solution were performed by measuring the u.v. absorbance of ozone at 258 rim. They were based on a molar absorptivity of 2900 M - * c m - * which was determined in other studies by a direct comparison with the iodine method, here considered as the primary standard (Kilpatrick & Herrick, 1965; Hoign6 & Bader, 1976). For these calibrations t h e samples of aqueous ozone was acidified by sulfuric acid to give 0.3-0.5 M acid. About 0.2 g o f potassium iodide were added. The liberated iodine was titrated with 0.1 M sodium thiosulfate using starch indicator. Care was taken that the aqueous ozone was always handled in exactly the same way, whether it was transferred into samples containing tndigo g ~ t or inlo the u.v. cells or into flasks for the iodide t i t r a t i ~ , All concentrations of ozone are therefore finally based on the iodide
2.2 Instrumentation Generally, the absorption measurements for the indigo method were performed at 600 nm on a Beckman DK 2A spectrophotometer which was equipped with a thermostated cell compartment maintained at 25°C. Cells of 0.5-5 cm path length were used. A VITATRON photometer equipped with a 4 cm cell and a 595 nm interference filter of 15 nm bandwidth was also used. In experiments to test the indigo method as a visual field method, pairs of graduated glass cylinders were used (volume 250 ml. height 25 cm). 2.3 Glassware Best reproducibilities were achieved when the glassware used for handling of aqueous ozone was conditioned by repetitive use for the same procedures. 2.4 Experimental procedures for the indigo method used with a spectrophotometer During this study, many varied procedures were tested. Only illustrative examples can be given here: Example for the determination of aqueous ozone in the concentration range 0.3-15 mg 1- t : Series of 100 ml volumetric flasks were prepared. To each flask of the series 10 ml of 0.5 M Phosphate Buffer Reagent for pH 2, 1 ml of 1 mM Indigo Reagent and, in case of the measurements of high concentrations of ozone, about 20 ml of distilled water were added. Some of these flasks were used to determine the standard curve. For this, individual flasks of the series were dosed with different amounts of diluted stock solutions of aqueous ozone containing about 2 m g l - ' ozone e.g. 0, 5, 10... 25 mt, at random to minimize systematic time drifts of ozone concentrations. This solution was dosed with a glass pipette whose tip was placed below the surface of the reagent solution while stirring with a Teflon covered magnetic bar. The concentration of the ozone in the diluted stock solution applied was calibrated simultaneously by measuring its u.v. absorbance at 258 nm in a 5 cm u.v. cell. Finally, distilled water was added to the 100 ml mark. During this operation the stirrer was temporarily raised above the mark. The absorbance of the residual indigo present in these series of test solutions was measured at 600 nm in a 5 cm cell. The values were plotted vs the amount of the ozone added. A typical standard curve is presented in Fig. 3(B). The determination of the ozone in the unknown samples is thereafter performed similarly by dosing specific volumes of these samples to others of the prepared flasks of the series. In order to be within the range of the amount of Indigo Reagent used the size of the sample was selected to contain an estimated 10--40#g ozone. For example, when the concentration of ozone was expected to be 0.5 mg 1-1 20-80 ml were injected and when the concentration was very high, such as 15 mg 1-t. 1-2.5 ml were dosed. The volumetric flasks were filled again with distilled water to the I00 ml mark. The residual absorption at 600 nm was compared with the standard curve and the original concentration of ozone was calculated. Comparable procedures were adapted to determine ozone in other concentration ranges. In cases of higher concentrations, higher amounts of the Indigo Reagent were dosed. In this case the determination of the residual absorbance of the Indigo Reagent was determined using 0.5 cm cells. An example of a standard curve is shown in Fig. 3(A). In this case the amounts of the Indigo Reagent and of the ozone applied were about 10 times higher than those in the example shown in Fig. 2. When lower concentrations of ozone were determined, less Indigo Reagent was used. Figure 3(C) gives an example in which the concentrations were 4 times lower than those for the example in
Determination of ozone in water by the indigo method
451
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Fig. 4. Standard curve absorbance vs ozone added to Indigo Reagent. Measurement with filterphotometer. Interference filter 595 nm. Cone. of Indigo Reagent 0.6 M. (Concentrations are based on final volume.)
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glass cylinders A and B. Cylinder A was filled to its 25 cm mark with the sample of aqueous ozone. Cylinder B was filled with distilled water. Cylinder B was then emptied into an Erlenmeyer flask and refilled with this reference solution until the color intensity, when observed from top, matched that of cylinder A. A standard curve was obtained by repeating such measurements using different known amounts of ozone. The height of the reference solution in cylinder B was plotted vs the concentration of the aqueous ozone applied in cylinder A. A standard curve is given in Fig. 5. Thereafter, the concentration of ozone in the samples was determined by comparing the height of solution in the reference cylinder with the heights given in the standard curve. The size of the sample was taken into account for the calculation of the concentration.
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2.6 Experimental procedures applied for other spectrometric
ICl
analysts of ozone For some tests, ozone had to be determined by the direct measurement of its u.v. absorption at 258 nm even in natural waters. In these waters, which sometimes exhibited significant background absorption at 258 nm, the measured absorbance was corrected by determining the background absorbance after immediately destroying the residual ozone with an approx. 10-fold excess of aqueous nitrite or butene-ol. The ACYK method was carried out as described by Masschelein & Fransolet (1977). The methods based on
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Added [031 mg I-' Fig. 3. Standard curves o f absorbance (A) at 600 nm vs ozone added to Indigo Reagent. Measurement with s t ~ r o p h o t o m e t e r . Conc. of Indigo Reagent added: (A): 0.1 raM; (B) 10 #M; (C) 2.5/~M. (Cone. of Indigo Reagent and ozone are based on final volumes.)
i
25 •
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~0.48
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ax
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Fig. 3(B). Figure 4 gives an example for a 14 times lower concentration range which has however measured on a filter photometer. If the rates of fast consumptions of ozone were measured (Hoign6 & Bad•r, 1981), the Indigo Reagent was injected directly into the flasks containing the aqueous ozone. Ozonation reactions were thus immediately stopped upon mixing with the Indigo Reagent. The concentration of the ozone was determined again from the decolorization of the Indigo Reagent as described above. 2.5 Visual method A visual method was developed for field studies: The same amount of the Indigo Reagent was added to a pair of w.R. 15/4--D
,~ ~
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Fig. 5. Standard curve for visual method. To both cylinders of 250 ml 0.4/zmol of indigo trisulfonate had been added, h* Is the height of reference solution in cylinder B giving the same visual color intensity as 25 cm of the solution in cylinder A which was decolorized by the addition of a sample of aqueous ozone. The right-hand scale indicates the approximate absorbance of the solution in cylinder A. (Cone. of ozone is based on final volume.)
452
H. BADERand J. HO1GNE
o-tolidine and DPD were performed as described in Stan-
dard Methodsfor the Examination of Water and Wastewater I1975).
3. RESULTS
3.1 Absorption spectra of the Indigo Reagent The Indigo Reagent shows an absorption maximum at 600 nm. The apparent absorption coefficient, based on the amount of total mass of commercial (non-purified) reagent added is 16,500 M - 1 cm- 1. The shape of the absorption curve and )~,x do not change with ozone addition (see Fig. 2). 3.2 Sensitivity and precision of the indigo method as
used with spectrophotometers The absorption of samples containing the Indigo Reagent decreased linearly with the amount of ozone added over all the concentration ranges investigated. From the slopes of standard curves the photometric sensitivity of the analytical method may be deduced. Values are presented in Table 1. The absorbance change at 600nm is 2.0 + 0.! x 104cm -1 per mol 1-1 of added ozone for all concentrations tested. Lack of precision may be due to uncertainties in the calibration of aqueous ozone applied for the determination of the standard curves. We estimate that this uncertainty can cover a range of 5% when series performed at different concentration ranges are considered. The precision of the slope of single standard curves (about 8 points measured) was generally within this range. Neither deterioration of the absorbance of the Indigo Reagent solution to 80% of its original value during extended storage (3 months) nor a partial decolorization by a preliminary ozonation affected these calibrations. The error of the ozone determination decreases with the relative amount of decolorization and increases in case of very low concentration ranges. However, the relative standard deviations calculated from extended series were generally about + 2% when the concentration of ozone was above 0.1 mg 1-1 and when the residual absorbance was in the region of 10-60% of the initial absorbance. The main part of
Table 1. Typical sensitivity values of the Indigo Method co
Range of ozone dose
AA*
(/~M)
(mg 1-1)
(M - l c m - l)
100 10 2.5
0.2-3.5 0.02-0.3 0.02q?.l
(2.02 _ 0.10)' 104 (2.02 _ 0.10)" 104 (1.99 +_0.10)" 104
Co = cone. of potassium indigo trisulfonate applied, based on final volume. AA = C h a n g e o f a b s o r b a n c e ( c m - 1) at 600 n m m o l - 1 1- t added o z o n e . * Estimated range of accuracy of calibrations = + 0 . 1 . 1 0 ' * M - l cm -1.
the variances is thereby expected to be due to the variance in ozone doses. The effect of the rate of mixing of the aqueous ozone with the Indigo Reagent solution was tested in special series. Only a small difference of the sensitivity factor (2°; between matched pairs of extended series of simultaneously prepared samples) could be found between normally and extremely slowly stirred samples. 3.3 Measurements with filter photometers Measurements of the residual absorbance of the Indigo Reagent with a photometer equipped with a 595 nm filter gave results and variances comparable to those determined with the spectrophotometer. An example of a standard curve is given in Fig. 4. The slope of this curve is 85°,0 of that expected from measurements using a spectrophotometer at 600nm, probably because the 595 nm filter does not quite match the maximum of the absorption spectrum and the filter band-width of about 15nm exceeds the maximum of the absorption band of indigo trisulfonate (c.f. Fig. 2). 3.4 Visual methods An example of results found by the visual method is presented in Fig. 5. An incremental ozone dose of 10 pg 1- 1 can be easily detected when 25 cm cylinders are used. 3.5 Stoichiometricfactor Extrapolation of the standard curves to the intercept on the absissa shows that only 80% of the commercial indigo product applied is effective in consuming ozone. This calculation is based on the assumptions that the potassium indigo trisulfonate applied was 100`0/0 pure and that 1 mol ozone added would decolorize 1 tool of indigo. Above pH 4 the stoichiometric factor often decreases and it seems to become somewhat erratic. 3.6 Rates of reactions For the application of the Indigo Method for kinetic measurements it is important to note that all the ozone is consumed by the reagent immediately upon mixing. This was observed on a 1 s time scale even in systems where the concentration of ozone decreases by fast concurrent reactions, e.g. 20% s-1, and even when the concentrations of both the indigo trisulfonate and the ozone were, at the time of mixing, as low a s 10 - 7 M {Hoign6 & Bader, 1981). That means that the rate constant for the reaction of ozone with indigo trisulfonate must be higher than 107M - is-1. Aqueous indigo trisulfonate which is preozonized until just decolorized consumes further ozone only slowly. The apparent reaction-rate constant of t h e s e ozonolytic products is about 2500 M - ts- 1 (Hoign6 & Bader, 1981). Based on this, we may assume that t h e indigo method will not be significantly disturbed by short temporary local overdoses of ozone.
Determination of ozone in water by the indigo method
453
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Fig. 6. Stability of residual absorbance of different reagents after addition of aqueous ozone. In case of ACVK and Indigo the amount of ozone added decolorized about 40% of the added reagent.
3.7 Temperature effect on absorbance
mgl-,
The absorbance of the Indigo Reagent and the sensitivity factor for the determination of ozone decreases 2.0% with an increase of 10°C.
03 ffteOS. O.fl
3.8 Stability of residual absorption after ozone addition
0.6
The residual absorbance of Indigo Reagent measured after an addition of aqueous ozone in distilled water does not change during a 6 h time period. A similar stability was only found with ACVK. In contrast, D P D and o-tolidine show significant absorbance changes even when observed within I h (ef. Fig. 6).
0.4
3.9 Determination of ozone in natural water Comparable results for the determination of the residual ozone present in ozonized natural water have always been found by measuring its absorbance at 258 nm (reference method) and by the indigo method. Comparisons are presented in Table 2. The direct u.v. measurement, however, required a rather complicated procedure to compensate for the background absorption of natural water at 258 n m (see Experimental), also its sensitivity is relatively low (see Discussion). Discrepancies between the two methods are therefore
j a 0.,~
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a Rowwoter,loke Zurich (3Ore) Row woter, G r e i f ~ ,
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Fig. 7. Stability of residual absorbance of Indigo Reagent in different types of raw waters, expressed in apparent concentrations of ozone determined. The residual absorbance was about 50% of the non-ozonized reference.
due rather to errors involved in the data of the reference method applied. In some types of ozonated surface water the residual indigo trisulfonate shows a slow post-decolorization when observed over several hours. Samples of
Table 2. Comparison of cone. of residual ozone determined in different types of ozonized waters by the direct method (u.v. 258 nm) and by the indigo method (600 nm) Water Bidistilled Lake Zurich (lake water, DOC 1.1 mg 1-1) (after sandfilter, DOC 1.2 mg 1-~)1" Greifensee (lake water, DOC 4 mg 1-1)
I rng fJDOC
U.V.2sanm Indigo6oonm (rag 1-1 ozone) (mg 1- l ozone)*
0.51
0.53
0.61 0.35 0.80
0.60 0.34 0.83
1.24
1.28
0.43
0.43
* All calibrations are based on a sensitivity factor of 20,000 M - ~ cm- 1. t Ozonizations performed in plant at Zurich-Lengg.
454
H. BADERand J. HOIGNt~
water from Greifensee--a highly eutrophic lake exhibited the maximum rate of change encountered in our survey studies (Fig. 7). Such post-decolorizations are, however, very small when purified drinking waters are considered.
found for this reagent when the absorbancc of the disulfonate was measured at 610nm, where this reagent shows its maximum absorption (Hoign6 & Bader, 19801.
3,10 Effect of other oxidants
4. DISCUSSION
At pH 2 hydrogen peroxide, chlorite, chlorate and perchlorate do not decolorize Indigo Reagent when observed within a few hours and when the concentrations considered are within a factor of 10 of that of the ozone to be determined. However, chlorine and iodine lead to a slow post-decolorization. Their effects increase with time. Chlorine can be successfully masked by adding 500 mg 1-1 malonic acid to the Indigo Reagent; malonic acid at pH 2 consumes ozone with a reaction-rate constant of only 3 M ~s(Hoign6 & Bader, 1981) and therefore does not compete with the Indigo Reagent for reaction with ozone when added simultaneously with the reagent. An example of the effect of chlorine in the absence and presence of malonic acid is given in Table 3. Products of ozonolysis of organic solutes seem generally not to interfere with the residual Indigo Reagent when tested within a few hours: no decolorization was observed in any of the solutions we used for kinetic measurements. This study covered many different types of aromatic hydrocarbons, phenols and other products (Hoign6 & Bader, 1981). However if, for example, l mM 1-hexene-4-ol has been preozonized with 0.5 mM 03 at pH 2, it decolorized a 0.01 mM solution of Indigo Reagent significantly within 40 min. Permanganate decolorized Indigo Reagent in a fast reaction.
4.1 Stoichiometric Jactor for the decolorization of
Indigo Reagent The sensitivity of the decolorization of Indigo Reagent stays constant over a wide range of ozone doses. This can only be expected if it is assumed that the sulfonated indigo is a spectroscopically uniform material with respect to its decolorization reaction; in the presence of reactive impurities (such as other indigo derivatives), both the absorption maximum and the sensitivity could shift during ozonation. An apparent stoichiometric factor of 1:0.8 for the decolorisation of the Indigo Reagent agrees with the microanalytical characterization of the commercial potassium indigo trisulfonate. The phenomenon of the stoichiometric factor decreasing above pH 4 can be explained: in the presence of organic materials with structures similar to that of the ozonolytic products of indigo, a catalytic chain reaction leads to an accelerated decomposition of ozone (Hoign6 & Bader, 1976). Such chain reactions are initiated by hydroxide ions and therefore proceed at elevated pH during mixing of the reagents wherever temporary local excess of ozone is present in the reaction mixture. The secondary oxidants formed from decomposed ozone, OH radicals, do not however selectively oxidize the chromophoric structure of residual indigo (Hoign6 & Bader, 1978).
3.11 Indigo Reagent based on indigo disulfonate All results reported here are based on an Indigo Reagent which was prepared from commercial potassium indigo trisulfonate. In response to inquiries received, some tests have been repeated with an Indigo Reagent based on commercial types of sodium indigo disulfonate. Comparable results have been
Table 3. Effect of chlorine on the determination of ozone. The residual absorption (A) was measured 4 rain and again I h after the ozone (and chlorine) was dosed to the Indigo Reagent. Malonic acid (m.a.+) was added directly to the Indigo Reagent before reaction 600 n m
CI2 m.a. Ascm 0.2 mg1-1 l m g l -~ 500mg1-1 4min lh 0 3
-
+ + + +
+ +
not added. + added.
-
+ +
0.88 0.51 0.53 0.44 0.48
0.88 0.52 0.52 0.24 0.48
4.2 The sensitivity and limits of detection The sensitivity, which amounts to a change of the absorbance of AA = - 20,000 cm- 1 mol- t ozone added l-~ is quite high. If a classical type of filter photometer or u.v. spectrograph is used, a limit of detection of about 5 pg I-1 can be achieved. When working with a high performance spectrophotometer, a detection limit of 1 #g l- ~ is approached (cf. Hoign6 & Bader, 1981a). Here the handling of the ozonated water and not the instrumental accuracy become limiting. The detection limit of the simple visual method is 10 pg l- 1 when 25-cm cylinders are used. This limit of detection fulfils the requirements for field studies on drinking water: e.g. according to a recommendation accepted in Switzerland, less than 50/~g1-t of residual ozone should appear at the consumers tap (Schweiz. Lebensmittelbuch, 1972). 4.3 Sensitivity of the Indigo Method compared with
other spectrophotometric methods In Table 4, the absorption regions and sensitivities of different photometric methods are listed for corn-
Determination of ozone in water by the indigo method Table 4. Sensitivities of different colorimetric methods for the determination of aqueous ozone expressed as AA (change of absorbance per 1 mol 1-t ozone added per I cm) Method u.v. of 03 Indigo ACVK Orthotolidine DPD Iodide/triiodide
Wavelength (nm)
AA (M- 1 cm- t)
Ref.
258 600 550 440 510 352
+2900 - 20,000 - 2500 + 32,000 + 15,000 +25,000
a b c d e f
a--Kilpatrick (1965) and Hoign6 & Bader (1976). b---this study. c--own measurement; method of Masschelein (1977). d--own measurement and Sulzer (1958). e--own measurement, method of Palin (1975). f---talc, from data of Shechter (1973). parison. The most straightforward method is the direct measurement of the absorbance of aqueous ozone at 258 nm. This absorption occurs, however, in a region where many types of other solutes interfere: in natural waters, e.g. humic materials often cause a significant background absorption which varies with ozonation. In addition, the instrumental measurement must be performed at the time at which the concentration of ozone is of interest. Moreover, due to the low molar absorptivity of ozone, the sensitivity of this method is relatively low. This direct method, however, must still be recommended as the best defined and useful reference method for the calibration of ozone concentrations whenever the limitations of sensitivity and background absorptions are not relevant. In contrast, the Indigo Reagent absorbs light in a spectral region where only a few other substances interfere. Natural waters, for example, have generally no background absorption in the 600 nm region. The sensitivity, when expressed as the change in absorbance per added ozone, is quite high. In addition, instrumental sensitivities are generally high at 600 nm and even the sensitivity of the human eye is relatively high at this wavelength. For most types of waters measurements can be performed even hours after the ozonated water has been added to the reagent solution. The sensitivity of the indigo method is about 8 times higher than that of the ACVK method. In addition, measurements at 600nm are preferable to measurements at 550 nm where more aqueous solutes may interfere. The o-tolidine method exhibits a sensitivity 1.6 times higher than the indigo method. Its non-specific response to many kinds of oxidants, its low wavelength region of absorption, and its temporal instability, however, disqualify this method when compared with the others (Sulzer, 1958; Hofman & Stern, 1969), The DPD method for the determination of ozone is based on a preliminary oxidation of iodide. However, all methods based on oxidations of iodide to iodine
455
have the disadvantage of being non-specific; iodine is also formed by secondary oxidants such as produced by ozonation of water below pH 6-7 that contains organic solutes. Moreover, the DPD method shows a large time drift and therefore cannot be recommended for extended series of measurements as required for kinetic studies. 4.4 Dynamic range of the Indigo Method The ozone dose must be adjusted to decolorize 20-903/o of the Indigo Reagent. For one fixed concentration of the reagent and one predetermined relative ratio of volume of reagent to volume of aqueous ozone, the dynamic range is therefore restricted to a factor of about 4.5. In non-automated systems, preliminary tests will help to locate the approximate range of concentration before specifying the analytical procedure with respect to ratios of volumes to be used. During our studies, we covered a region of measurement of 0.005-30 mg l-1 of aqueous ozone by merely adjusting the relative volume of aqueous ozone and of Indigo Reagent applied. This entire range exhibits constant sensitivity. It covers the requirements generally encountered in ozone applications. 4.5 Calibrations In this study we used the u.v. absorbance of ozone in distilled water at 258 nm as a secondary standard to calibrate the analytical standard curves. At the present time we still recommend such calibrations, although different commercial products of indigo trisulfonate tested so far have shown rather comparable sensitivity factors (Hoign6 & Bader, 1980). Sensitivity factors can, however, only be exchanged between laboratories which perform the measurements on calibrated spectrophotometers at 600 nm. Filter instruments will require individual calibrations. 5. CONCLUSIONS The Indigo method covers the range of concentrations between 0.005 and 30 rag l-t. The method is relatively straightforward and does not require any difficult or time-consuming operations. It can be performed with the classical instrumentation of waterwork laboratories. The Indigo Reagent is stable: it can be stored for months as a reagent which is "ready to use". The absorption to be measured at 600 nm is situated in a range where solutes of natural waters do not interfere and where even simple visual methods can be used for measurement. The change of absorbance reflects the amount of ozone added with high and constant sensitivity factor of AA = - 2 0 , 0 0 0 c m - 1 ( m o l . l - l ) - I ozone added. All standard curves are linear. The sensitivity does not vary with ozone concentration, with small changes of temperature of reaction, or with the chemical composition of the water whenever the pH is below 4. For many types of waters the instrumental or visual
456
H. BADERand J. HOI,GNF'~
measurements can be performed even a few hours after the ozone has reacted with the Indigo Reagent. The method is relatively selective. Secondary oxidation products do generally not interfere. The reaction of ozone with the Indigo Reagent is so fast that it can be applied to stop and measure even very fast ozonation reactions. The method has been well tested and applied for the determination of ozone in many different types of waters. Unexpected difficulties have not arisen. More practical experiences with this method are now being accumulated in many waterworks and laboratories. However, this Indigo M e t h o d cannot be used as a primary standard. At the present time the reagent must still be calibrated. A standard curve can be produced using an aqueous solution of ozone in distilled water which is calibrated by the direct u.v. absorption measured at 258 nm or by the classical iodometric procedure. It is, however, quite possible that a wider application of the indigo m e t h o d will stimulate central national laboratories or a chemical company to market an indigo reagent with a calibrated sensitivity factor for the determination of ozone and chlorine dioxide (see Hoignb & Bader, 1980). Acknowledgements--We thank Professor Werner Stumm for his continuous encouragement, and Dr Jiirg Zobrist and Dr Joan Davis for the valuable reviewing and discussions. Andreas Locher is thanked for his assistance in performing many of the experiments during his apprenticeship stage. REFERENCES
Dorta-Schaeppi Y. & Treadwell W. D. (1949) Ueber eine kolorimetrische Bestimmung von hochverdiinntem Ozon in Sauerstoff. Heir. chim. Acta 32, 356-364. Dr~iger H. & Dr~iger B. (1966) Anzeigepr~iparat f'tir PriifriShrchen zur Anzeige von Ozon und Verfahren zu seiner Herstellung. German Patent No. 1'204'854 (26.5.1966). Fonrobert E. (1916) Chemie in Einzeldarstellungen, Vol. IX, p. 231. Das Ozon, Verlag Ferd. Enke, Stuttgart. Hofmann P. & Stern P. (1969) Photometrische Bestimmung niedriger Ozonkonzentrationen in W~issern. Analyt. Chim. Acta 47, 113-120.
Hoigne J. & Bader H. (1976) The role of hydroxyl radical reactions in ozonation processes in aqueous solutions. Water Res. 10, 377--386. Hoign6 J. & Bader H. (1978) Ozone and hydroxyl-radical initiated oxidations of organic and organometallic trace impurities in water. Orqanometals and Organometalloids: Occurrence and .late in the Environment. (Edited by Brinckman F. E. & Bellama J. M.), Am. Chem. Soc. Symp. Ser. 82, pp. 292 313. Hoign6 J. & Bader H. (1979a) Ozonation of water: Selectivity and rate of oxidation of solutes. Ozone Sci. Enqng. 1, 73 85. Hoign6 J. & Bader H. (1979b). Ozonation of water: "Oxidation-Competion values" of different types of waters used in Switzerland. Ozone Sci. En,qno. !, 357 372. Hoign6 J. & Bader H. (1980) Bestimmung yon Ozon und Chlordioxid in Wasser mit der lndigo-Methode. Vom Wasser. 55, 261--280. Hoign6 J. & Bader H. (1981) Rate Constants of direct reactions of ozone with organic compounds in water: In preparation. Kilpatrick M. L. & Herrick C, D. (1965) The decomposition of ozone in aqueous solution. J. Am. chem. Soc. 78, 1784-1889. Masschelein W. J. & Fransolet G. (1977) Spectrophotometric determination of residual ozone in water with ACVK. J. Am. Wat. Wks. Ass. 69, 461-462. Palin A. T. (1975) Current DPD method for residual halogen compounds and ozone in water. J. Am. Wat. Wks Ass. 67, 32-33. Preisler P. W., Hill E. S., Loeffel R. G. & Shaffer Ph.A. (1959) Oxidation reduction potentials, ionization constants and semiquinone formation of indigo solfonates and their reduction products. J. Am. chem. Soc. 81, 1991-1995. Schweizerisches Lebensmittelbuch (1972) Trinkwasser und Mineralwasser. Kap. 27, p. 8. Shechter H. (1973) Spectrophotometric method for determination of ozone in aqueous solutions. Water Res 7, 729-739. Standard Methods for the Examination of Water and Wastewater (1975) 14th Edition. APHA-AWWA-WPCF. Sulzer F. (1958) Ueber das Verhalten w~isseriger Ozonl6sungen. Schweiz. Z. Hydrol. 20, 16-29. Thenard A. & Thenard P. (1872) M6moire sur Faction de l'ozone sur le sulfate d'indigo et I'acide ars6nieux. C. r. Acad. Sci. 75, 458-465. Verein Deutscher Ingenieure, VDI, 2468 (1977) Messen gasf6rmiger Immissionen. Messen der Ozon-Konzentration. Manuelles photometrisches Verfahren. Indigosulfons~iure-Verfahren.
0043-1354/81/040449-08102.00/0 Pergamon Press Ltd
DETERMINATION OF OZONE IN WATER BY THE INDIGO METHOD H. BADER and J. HOIGNI~ Federal Institute of Water Resources and Water Pollution Control (EAWAG), Swiss Federal Institute of Technology, CH-8600 Diibendorf, Switzerland (Received 20 May 1980) Al~tract--The concentration of aqueous ozone can best be determined by the decolorization of indigo trisulfonate (600 nm, pH below 4) whenever the ozone cannot be measured directly by its u.v. absorption. The method is stoichiometric and extremely fast. The change of absorbance vs ozone added is -2.0 _ 0.1 x 104 M -1 cm -~ and is independent of the concentration of aqueous ozone in the range 0.005-30 mg 1- t. The precision of the analysis is 2% or 3/~g 1-1 for low concentrations if a spectrophotometer or a good filter instrument is used. Visual methods can be used to measure 0.01 mg 1-1 ozone. Secondary oxidants produced by ozone in natural water, including hydrogen peroxide or chlorite, do not interfere; chlorine can be masked. The reagent solution is stable for 3 months. The method is recommended for kinetic measurements, for studies of ozonation processes and for visual field methods.
1. INTRODUCTION Approximately a dozen analytical methods for the determination of aqueous ozone have been described in the literature and about half of them are still applied. However, most of these methods are not specific and often give incorrect results. Among the first reagents used to measure ozone in air or exhaust gases were indigo and its water soluble derivatives, the sulfonated indigo compounds such as indigo disulfonate, indigocarmine and indigo trisulfohate (Thenard & Thenard, 1872; Fonrobert, 1916; Dorta-Sehaeppi & Treadwell, 1949; Dr~iger & Driiger, 1966; Verein Deutscher Ingenieure, 1977). In contrast, the application of the indigo methodfor the analysis of ozone in drinking water has been neglected. We can cite 3 reasons for this: (1) In most waterworks ozone is measured by merely adapting analytical methods already accepted for determination of chlorine or other oxidants; (2) Most analytical chemists are sceptical about methods based on decolorizations; and (3) It was not recognized that erraneous results are often only due to a preliminary decomposition of ozone and that this can easily be avoided by maintaining pH values below 4 (Hoign6 & Bader, 1976). In the past few years we have accumulated experience on the application of the indigo method for analyzing aqueous ozone in many different types of waters (Hoign6 & Bader, 1979a, b, 1980). For these studies this method showed many advantages: it is very sensitive, precise, fast, specific, and easy to handle. Since other analytical methods have presented difficulties, many research and process chemists have approached us for more detailed information concerning this method. Indigo is a well known classical blue vat dye (example: Blue Jeans). Its water-soluble derivatives, indigo disulfonate and indigo trisulfonate, are corn449
merciaUy available reagents, listed as pH or redox indicators. Aqueous solutions absorb light at about 600 n m with a rather high molar absorptivity (Preisler et al., 1959, reports 2max = 605 rim, ~ = 23,800 M - 1 cm-1 for indigo trisulfonate). The indigo molecule contains only one C----C double bond which can be expected to react with ozone with a very high reaction-rate constant (see Fig. 1). At low pH the amino groups are protonated and therefore unreactive (Hoign6 & Bader, 1978, 1979a). Thus, it can be assumed that one molecule of sulfonated indigo reacts with one molecule of ozone under proper reaction conditions including high rate of mixing. Ozonolysis of the reactive ~ bond produces sulfonated isatin and similar products and eliminates the absorbance at 600 nm as shown in Fig. 2 (Dorta-Schaeppi & Treadwell, 1949). 2. EXPERIMENTAL
2.1 Chemicals Indigo recu3entfor determination of ozone. Generally, the stock solutions of Indigo Reagent were prepared by dissolving 0.6 g l- ~ (1 mM) potassium indigo trisulfonate in 20mM phosphoric acid. These solutions were replaced when their absorbance at 600 nm had decreased to below 80% of the initial value (typically after 3-4 months storage in transparent glass bottles). Potassium indigo trisulfonate was generally purchased from Riedel-de Hahn AG (Redox Indicator), but a product from Merck, which had been stored for 10-20 years, gave similar results. The microanalysis for C/N/O and titrations with permanganate indicated an indigo content of about 80% of the theoretical value. (An uncertainty of the concentration of the Indigo Reagent does not influence the accuracy of the analytical method which is based on standard curves for changes of absorbance achieved per added mole of ozone.) o-Tolidine hydrochloride and DPD (N,N-diethyl-l,4 phenylene-diamine sulfate (GR) were Merck p.A. products. (Comparable results were also achieved with the DPD test kit "AQUAMERCK Nr. 11100 for chlorine".) ACVK (Acid chrome violet K, Michrome Nr. 1047) was an Edward
450
H. BADER and J. HOIGNI~ so~
method; the u.v. method was applied only as a more convenient secondary standard.
0 H~Nr--~ s o , . I", . . , ~IIl , ~ =| L
"'1~j~' ' ~ ~" s o ,
o ~'03
1
0
isatin sutfonic acid and corres~ndimg products
sc~../,~.~k..~.O
2x
II
indigo trisuffomc acid
[, o.e2oooo .-' cm-' 1
".T
r
I~.,-0.0 M-' cm-' I
"¢'----"'" (so~) etc
Fig. 1. Indigo trisulfonate and its ozonation product. n.Ri -I ozone o~ded
Ascff 0.~
O~
0,~
02 i0.0 4O0
[]11
,, 500
. 600
700 nm
Fig. 2. Absorption spectra of Indigo Reagent after stepwise ozonation (Indigo trisulfonate added: 10/tM).
Gurr Ltd (London) product; Phosphate Buffer Reagent for pH 2: 28g N a H 2 P O 4 ' H 2 0 and 35g H s P O , (85V.) dissolved in t 1. dist. water. Other reagents were of commercial pro analysis grade. Aqueous ozone. Stock solutions of aqueous ozone were prepared by continuously bubbling ozonated oxygen (about 4%) through a gas-washing bottle into distilled water chilled to 2°C. This stock solution generally contained a steady-state concentration of ozone of 40 mg 1- ~. Lower concentrations were achieved by dilution with distilled water. Whenever not required for later use, this water was acidified with phosphoric acid to pH 2. When the bottle was kept closed the concentration of ozone decreased less than 0.2~ per min (Hoign6 & Bader, 1978). The aqueous ozone was transferred into the individual test bottles with glass pipettes or by plastic dispensers. The concentration of the diluted ozone water was determined before and after the samples were withdrawn. These calibrations of the stock solution were performed by measuring the u.v. absorbance of ozone at 258 rim. They were based on a molar absorptivity of 2900 M - * c m - * which was determined in other studies by a direct comparison with the iodine method, here considered as the primary standard (Kilpatrick & Herrick, 1965; Hoign6 & Bader, 1976). For these calibrations t h e samples of aqueous ozone was acidified by sulfuric acid to give 0.3-0.5 M acid. About 0.2 g o f potassium iodide were added. The liberated iodine was titrated with 0.1 M sodium thiosulfate using starch indicator. Care was taken that the aqueous ozone was always handled in exactly the same way, whether it was transferred into samples containing tndigo g ~ t or inlo the u.v. cells or into flasks for the iodide t i t r a t i ~ , All concentrations of ozone are therefore finally based on the iodide
2.2 Instrumentation Generally, the absorption measurements for the indigo method were performed at 600 nm on a Beckman DK 2A spectrophotometer which was equipped with a thermostated cell compartment maintained at 25°C. Cells of 0.5-5 cm path length were used. A VITATRON photometer equipped with a 4 cm cell and a 595 nm interference filter of 15 nm bandwidth was also used. In experiments to test the indigo method as a visual field method, pairs of graduated glass cylinders were used (volume 250 ml. height 25 cm). 2.3 Glassware Best reproducibilities were achieved when the glassware used for handling of aqueous ozone was conditioned by repetitive use for the same procedures. 2.4 Experimental procedures for the indigo method used with a spectrophotometer During this study, many varied procedures were tested. Only illustrative examples can be given here: Example for the determination of aqueous ozone in the concentration range 0.3-15 mg 1- t : Series of 100 ml volumetric flasks were prepared. To each flask of the series 10 ml of 0.5 M Phosphate Buffer Reagent for pH 2, 1 ml of 1 mM Indigo Reagent and, in case of the measurements of high concentrations of ozone, about 20 ml of distilled water were added. Some of these flasks were used to determine the standard curve. For this, individual flasks of the series were dosed with different amounts of diluted stock solutions of aqueous ozone containing about 2 m g l - ' ozone e.g. 0, 5, 10... 25 mt, at random to minimize systematic time drifts of ozone concentrations. This solution was dosed with a glass pipette whose tip was placed below the surface of the reagent solution while stirring with a Teflon covered magnetic bar. The concentration of the ozone in the diluted stock solution applied was calibrated simultaneously by measuring its u.v. absorbance at 258 nm in a 5 cm u.v. cell. Finally, distilled water was added to the 100 ml mark. During this operation the stirrer was temporarily raised above the mark. The absorbance of the residual indigo present in these series of test solutions was measured at 600 nm in a 5 cm cell. The values were plotted vs the amount of the ozone added. A typical standard curve is presented in Fig. 3(B). The determination of the ozone in the unknown samples is thereafter performed similarly by dosing specific volumes of these samples to others of the prepared flasks of the series. In order to be within the range of the amount of Indigo Reagent used the size of the sample was selected to contain an estimated 10--40#g ozone. For example, when the concentration of ozone was expected to be 0.5 mg 1-1 20-80 ml were injected and when the concentration was very high, such as 15 mg 1-t. 1-2.5 ml were dosed. The volumetric flasks were filled again with distilled water to the I00 ml mark. The residual absorption at 600 nm was compared with the standard curve and the original concentration of ozone was calculated. Comparable procedures were adapted to determine ozone in other concentration ranges. In cases of higher concentrations, higher amounts of the Indigo Reagent were dosed. In this case the determination of the residual absorbance of the Indigo Reagent was determined using 0.5 cm cells. An example of a standard curve is shown in Fig. 3(A). In this case the amounts of the Indigo Reagent and of the ozone applied were about 10 times higher than those in the example shown in Fig. 2. When lower concentrations of ozone were determined, less Indigo Reagent was used. Figure 3(C) gives an example in which the concentrations were 4 times lower than those for the example in
Determination of ozone in water by the indigo method
451
5iS A4©m 08'
~,
0.04
(A)
0.03 0.02
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06
0A
001
~
I 5
(200
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I I
I 2
i
! IL 25
"\.\
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Fig. 4. Standard curve absorbance vs ozone added to Indigo Reagent. Measurement with filterphotometer. Interference filter 595 nm. Cone. of Indigo Reagent 0.6 M. (Concentrations are based on final volume.)
,.I 4~
3
Added[03]
n ~ f'
glass cylinders A and B. Cylinder A was filled to its 25 cm mark with the sample of aqueous ozone. Cylinder B was filled with distilled water. Cylinder B was then emptied into an Erlenmeyer flask and refilled with this reference solution until the color intensity, when observed from top, matched that of cylinder A. A standard curve was obtained by repeating such measurements using different known amounts of ozone. The height of the reference solution in cylinder B was plotted vs the concentration of the aqueous ozone applied in cylinder A. A standard curve is given in Fig. 5. Thereafter, the concentration of ozone in the samples was determined by comparing the height of solution in the reference cylinder with the heights given in the standard curve. The size of the sample was taken into account for the calculation of the concentration.
(B)
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2.6 Experimental procedures applied for other spectrometric
ICl
analysts of ozone For some tests, ozone had to be determined by the direct measurement of its u.v. absorption at 258 nm even in natural waters. In these waters, which sometimes exhibited significant background absorption at 258 nm, the measured absorbance was corrected by determining the background absorbance after immediately destroying the residual ozone with an approx. 10-fold excess of aqueous nitrite or butene-ol. The ACYK method was carried out as described by Masschelein & Fransolet (1977). The methods based on
0.03
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I
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I
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Added [031 mg I-' Fig. 3. Standard curves o f absorbance (A) at 600 nm vs ozone added to Indigo Reagent. Measurement with s t ~ r o p h o t o m e t e r . Conc. of Indigo Reagent added: (A): 0.1 raM; (B) 10 #M; (C) 2.5/~M. (Cone. of Indigo Reagent and ozone are based on final volumes.)
i
25 •
-~o. eo
2o
.!o.
15~
~0.48
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ax
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Fig. 3(B). Figure 4 gives an example for a 14 times lower concentration range which has however measured on a filter photometer. If the rates of fast consumptions of ozone were measured (Hoign6 & Bad•r, 1981), the Indigo Reagent was injected directly into the flasks containing the aqueous ozone. Ozonation reactions were thus immediately stopped upon mixing with the Indigo Reagent. The concentration of the ozone was determined again from the decolorization of the Indigo Reagent as described above. 2.5 Visual method A visual method was developed for field studies: The same amount of the Indigo Reagent was added to a pair of w.R. 15/4--D
,~ ~
S 0
I
I 20
I
I 40
i
I 60
%** 1%
,,o.)6 I i
Added [0~]/~0 I-I
Fig. 5. Standard curve for visual method. To both cylinders of 250 ml 0.4/zmol of indigo trisulfonate had been added, h* Is the height of reference solution in cylinder B giving the same visual color intensity as 25 cm of the solution in cylinder A which was decolorized by the addition of a sample of aqueous ozone. The right-hand scale indicates the approximate absorbance of the solution in cylinder A. (Cone. of ozone is based on final volume.)
452
H. BADERand J. HO1GNE
o-tolidine and DPD were performed as described in Stan-
dard Methodsfor the Examination of Water and Wastewater I1975).
3. RESULTS
3.1 Absorption spectra of the Indigo Reagent The Indigo Reagent shows an absorption maximum at 600 nm. The apparent absorption coefficient, based on the amount of total mass of commercial (non-purified) reagent added is 16,500 M - 1 cm- 1. The shape of the absorption curve and )~,x do not change with ozone addition (see Fig. 2). 3.2 Sensitivity and precision of the indigo method as
used with spectrophotometers The absorption of samples containing the Indigo Reagent decreased linearly with the amount of ozone added over all the concentration ranges investigated. From the slopes of standard curves the photometric sensitivity of the analytical method may be deduced. Values are presented in Table 1. The absorbance change at 600nm is 2.0 + 0.! x 104cm -1 per mol 1-1 of added ozone for all concentrations tested. Lack of precision may be due to uncertainties in the calibration of aqueous ozone applied for the determination of the standard curves. We estimate that this uncertainty can cover a range of 5% when series performed at different concentration ranges are considered. The precision of the slope of single standard curves (about 8 points measured) was generally within this range. Neither deterioration of the absorbance of the Indigo Reagent solution to 80% of its original value during extended storage (3 months) nor a partial decolorization by a preliminary ozonation affected these calibrations. The error of the ozone determination decreases with the relative amount of decolorization and increases in case of very low concentration ranges. However, the relative standard deviations calculated from extended series were generally about + 2% when the concentration of ozone was above 0.1 mg 1-1 and when the residual absorbance was in the region of 10-60% of the initial absorbance. The main part of
Table 1. Typical sensitivity values of the Indigo Method co
Range of ozone dose
AA*
(/~M)
(mg 1-1)
(M - l c m - l)
100 10 2.5
0.2-3.5 0.02-0.3 0.02q?.l
(2.02 _ 0.10)' 104 (2.02 _ 0.10)" 104 (1.99 +_0.10)" 104
Co = cone. of potassium indigo trisulfonate applied, based on final volume. AA = C h a n g e o f a b s o r b a n c e ( c m - 1) at 600 n m m o l - 1 1- t added o z o n e . * Estimated range of accuracy of calibrations = + 0 . 1 . 1 0 ' * M - l cm -1.
the variances is thereby expected to be due to the variance in ozone doses. The effect of the rate of mixing of the aqueous ozone with the Indigo Reagent solution was tested in special series. Only a small difference of the sensitivity factor (2°; between matched pairs of extended series of simultaneously prepared samples) could be found between normally and extremely slowly stirred samples. 3.3 Measurements with filter photometers Measurements of the residual absorbance of the Indigo Reagent with a photometer equipped with a 595 nm filter gave results and variances comparable to those determined with the spectrophotometer. An example of a standard curve is given in Fig. 4. The slope of this curve is 85°,0 of that expected from measurements using a spectrophotometer at 600nm, probably because the 595 nm filter does not quite match the maximum of the absorption spectrum and the filter band-width of about 15nm exceeds the maximum of the absorption band of indigo trisulfonate (c.f. Fig. 2). 3.4 Visual methods An example of results found by the visual method is presented in Fig. 5. An incremental ozone dose of 10 pg 1- 1 can be easily detected when 25 cm cylinders are used. 3.5 Stoichiometricfactor Extrapolation of the standard curves to the intercept on the absissa shows that only 80% of the commercial indigo product applied is effective in consuming ozone. This calculation is based on the assumptions that the potassium indigo trisulfonate applied was 100`0/0 pure and that 1 mol ozone added would decolorize 1 tool of indigo. Above pH 4 the stoichiometric factor often decreases and it seems to become somewhat erratic. 3.6 Rates of reactions For the application of the Indigo Method for kinetic measurements it is important to note that all the ozone is consumed by the reagent immediately upon mixing. This was observed on a 1 s time scale even in systems where the concentration of ozone decreases by fast concurrent reactions, e.g. 20% s-1, and even when the concentrations of both the indigo trisulfonate and the ozone were, at the time of mixing, as low a s 10 - 7 M {Hoign6 & Bader, 1981). That means that the rate constant for the reaction of ozone with indigo trisulfonate must be higher than 107M - is-1. Aqueous indigo trisulfonate which is preozonized until just decolorized consumes further ozone only slowly. The apparent reaction-rate constant of t h e s e ozonolytic products is about 2500 M - ts- 1 (Hoign6 & Bader, 1981). Based on this, we may assume that t h e indigo method will not be significantly disturbed by short temporary local overdoses of ozone.
Determination of ozone in water by the indigo method
453
03oddN ~nm As. ~
--
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--
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/
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Fig. 6. Stability of residual absorbance of different reagents after addition of aqueous ozone. In case of ACVK and Indigo the amount of ozone added decolorized about 40% of the added reagent.
3.7 Temperature effect on absorbance
mgl-,
The absorbance of the Indigo Reagent and the sensitivity factor for the determination of ozone decreases 2.0% with an increase of 10°C.
03 ffteOS. O.fl
3.8 Stability of residual absorption after ozone addition
0.6
The residual absorbance of Indigo Reagent measured after an addition of aqueous ozone in distilled water does not change during a 6 h time period. A similar stability was only found with ACVK. In contrast, D P D and o-tolidine show significant absorbance changes even when observed within I h (ef. Fig. 6).
0.4
3.9 Determination of ozone in natural water Comparable results for the determination of the residual ozone present in ozonized natural water have always been found by measuring its absorbance at 258 nm (reference method) and by the indigo method. Comparisons are presented in Table 2. The direct u.v. measurement, however, required a rather complicated procedure to compensate for the background absorption of natural water at 258 n m (see Experimental), also its sensitivity is relatively low (see Discussion). Discrepancies between the two methods are therefore
j a 0.,~
0£
I 20
I 40
I 60
Time offer indigo odd. ~h
a Rowwoter,loke Zurich (3Ore) Row woter, G r e i f ~ ,
surfoce ~4rag F'DOC
Fig. 7. Stability of residual absorbance of Indigo Reagent in different types of raw waters, expressed in apparent concentrations of ozone determined. The residual absorbance was about 50% of the non-ozonized reference.
due rather to errors involved in the data of the reference method applied. In some types of ozonated surface water the residual indigo trisulfonate shows a slow post-decolorization when observed over several hours. Samples of
Table 2. Comparison of cone. of residual ozone determined in different types of ozonized waters by the direct method (u.v. 258 nm) and by the indigo method (600 nm) Water Bidistilled Lake Zurich (lake water, DOC 1.1 mg 1-1) (after sandfilter, DOC 1.2 mg 1-~)1" Greifensee (lake water, DOC 4 mg 1-1)
I rng fJDOC
U.V.2sanm Indigo6oonm (rag 1-1 ozone) (mg 1- l ozone)*
0.51
0.53
0.61 0.35 0.80
0.60 0.34 0.83
1.24
1.28
0.43
0.43
* All calibrations are based on a sensitivity factor of 20,000 M - ~ cm- 1. t Ozonizations performed in plant at Zurich-Lengg.
454
H. BADERand J. HOIGNt~
water from Greifensee--a highly eutrophic lake exhibited the maximum rate of change encountered in our survey studies (Fig. 7). Such post-decolorizations are, however, very small when purified drinking waters are considered.
found for this reagent when the absorbancc of the disulfonate was measured at 610nm, where this reagent shows its maximum absorption (Hoign6 & Bader, 19801.
3,10 Effect of other oxidants
4. DISCUSSION
At pH 2 hydrogen peroxide, chlorite, chlorate and perchlorate do not decolorize Indigo Reagent when observed within a few hours and when the concentrations considered are within a factor of 10 of that of the ozone to be determined. However, chlorine and iodine lead to a slow post-decolorization. Their effects increase with time. Chlorine can be successfully masked by adding 500 mg 1-1 malonic acid to the Indigo Reagent; malonic acid at pH 2 consumes ozone with a reaction-rate constant of only 3 M ~s(Hoign6 & Bader, 1981) and therefore does not compete with the Indigo Reagent for reaction with ozone when added simultaneously with the reagent. An example of the effect of chlorine in the absence and presence of malonic acid is given in Table 3. Products of ozonolysis of organic solutes seem generally not to interfere with the residual Indigo Reagent when tested within a few hours: no decolorization was observed in any of the solutions we used for kinetic measurements. This study covered many different types of aromatic hydrocarbons, phenols and other products (Hoign6 & Bader, 1981). However if, for example, l mM 1-hexene-4-ol has been preozonized with 0.5 mM 03 at pH 2, it decolorized a 0.01 mM solution of Indigo Reagent significantly within 40 min. Permanganate decolorized Indigo Reagent in a fast reaction.
4.1 Stoichiometric Jactor for the decolorization of
Indigo Reagent The sensitivity of the decolorization of Indigo Reagent stays constant over a wide range of ozone doses. This can only be expected if it is assumed that the sulfonated indigo is a spectroscopically uniform material with respect to its decolorization reaction; in the presence of reactive impurities (such as other indigo derivatives), both the absorption maximum and the sensitivity could shift during ozonation. An apparent stoichiometric factor of 1:0.8 for the decolorisation of the Indigo Reagent agrees with the microanalytical characterization of the commercial potassium indigo trisulfonate. The phenomenon of the stoichiometric factor decreasing above pH 4 can be explained: in the presence of organic materials with structures similar to that of the ozonolytic products of indigo, a catalytic chain reaction leads to an accelerated decomposition of ozone (Hoign6 & Bader, 1976). Such chain reactions are initiated by hydroxide ions and therefore proceed at elevated pH during mixing of the reagents wherever temporary local excess of ozone is present in the reaction mixture. The secondary oxidants formed from decomposed ozone, OH radicals, do not however selectively oxidize the chromophoric structure of residual indigo (Hoign6 & Bader, 1978).
3.11 Indigo Reagent based on indigo disulfonate All results reported here are based on an Indigo Reagent which was prepared from commercial potassium indigo trisulfonate. In response to inquiries received, some tests have been repeated with an Indigo Reagent based on commercial types of sodium indigo disulfonate. Comparable results have been
Table 3. Effect of chlorine on the determination of ozone. The residual absorption (A) was measured 4 rain and again I h after the ozone (and chlorine) was dosed to the Indigo Reagent. Malonic acid (m.a.+) was added directly to the Indigo Reagent before reaction 600 n m
CI2 m.a. Ascm 0.2 mg1-1 l m g l -~ 500mg1-1 4min lh 0 3
-
+ + + +
+ +
not added. + added.
-
+ +
0.88 0.51 0.53 0.44 0.48
0.88 0.52 0.52 0.24 0.48
4.2 The sensitivity and limits of detection The sensitivity, which amounts to a change of the absorbance of AA = - 20,000 cm- 1 mol- t ozone added l-~ is quite high. If a classical type of filter photometer or u.v. spectrograph is used, a limit of detection of about 5 pg I-1 can be achieved. When working with a high performance spectrophotometer, a detection limit of 1 #g l- ~ is approached (cf. Hoign6 & Bader, 1981a). Here the handling of the ozonated water and not the instrumental accuracy become limiting. The detection limit of the simple visual method is 10 pg l- 1 when 25-cm cylinders are used. This limit of detection fulfils the requirements for field studies on drinking water: e.g. according to a recommendation accepted in Switzerland, less than 50/~g1-t of residual ozone should appear at the consumers tap (Schweiz. Lebensmittelbuch, 1972). 4.3 Sensitivity of the Indigo Method compared with
other spectrophotometric methods In Table 4, the absorption regions and sensitivities of different photometric methods are listed for corn-
Determination of ozone in water by the indigo method Table 4. Sensitivities of different colorimetric methods for the determination of aqueous ozone expressed as AA (change of absorbance per 1 mol 1-t ozone added per I cm) Method u.v. of 03 Indigo ACVK Orthotolidine DPD Iodide/triiodide
Wavelength (nm)
AA (M- 1 cm- t)
Ref.
258 600 550 440 510 352
+2900 - 20,000 - 2500 + 32,000 + 15,000 +25,000
a b c d e f
a--Kilpatrick (1965) and Hoign6 & Bader (1976). b---this study. c--own measurement; method of Masschelein (1977). d--own measurement and Sulzer (1958). e--own measurement, method of Palin (1975). f---talc, from data of Shechter (1973). parison. The most straightforward method is the direct measurement of the absorbance of aqueous ozone at 258 nm. This absorption occurs, however, in a region where many types of other solutes interfere: in natural waters, e.g. humic materials often cause a significant background absorption which varies with ozonation. In addition, the instrumental measurement must be performed at the time at which the concentration of ozone is of interest. Moreover, due to the low molar absorptivity of ozone, the sensitivity of this method is relatively low. This direct method, however, must still be recommended as the best defined and useful reference method for the calibration of ozone concentrations whenever the limitations of sensitivity and background absorptions are not relevant. In contrast, the Indigo Reagent absorbs light in a spectral region where only a few other substances interfere. Natural waters, for example, have generally no background absorption in the 600 nm region. The sensitivity, when expressed as the change in absorbance per added ozone, is quite high. In addition, instrumental sensitivities are generally high at 600 nm and even the sensitivity of the human eye is relatively high at this wavelength. For most types of waters measurements can be performed even hours after the ozonated water has been added to the reagent solution. The sensitivity of the indigo method is about 8 times higher than that of the ACVK method. In addition, measurements at 600nm are preferable to measurements at 550 nm where more aqueous solutes may interfere. The o-tolidine method exhibits a sensitivity 1.6 times higher than the indigo method. Its non-specific response to many kinds of oxidants, its low wavelength region of absorption, and its temporal instability, however, disqualify this method when compared with the others (Sulzer, 1958; Hofman & Stern, 1969), The DPD method for the determination of ozone is based on a preliminary oxidation of iodide. However, all methods based on oxidations of iodide to iodine
455
have the disadvantage of being non-specific; iodine is also formed by secondary oxidants such as produced by ozonation of water below pH 6-7 that contains organic solutes. Moreover, the DPD method shows a large time drift and therefore cannot be recommended for extended series of measurements as required for kinetic studies. 4.4 Dynamic range of the Indigo Method The ozone dose must be adjusted to decolorize 20-903/o of the Indigo Reagent. For one fixed concentration of the reagent and one predetermined relative ratio of volume of reagent to volume of aqueous ozone, the dynamic range is therefore restricted to a factor of about 4.5. In non-automated systems, preliminary tests will help to locate the approximate range of concentration before specifying the analytical procedure with respect to ratios of volumes to be used. During our studies, we covered a region of measurement of 0.005-30 mg l-1 of aqueous ozone by merely adjusting the relative volume of aqueous ozone and of Indigo Reagent applied. This entire range exhibits constant sensitivity. It covers the requirements generally encountered in ozone applications. 4.5 Calibrations In this study we used the u.v. absorbance of ozone in distilled water at 258 nm as a secondary standard to calibrate the analytical standard curves. At the present time we still recommend such calibrations, although different commercial products of indigo trisulfonate tested so far have shown rather comparable sensitivity factors (Hoign6 & Bader, 1980). Sensitivity factors can, however, only be exchanged between laboratories which perform the measurements on calibrated spectrophotometers at 600 nm. Filter instruments will require individual calibrations. 5. CONCLUSIONS The Indigo method covers the range of concentrations between 0.005 and 30 rag l-t. The method is relatively straightforward and does not require any difficult or time-consuming operations. It can be performed with the classical instrumentation of waterwork laboratories. The Indigo Reagent is stable: it can be stored for months as a reagent which is "ready to use". The absorption to be measured at 600 nm is situated in a range where solutes of natural waters do not interfere and where even simple visual methods can be used for measurement. The change of absorbance reflects the amount of ozone added with high and constant sensitivity factor of AA = - 2 0 , 0 0 0 c m - 1 ( m o l . l - l ) - I ozone added. All standard curves are linear. The sensitivity does not vary with ozone concentration, with small changes of temperature of reaction, or with the chemical composition of the water whenever the pH is below 4. For many types of waters the instrumental or visual
456
H. BADERand J. HOI,GNF'~
measurements can be performed even a few hours after the ozone has reacted with the Indigo Reagent. The method is relatively selective. Secondary oxidation products do generally not interfere. The reaction of ozone with the Indigo Reagent is so fast that it can be applied to stop and measure even very fast ozonation reactions. The method has been well tested and applied for the determination of ozone in many different types of waters. Unexpected difficulties have not arisen. More practical experiences with this method are now being accumulated in many waterworks and laboratories. However, this Indigo M e t h o d cannot be used as a primary standard. At the present time the reagent must still be calibrated. A standard curve can be produced using an aqueous solution of ozone in distilled water which is calibrated by the direct u.v. absorption measured at 258 nm or by the classical iodometric procedure. It is, however, quite possible that a wider application of the indigo m e t h o d will stimulate central national laboratories or a chemical company to market an indigo reagent with a calibrated sensitivity factor for the determination of ozone and chlorine dioxide (see Hoignb & Bader, 1980). Acknowledgements--We thank Professor Werner Stumm for his continuous encouragement, and Dr Jiirg Zobrist and Dr Joan Davis for the valuable reviewing and discussions. Andreas Locher is thanked for his assistance in performing many of the experiments during his apprenticeship stage. REFERENCES
Dorta-Schaeppi Y. & Treadwell W. D. (1949) Ueber eine kolorimetrische Bestimmung von hochverdiinntem Ozon in Sauerstoff. Heir. chim. Acta 32, 356-364. Dr~iger H. & Dr~iger B. (1966) Anzeigepr~iparat f'tir PriifriShrchen zur Anzeige von Ozon und Verfahren zu seiner Herstellung. German Patent No. 1'204'854 (26.5.1966). Fonrobert E. (1916) Chemie in Einzeldarstellungen, Vol. IX, p. 231. Das Ozon, Verlag Ferd. Enke, Stuttgart. Hofmann P. & Stern P. (1969) Photometrische Bestimmung niedriger Ozonkonzentrationen in W~issern. Analyt. Chim. Acta 47, 113-120.
Hoigne J. & Bader H. (1976) The role of hydroxyl radical reactions in ozonation processes in aqueous solutions. Water Res. 10, 377--386. Hoign6 J. & Bader H. (1978) Ozone and hydroxyl-radical initiated oxidations of organic and organometallic trace impurities in water. Orqanometals and Organometalloids: Occurrence and .late in the Environment. (Edited by Brinckman F. E. & Bellama J. M.), Am. Chem. Soc. Symp. Ser. 82, pp. 292 313. Hoign6 J. & Bader H. (1979a) Ozonation of water: Selectivity and rate of oxidation of solutes. Ozone Sci. Enqng. 1, 73 85. Hoign6 J. & Bader H. (1979b). Ozonation of water: "Oxidation-Competion values" of different types of waters used in Switzerland. Ozone Sci. En,qno. !, 357 372. Hoign6 J. & Bader H. (1980) Bestimmung yon Ozon und Chlordioxid in Wasser mit der lndigo-Methode. Vom Wasser. 55, 261--280. Hoign6 J. & Bader H. (1981) Rate Constants of direct reactions of ozone with organic compounds in water: In preparation. Kilpatrick M. L. & Herrick C, D. (1965) The decomposition of ozone in aqueous solution. J. Am. chem. Soc. 78, 1784-1889. Masschelein W. J. & Fransolet G. (1977) Spectrophotometric determination of residual ozone in water with ACVK. J. Am. Wat. Wks. Ass. 69, 461-462. Palin A. T. (1975) Current DPD method for residual halogen compounds and ozone in water. J. Am. Wat. Wks Ass. 67, 32-33. Preisler P. W., Hill E. S., Loeffel R. G. & Shaffer Ph.A. (1959) Oxidation reduction potentials, ionization constants and semiquinone formation of indigo solfonates and their reduction products. J. Am. chem. Soc. 81, 1991-1995. Schweizerisches Lebensmittelbuch (1972) Trinkwasser und Mineralwasser. Kap. 27, p. 8. Shechter H. (1973) Spectrophotometric method for determination of ozone in aqueous solutions. Water Res 7, 729-739. Standard Methods for the Examination of Water and Wastewater (1975) 14th Edition. APHA-AWWA-WPCF. Sulzer F. (1958) Ueber das Verhalten w~isseriger Ozonl6sungen. Schweiz. Z. Hydrol. 20, 16-29. Thenard A. & Thenard P. (1872) M6moire sur Faction de l'ozone sur le sulfate d'indigo et I'acide ars6nieux. C. r. Acad. Sci. 75, 458-465. Verein Deutscher Ingenieure, VDI, 2468 (1977) Messen gasf6rmiger Immissionen. Messen der Ozon-Konzentration. Manuelles photometrisches Verfahren. Indigosulfons~iure-Verfahren.