The Effect of Indigo Purity on Measuring the Concentration of Aqueous Ozone Gilbert Gordon*, Bryan Walters and Bernard Bubnis Department of Chemistry Miami University Oxford, OH USA Abstract The IOA Guideline for measuring residual ozone concentrations assumes a ! ! sensitivity coefficient ( f ) of 0.42 L mg 1cm 1. The value of the sensitivity coefficient depends on the purity of the indigotrisulfonate reagent. There is evidence indicating that solid indigotrisulfonate will eventually decompose and the molar absorptivity of indigo solutions slowly decreases on the days timescale. The data presented in this paper shows that the indigo sensitivity coefficient is not constant and can result in concentration errors as great as 25%. Information is presented describing a simple procedure for calculating the sensitivity coefficient using an initial absorbance measurement of the indigo reagent. Introduction The International Ozone Association Guideline (1) for measuring residual ozone is based on the decolorization of indigotrisulfonate at 600 nm. The calculation assumes a sensitivity ! ! ! ! coefficient ( f ) of 0.42 L mg 1cm 1 (e600 nm = -20,000 M 1cm 1) as reported by Bader and Hoigné (2,3,4). In principle, once the sensitivity coefficient is determined for an indigo source, ozone calibration should not be required on a daily basis provided the reagent is of high purity and stable both in the solid form and following preparation of a reagent stock solution. The value of the sensitivity coefficient is dependent on the purity of the indigotrisulfonate reagent. Preisler reports (5) that high purity indigo has a molar absorptivity of 23,800 !1 !1 M cm . However, the purity of commercially available indigotrisulfonate is typically at the ! ! 80-85% level. Thus, an average value of -20,000 M 1cm 1 has been used to adjust for the purity of the indigo reagent. In addition, there is some evidence indicating that solid indigotrisulfonate will eventually decompose after sitting for many years on the laboratory shelf. In solution, the molar absorptivity of indigo will slowly decrease on the days timescale. In fact, Bader and Hoigne observed this degradation and urged caution when indigo solutions are to be used over extended periods of time. The rule-of-thumb is that a fresh solution of

indigo should be prepared when a 20% loss in absorbance is observed in the stock solution based on an initial absorbance measurement at the time of preparation. In the original report, Bader and Hoigne address the purity issue by stating that ... "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". This observation is valid, however, in practice there is a reluctance to undertake the tedious and time consuming process of preparing an ozone calibration curve on a routine basis. The normal practice is to either determine the sensitivity coefficient upon purchase and use this experimentally determined value or to simply use the ! ! generally accepted value of 0.42 L mg 1cm 1. Typically, the sensitivity coefficient is not evaluated on a routine basis. The data presented in this paper shows that the indigo sensitivity coefficient is not constant. Information is also presented describing a simple procedure for calculating the sensitivity coefficient using a new initial absorbance measurement of the indigo reagent at the time of each measurement. This procedure eliminates the tedious ozone calibration and provides a way for water utility engineers to recalculate the sensitivity coefficient on a routine basis. Experimental All solutions were prepared using double distilled deionized water (DDW) unless otherwise noted. All chemicals were used without further purification. Preparation of Indigo Stock Solutions An indigo stock reagent for each source was prepared following the Standard Method (6). 770 mg of potassium indigotrisulfonate is dissolved in 500 mL of distilled water and 1 mL of concentrated phosphoric acid in a 1L flask. The solution is mixed and diluted to volume with distilled water. A 10:1 working stock solution containing appropriate amounts of sodium dihydrogen phosphate and concentrated phosphoric acid is prepared. All stock and working indigo solutions should be stored in the dark. Indigo Stock Solution Molar Absorptivity Calculation The calculation of the molar absorptivity is based on the Beer-Lambert Law:

Absorbance = molar absorptivity x cell pathlength x indigo concentration

10 mL of each indigo stock solution was pipetted into a 100 mL volumetric flask and diluted to volume. The absorbance of the solution was measured a minimum of 5 times at 600 nm in a 2 cm measuring cell and the molar absorptivity calculated. Sensitivity Coefficient Calculation: Ozone Calibration Curve The calculation of the sensitivity coefficient from an ozone calibration curve is tedious and time consuming. Ozone readily decomposes and is volatile. Thus, prior to each calibration, fresh ozone was generated directly in a shrinking bottle (7) containing a pH 4 acetic acid buffer (8). Because there is zero headspace in the shrinking bottle, problems caused by ozone volatility are eliminated. The absorbance of the ozone stock solution was measured at ! ! 260 nm and the concentration calculated using a molar absorptivity of 3,000 M 1cm 1. Ozone calibration curves were prepared by pipetting 10 mL of the working indigo stock solution into a 100 mL flask and adding appropriate aliquots of the ozone stock solution. The absorbance of each ozone "standard" was measured at 600 nm. By plotting the ? Absorbance vs. mg/L O3, a calibration curve can be drawn and the sensitivity coefficient calculated: Sensitivity Coefficient ( f ) =

calibration curve slope cell pathlength

Sensitivity Coefficient Calculation: Initial Absorbance As a matter of convenience, the Beer-Lambert Law can be rearranged to calculate the sensitivity coefficient ( f ) based on the initial absorbance of the indigo stock solution at 600 nm (a minimum of 3 measurements is recommended): Sensitivity Coefficient ( f ) =

Indigo Molar Absorptivity SampleVolume x 616.74 x g Indigo Total Volume

Results and Discussion The data in Table 1 show that the sensitivity coefficient can vary significantly depending on the source of the indigo.

Table 1. Sensitivity Coefficients: Ozone Calibration Curve Indigo Source

Trial 1

Trial 2

Trial 3

Avg

STD

%RSD

Aldrich 1999

0.424

0.435

0.421

0.427

0.006

1.41

Fluka 1998

0.408

0.427

0.420

0.418

0.008

1.88

Acros 1999

0.405

0.403

0.407

0.405

0.002

0.40

Aldrich 1998

0.390

0.398

0.385

0.391

0.005

1.37

Acros 1998

0.382

0.379

0.392

0.384

0.006

1.45

Sigma 1998

0.361

0.360

0.358

0.360

0.001

0.35

Riedel 1983

0.364

0.353

0.371

0.363

0.007

2.04

All Data Points

0.393

0.025

6.4

The average calculated sensitivity coefficient for the 7 indigo sources was 0.393 " 0.025 L ! ! mg 1cm 1. In the case of the oldest source of indigo, the sensitivity coefficient differs by ! ! 15% compared to the generally accepted value of 0.42 L mg 1cm 1. We now propose a simplification and an improvement in the calculation of the sensitivity coefficient based on the initial absorbance of the indigo stock solution. The data in Table 2 show the molar absorptivity for each indigo dye calculated using the simplified procedure. However, in this context, it should be noted that only 2 dyes (Aldrich 1999 and Fluka 199) ! ! meet the molar absorptivity criteria (-20,000 M 1cm 1) established by Bader and Hoigne for stock indigo solutions at the time of preparation. Table 2. Molar Absorptivities: Initial Stock Solution Absorbance Indigo Source

Trial 1

Trial 2

Trial 3

Avg

STD

%RSD

Aldrich 1999

20,340

20,318

20,406

20,355

37

0.18

Fluka 1998

20,480

19,920

19,560

19,987

379

1.89

Acros 1999

18,923

19,209

18,998

19,043

121

0.64

Aldrich 1998

18,808

18,160

17,960

18,067

82

0.45

Acros 1998

17,640

17,560

17,920

17,707

154

0.87

Sigma 1998

17,328

17,210

17,441

17,326

94

0.54

Riedel 1983

15,760

15,600

16,040

15,800

182

1.15

All Data Points

18,360

1,489

8.1

!

!

The molar absorptivities in this study ranged from 15,600 to 20,406 M 1cm 1. Deviations ! ! from the -20,000 M 1cm 1 value are significant because indigo solutions decompose at faster rates compared to solid indigo on the lab shelf. This means that the useable lifetime of the stock solution can be much less than the original 4 month projection recommended in the early indigo papers. Table 3 shows the calculated sensitivity coefficients based on the simplified initial absorbance

method. Table 3. Sensitivity Coefficients: Initial Stock Solution Absorbance Indigo Source

Trial 1

Trial 2

Trial 3

Avg

STD

%RSD

Aldrich 1999

0.428

0.428

0.430

0.429

0.001

0.18

Fluka 1998

0.431

0.419

0.412

0.421

0.008

1.89

Acros 1999

0.398

0.404

0.400

0.401

0.003

0.64

Aldrich 1998

0.381

0.382

0.378

0.380

0.002

0.45

Acros 1998

0.371

0.370

0.377

0.373

0.003

0.87

Sigma 1998

0.365

0.362

0.367

0.365

0.002

0.54

Riedel 1983

0.332

0.328

0.338

0.333

0.004

1.15

All Data Points

0.386

0.031

8.0

The range of sensitivity coefficient values is similar to those calculated using ozone calibration curves presented in Table 1. The average of the sensitivity coefficients for all the sources of indigo using the calibration curve method and the initial absorbance method are indistinguishable, 0.393 " 0.025 vs. 0.386 " 0.031 respectively (-1.8%). The error analysis shows that by neglecting the oldest source of indigo (circa. 1983), the difference in sensitivity coefficients between the two calculation methods is -0.7%. The correlation (r2 = 0.994) between the 2 methods is shown in Figure 1.

Figure 1. Correlation of the Methods for Calculating the Indigo Sensitivity Coefficient The significance of this data set is 2-fold: 1) a potentially large deviation from the widely

!

!

reported ozone sensitivity coefficient of 0.42 L mg 1cm 1 is observed depending on the source and/or age of the indigo dye; and 2) the calculation of the indigo sensitivity coefficient does not require the preparation of an ozone calibration curve. The data presented here shows that the sensitivity coefficient of freshly made indigo stock ! ! solutions can range from -0.360 to 0.429 L mg 1cm 1. The cause of the large differences can be the result of ! ! ! ! !

differences in reagent purity the ratio of disulfonate to trisulfonate indigo species in the solid material varying molar absorptivities of chromophoric species in the solid potentially different rates of reactions with ozone possibly a continuing slow decomposition of the indigo dye during storage

The data for indigo purchased from a single supplier in different years (Acros 1998/1999 and Aldrich 1998/1999) suggests that solid indigo stored in the dark decomposes. In 1983, ! ! we determined that the molar absorptivity of the Riedel dye was -20,000 M 1cm 1 with a ! ! sensitivity coefficient of 0.42 L mg 1cm 1. The data in Tables 1 and 2 show an -20% decrease in the sensitivity coefficient since our original measurements. The degradation of indigo "on-the-shelf" is unambiguous. Standard Method 4500 - O3 B states that solutions of indigo have limited stability (-4 months when stored in the dark). To verify the extent of indigo degradation in solution, studies were designed to monitor the change in the molar absorptivity of the Sigma, Acros and Aldrich dyes over a 10-day holding period. The data in Table 4 show that the average change in the molar absorptivity was 0.94 " 0.09% per day or about 1% per day. Table 4. Indigotrisulfonate Stock Solution Decomposition Sigma 1998

% Decomposition

Acros 1999

% Decomposition

19,043

Aldrich 1999

% Decomposition

Initial

17,326

20,355

Day 1

17,204

0.70

18,829

1.12

20,133

1.09

Day 2

17,002

1.87

18,664

1.99

19,991

1.79

Day 3

16,734

3.42

18,419

3.28

19,772

2.86

Day 4

16,650

3.90

18,281

4.00

19,653

3.45

Day 10

15,697

9.40

17,462

8.30

18,320

10.6

Conclusions The data confirm that solid indigo reagent upon dissolution does not necessarily have a ! ! ! ! sensitivity coefficient of 0.42 L mg 1cm 1 (e600 nm = -20,000 M 1cm 1). The data also show that indigo stock solutions are not very stable and that different sources of indigo decompose at different rates. Based on the study of 7 sources of indigo, we recommend that the indigo sensitivity coefficient be recalculated on a regular basis. The recalculation does not require the preparation of an ozone calibration curve. Instead, it can be calculated by using an average of at least 3 initial absorbance measurements. Because the initial absorbance method is simple and straight-forward, it is ideal for water treatment laboratories. Key Words Ozone; Indigo; Sensitivity Coefficient; Molar Absorptivity; Purity References International Ozone Association Revised Guideline Document - 1998. "III. Colorimetric Method for Manual Determination of Ozone Concentration in Water". Ozone Sci. & Eng., Vol. 20.6, p.443. Bader, H.; Hoigne, J. "Determination of Ozone in Water by the Indigo Method". Water Res., 15:449 (1981).

Bader, H.; Hoigne, J. Ozonation Manual for Water and Wastewater Treatment, Chapter 38. John Wiley & Sons, New York (1982). Bader, H.; Hoigne, J. "Determination of Ozone in Water by the Indigo Method; A Submitted Standard Method", Ozone Sci. & Eng., 4:169 (1982).

Preisler, P.W.; Hill, E.S.; Loeffel, R.G.; Shaffer, Ph.A. "Oxidation Reduction Potentials, Ionization Constants and Semiquinone Formation of Indigo Sulfonates and Their Reduction Products", J. Am. Chem. Soc., 81, 1991-1995 (1959)

Standard Methods for the Examination of Water and Wastewater, 18th Edition, 1992, Method 4500-O3 B Indigo Colorimetric Method, p. 4-106. Silverman, R.A.; Gordon, G. "Use of the Syringe as a Shrinking Bottle", Anal. Chem., 46:1:178 (1974).

Hoigne, J.; Bader, H. The Role of Hydroxyl Radical Reactions in Ozonation Processes in Aqueous Solutions. Water Res, 10:377 (1976).