Potassium Bromate Assay by Redox Titrimetry Using Arsenic Trioxide

Bromate, a disinfectant, is one of the analytes of interest in wastewater analysis. Environmental laboratories have a regulatory need for their measurements to be traceable to NIST standards. Bromate is not currently certified as a NIST Standard Reference Material (SRM). Therefore, a traceable assay of potassium bromate (KBrO3) is needed. KBrO3 was dissolved in water and assayed by redox titrimetry using arsenic trioxide (As2O3). A nominal (0.1 g) sample of As2O3 was dissolved in 10 mL of 5 mol/L sodium hydroxide. The solution was acidified with hydrochloric acid and about 95 % of the KBrO3 titrant was added gravimetrically. The end point was determined by addition of dilute (1:3) titrant using an automated titrator. The KBrO3 assay was determined to be 99.76 % ± 0.20 %. The expanded uncertainty considered the titrations of three independently prepared KBrO3 solutions.


Procedure
Three solutions were prepared from the dried KBrO 3 to a nominal mass fraction of 0.012 g/g. Each solution was titrated on a separate day. The assay procedure [8,9,10] was a redox titration in which As 2 O 3 was titrated with potassium bromate according to Eq. (1) and Eq. (2).
According to Eq. (2), after all the As 2 O 3 has been consumed, the end point (first appearance of free bromine) is detected by irreversible decolorization of the indicator and/or change in potential. A nominal 0.1 g sample of As 2 O 3 was weighed (± 0.00001 g) in a platinum boat. After transferring the sample to a 150 mL beaker, 10 mL of 5 mol/L NaOH was added. The concentration of NaOH is important to insure complete dissolution. It takes about 5 min to 10 min for the As 2 O 3 to dissolve, and difficulty in dissolution occurs with more dilute NaOH. A magnetic stir bar, 50 mL of water, and 10 mL of 10 mol/ L HCl were added to the solution. The resulting acidic medium is required for the titration method. The indicator, two drops of methyl red indicator, was added just before the start of the titration. At the end point, the indicator turns from red to colorless.
Approximately 95 % of the KBrO 3 titrant (gravimetric KBrO 3 ) was added gravimetrically to the solution from a weighed (± 0.00001 g) plastic 5 mL or 10 mL syringe. This initial titrant addition (gravimetric KBrO 3 ) is added quickly with visual help from the indicator change.
The remainder of the KBrO 3 (volumetric KBrO 3 ), about 0.4 mL of a more dilute solution with a nominal dilution factor of three, was titrated volumetrically to a potentiometric end point using an automated titrator. A visual end point from the indicator was also observed at this time. A combination platinum electrode (Schott Blue line 31 RX) 1 was immersed in the solution on a sample changer and the titrant (dilute KBrO 3 ) was added from a 10 mL buret of an automated titrator. As the solution was mixed by the rotating stir bar, the automated titrator added equal-volume (0.006 mL) increments of dilute KBrO 3 titrant. Data stored included the volume of titrant added, V, with a corresponding measured potential, E, and numerical estimates of the first derivative (dE/dV). The end point was determined as the maximum of this first derivative. The amount of dilute KBrO 3 added to reach the end point was the volumetric KBrO 3 . At least two blanks (reagents only, omitting As 2 O 3 ) were titrated volumetrically with the dilute KBrO 3 titrant each day.
The amount of gravimetric KBrO 3 (g) and volumetric KBrO 3 (mL) were added to calculate the titrant (total KBrO 3 ) using Eq. where m total titrant = mass of total KBrO 3 (g) at the end point m conc KBrO 3 = mass of concentrated KBrO 3 solution (gravimetric KBrO 3 ) (g) ρ = density of dilute KBrO 3 solution (g/mL) V dil KBrO 3 = volume of dilute KBrO 3 solution (mL) V blank = volume of dilute KBrO 3 solution titrated for the blank (mL) DF = dilution factor. According to Eq. (4) below, the mass fraction (w), in %, of KBrO 3 was calculated as the ratio of the KBrO 3 (g/g) from the titration with As 2 O 3 (1st factor) to the KBrO 3 (g/g) from the preparation of the gravimetric solution (2nd factor) as follows: where w KBrO 3 = mass fraction of KBrO 3 (%) m As 2 O 3 = mass of As 2 O 3 (g) w As 2 O 3 = mass fraction of As 2 O 3 (g/g) M KBrO 3 = molecular weight of KBrO 3 (g/mol) M As 2 O 3 = molecular weight of As 2 O 3 (g/mol) m total titrant = mass of total KBrO 3 (g) m grav KBrO 3 soln = mass of KBrO 3 gravimetric solution prepared from KBrO 3 salt(g) m grav KBrO 3 salt = mass of KBrO 3 (salt) for preparation of gravimetric solution (g). The molecular weights (relative molecular masses) of KBrO 3 and As 2 O 3 are 167.001 g/mol and 197.8412 g/mol, respectively [11]. The mass measurements were corrected for air buoyancy. The densities of the dilute and concentrated KBrO 3 solutions were determined. Corrections for air buoyancy were calculated based on densities [12] of 3.27 g/mL for KBrO 3 , 3.738 g/mL for As 2 O 3 , 0.00117 g/mL for air, and 8.0 g/mL for the stainless steel calibration weights in the microbalance [13].

Purity Analysis of KBrO 3
A potassium bromate sample was analyzed by glow discharge mass spectrometry (GDMS) [14]. Among the element impurities found were arsenic and chlorine, present at 1 µg/g and 10 µg/g, respectively. Assuming the worst situation that all arsenic is present as As (III), and all chlorine as Cl (V), the maximum relative effects on the KBrO 3 assay (mass fraction, %) of these two impurities are no greater than 0.001 % and 0.005 %, respectively, which is insignificant compared to the final expanded uncertainty (0.20 %) of the KBrO 3 assay (mass fraction, %). The arsenic impurity is probably present as As (V), since As(III), if present, would be oxidized to As (V) by the bromate matrix. However, to estimate the worst possible effect, arsenic (determined by GDMS) is assumed to be As (III). No correction or further consideration regarding the GDMS analysis is given.

Results and Discussion
The recommended mass fraction value for KBrO 3 and its uncertainty are summarized in Table 1. There is a difference among the titration results of the three solutions. The recommended value represents the combined mean mass fractions of the KBrO 3 in solutions 1, 2, and 3. The uncertainty assigned to the recommended value is calculated by combining the uncertainties of the measurements of KBrO 3 in the three solutions [15]. The resulting expanded uncertainty makes use of both within and between estimates of uncertainty. The within measurement uncertainty is calculated according to Eq. (5).
where u within = within measurement uncertainty u 1 = combined uncertainty (u c ) of solution 1 u 2 = combined uncertainty (u c ) of solution 2 u 3 = combined uncertainty (u c ) of solution 3. The between measurement uncertainty component is determined according to Eq. (6).
where u between = between measurement uncertainty |range| = absolute value of the difference between the maximum mean value for a solution (2) and the minimum mean value for a solution (3). The expanded uncertainty is found according to Eq. (7) using a coverage factor of 2 [15].   Table 2. Uncertainties were determined using the ISO Guidelines [16]. The individual components of uncertainty (Type A and Type B) are listed in Table 3 for solution 1. The u i represent the standard uncertainties associated with each of the uncertainty components, and the c i represent the associated sensitivity coefficients [17]. Since the Type B uncertainty components for each solution are similar, only the uncertainty components of solution 1 are listed in Table 3.
Comparisons of the individual uncertainty components are discussed later. Type A uncertainties are calculated from the standard deviations of the mean. Type A uncertainties represent the random variation in the following measurands: titration of KBrO 3 , titration of blanks, density, and the assay of As 2 O 3 [18]. The combined Type A uncertainty is calculated using the rootsum-of-square (RSS). The combined Type B uncertainty is calculated in a manner similar to that used to calculate the Type A uncertainty. The components of Type B uncertainty include the following: mass of As 2 O 3 , molecular weight of both As 2 O 3 and KBrO 3 , mass of concentrated KBrO 3 solution (titrant), volume of dilute KBrO 3 solution, dilution factor of the dilute titrant (KBrO 3 solution), mass of concentrated KBrO 3 solution, mass of KBrO 3 in a beaker, drift, and possible evaporation. It is calculated as the sum in quadrature of the uncertainty of the syringe before and after delivery of the titrant, and equals 141 µg. Because the actual mass value is most likely near the center of this range, the uncertainty distribution is best modeled as a triangular distribution. The standard uncertainty is then 58 µg The mass measurement uncertainty of As 2 O 3 is estimated to be 60 µg. Its uncertainty is calculated as the sum in quadrature of the uncertainty of each mass measurement (As 2 O 3 was weighed by difference) and equals 85 µg. The corresponding standard uncertainty, using a triangular distribution, is 35 µg To calculate the uncertainty of the volume of dilute KBrO 3 solution, the uncertainty in the accuracy of the buret and the uncertainty associated with the volume  [15]; k = 2.  . Assuming a uniformly probable distribution for buret error, this value is converted to a standard uncertainty by division by The volume of dilute KBrO 3 solution additions from the titrator was 0.006 mL for solutions 2 and 3, and 0.01 mL for solution 1. Uncertainties for volume increments were computed as standard errors for assumed underlying triangular distributions (0.006 mL / for solutions 2 and 3, and 0.01 mL / for solution 1). The standard uncertainty of the volume of dilute KBrO 3 was larger for solution 1 than for solutions 2 and 3.
ments. The mass of KBrO 3 salt was measured at the end of a drying study (about 50 h drying time). In Fig. 2, the loss of mass of the KBrO 3 salt on drying is plotted versus the drying time (h). The WB plot symbol identifies the weighing bottle for each sample and the ordinate identifies its corresponding mass loss. The four samples, taken from one bottle of KBrO 3 , were dried, and then used in the solution preparation for the samples to be titrated. Between 80 % and 90 % of the total mass loss is observed after 21 h. We have recommended a drying time of 24 h at 150°C for KBrO 3 , unless this mass loss becomes a significant uncertainty component. Thus, the uncertainty of the mass of KBrO 3 salt for each solution (solution 1, 2, and 3) is calculated to account for the difference between the mass loss at about 21 h of drying and the average mass loss at about 50 h. The uncertainty applies to the specific mass loss differences of a specific weighing bottle and the solution (solution 1, 2, and 3) that was prepared.
The uncertainty of the mass of the concentrated KBrO 3 solution (preparation of solutions 1, 2, and 3) with a 1 mg resolution balance is 0.002 g. Assuming a rectangular distribution for the error in weighing (0.002 / ) and considering that the mass of the concentrated KBrO 3 solution was determined from two mass measurements (multiplied by ) the standard uncertainty is 0.00163 g.  3.

6
The uncertainties in the molecular weight of both As 2 O 3 and KBrO 3 are calculated from the recommended uncertainties in the IUPAC assigned relative atomic masses [11] of the elements (As, O, K, Br) combined in quadrature. The corresponding standard uncertainty was calculated by dividing the IUPAC recommended uncertainty (99.7 % confidence interval) by 3. This estimation was based on interpretation by the NIST Statistical Engineering Division [19] of the language used in the IUPAC explanation [20].
The uncertainty of the mass of KBrO 3 salt used to prepare the concentrated KBrO 3 solution was calculated in a different way than the other mass measure- 3 2 The most significant sources of uncertainty are the following: measurement replication of the titration of KBrO 3 , mass of As 2 O 3 , volume of dilute KBrO 3 solution and, to a lesser extent, mass of KBrO 3 salt. Generally, the Type A uncertainty varied the most. The uncertainty associated with measurement replication of solution 3 was greater than the measurement replication uncertainties of solution 1 (Table 3) and solution 2 because the uncertainties of the mass fractions of the titrant (KBrO 3 ) and dilute titrant were greater for solution 3. The combined Type A uncertainty for solution 3 was 2.3 times greater than its combined Type B uncertainty. The uncertainty associated with measurement replication of solution 2 was the lowest. The combined Type B uncertainty for solution 2 was 2.0 times greater than its combined Type A uncertainty. Better measurement agreement across replications might have been obtained with solution 1 if the automated titrator had added dilute titrant in smaller increments (0.006 mL instead of 0.01 mL). The Type B uncertainties for all 3 solutions were similar. The uncertainty of the mass of As 2 O 3 is greater than the other mass measurements because of the small sample mass (0.1 g). The small mass is important to insure complete dissolution. However, the use of a microbalance with better than 10 µg resolution might improve this measurement. The uncertainty of the volume of dilute KBrO 3 might be decreased by smaller volume increments of the automated titrator, and/or a larger dilution factor of the dilute titrant.
Individual titration assay results for solutions 1, 2, and 3 are listed in Table 4.