1725
Anal. Chem. 1981, 53, 1725-1726
0.3 mm for the conventional ruler. As expected, precision was poorest for those peaks measured from the sloping base line with the highest standard deviation value being 0.8 mm for one set of primadone readings. This measurement precision was deemed satisfactory. The average bias between the two measurements was +0.2 mm. CONCLUSIONS The electronic ruler is easy to use, is mechanically reliable,
and has significantly reduced time spent in AED data reduction in our laboratory over the past 6 months. LITERATURE CITED ( I ) Soldln, S. J.; Hill, J. G. Ciin. Chem. 1976, 22, 856-859. (2) Soldin, S. J. Clin. Biochem. 1880 73, 99.
RECEIVED for review March 9,1981. Accepted April 4, 1981.
Reactions during the Titration of Ammonia with Hypoclhlorite in the Presence of Bromide Alan H. B. Wu' and Howard V. Malmstadt" School of Chemical Sciences, Department of Chemistry, University of Illinois, Urbana, Mnois 6 180 1
There have been many titration investigations for determining ammonia with hypochlorite titrant in the presence of bromide (I-7),and these have indicated that hypochlorite reacts with bromide to form hypobromite that reacts subsequently with ammonia according to eq 1 and 2. However,
Br2NH3
+ - + +
+ OC1-
+ 30Br-'
OBr- GI\Nz 3HzO 3Br-
(1) (2)
it has been found in the present investigation that reactions 1 and 2 occur only at pH values higher than those normally used for the titration. For example, by monitoring the titration in solutions buffered a t p€I 10.5, reactions 1 and 2 are shown to proceed as reported. However, by monitoring the titration a t the more typical pH 8,6 with a photodiode array rapidscanning spectrophotometric titrator @), whereby spectral regions can be monitored without titration interruption, it is apparent that bromine is the reactant with ammonia. Data are presented to show that in a solution buffered a t pH 8.6, hypochlorite reacts with bromide to form bromine that reacts with ammonia in the titration procedure, according to eq 3 and 4.
+ HzO + OCl60H- + 2NH3 + 31312' 2Br-
--
Br2 + C1- + 20H-
(3)
N2 + 6H2Q + 6Br-
(4)
Under a third set of conlditions, the titration of ammonia with hypochlorite and bromide in a lightly buffered solution at pH 8.6, a combination of reactions 1-4 are shown to occur. The reasons for this are discussed and are based on both spectrophotometric and pH data. EXPERIMENTAL SECTION Reagents. The 0.05 M hypochlorite solutions are prepared from commercial bleach (the Chlorox Co., Oakland, CA). Standard ammonia solutions are prepared from primary ammonium sulfate (Mallinckrodt Chemical Works, St. Louis, MO). The 5 M sodium bromide solutions are also prepared in the usual manner. Two borate buffer solutions are prepared at pH 8.6. One lightly buffered solution is prepared by dissolving 7.51 g of sodium tetraborate decahydrate (Mallinckrodt Chemicd Works) in 1 L of water, and another solution is heavily buffered by dissolving 38.1 g of borate in 1 L of water. Both are adjusted to pH 8.6 with perchloric acid, A third buffer is prepared at pH 10.5 by dissolving 6.6 g of borate in 1 L of water and adjusting the pH to 10.5 by adding sodium hydroxide. Present address: Clinical Chemistry Laboratory, Hartford Hospital, CT 06115. 0003-2700/81/O353-1725$0 1.25/0
Procedure. Titrants are standardized by using 0.10 N arsenic trioxide (Mallinckrodt). Titrations proceed using 10 mL of buffer, 1 mL of 5 Pvl sodium bromide, 4 mL of water, and 5 mL of standard ammonium sulfate solutions varying in concentrations from 7 to 50 MM. RESULTS AND DISCUSSION The titration of ammonia with hypochlorite in the presence of bromide zit pH 10.5 produces the curve shown in Figure 1. The reaction is monitored at 330 nm, the absorbance maximum for hypobromite. Figure 2 shows spectral scans from 240 to 3\50nm taken during various times of the titration of Figure 1 where these scans were taken. No traces of bromine, which has an absorption maximum of 267 nm, can be seen for this titration at pH 10.5. The apparent absorption before the end point of Figure 1 corresponds largely to the scattering of light by the microbubbles of nitrogen formed during the reaction shown in eq 2. In separate studies where the delivery of titrant is stopped at several times before the end point, it i3 noted that absorbance of the solution gradually returns to the base line as the last visible nitrogen escapes from solution. The titration of ammonia with hypochlorite in the presence of bromide that is heavily buffered at pH 8.6 produces the titration curve shown in Figure 3. The reaction is monitored at 267 nm, the absorbance maximum for bromine. Before the end point, the apparent absorbance is partly due to the scattering of light from nitrogen and partly to a trace of unreacted bromine produced during the continuous titration. Because of the high molar absorptivity of bromine in solution, trace amounte,can be spectrophotometrically detected. The presence of bromine was confirmed by the photodiode array titrator, where the absorbance curves are obtained at the various numbered points during the titration as shown in Figure 4. Curve 1 shows the presence of residual bromine before the end point. The presence of a large absorption band, shown in curve 2, is from excess bromine produced from reaction, eq 3, slightly after the end point, after all of the ammonia has bleen titrated. The high absorptivity of bromine produces the very sharp end point exhibited in Figure 3, distinctly different from the end point of Figure 1. The abrupt leveling off of absorbance is caused by an instrumental deviation from Beer's law. No traces of hypobromite can be seen in the curves of Figure 4. The titration of ammonia with hypochlorite in the presence of bromide in a solution lightly buffered at pH 8.6 produces the titration curve shown in Figure 5A. Before the end point, the reactions summarized in eq 3 and 4 occur as confirmed 0 1981 American Chemlcal Society
1726
ANALYTICAL CHEMISTRY, VOL. 53, NO. 11, SEPTEMBER 1981 1.51
7
I
0.5
1
2
1.5
1
Amount OCI- Added
(mL)
Flgure 1. Titration curve for ammonia with hypochlorite and bromide monitored at 330 nm at pH 10.5.
g 1.5
0.5
2t
Amount OCI- Added (mL)
Flgure 5. (A) Titration curve from ammonla with hypochlorite and bromide monitored at 330 nm lightly buffered at pH 8.6. (B) pH determination during the titration of ammonia.
1
/
1
1
1
260
241
1
1
1
280
1
300
1
320
1
1
1
340
$ Q5
Wavelength (nm)
Flgure 2. UV spectrum at various points during the tltratlon of ammonia with hypochlorite and bromide at pH 10.5.
240
260
280
Waveiength (nm)
320
3bO
Flgure 6. UV spectrum at various points during the titration of ammonia with hypochlorite and bromide lightly buffered at pH 8.6.
2 t
v
300
0.5
15
1
Amount OCI- Added (mi)
Flgure 3. Titration curve for ammonia with hypochlorite and bromide monitored at 267 nm heavily buffered at pH 8.6.
3) wherein hypobromite is formed because the pH rises as excess titrant is added. The results by simultaneous pH measurements are shown in Figure 5B. Before the end point, the pH of lightly buffered reaction does not change, consistent with eq 3 and 4 where no net gain or loss of H+ is seen. Because the titration is continuous there is always a trace excess of unreacted bromine during the titration, which causes the initial pH rise. After the end point, there is an increase in pH caused by the removal of H+ according to eq 3 as excess titrant is added. After the pH rises sufficiently there is only the production of hypobromite according to eq 1 so the pH remains constant. The absorption curve 3 of Figure 6 shows the production of OBr- after the titrant has been added in large excess. LITERATURE CITED (1) Willard. H. H.: Cake. W. E. J. Am. Chem. SOC. 1920, 42, 2646-2650. . I. M.; Stenger, V. A. Ind. fng. Chem., Anal. Ed. 1935, 7 , (2) Kolthoff, XLRI >
I
.., "..
240
260
280
300
320
30
Waveiength (nm)
Flgure 4. UV spectrum at various points during the titration of ammonia with hypochlorite and bromide heavily buffered at pH 8.6.
by the similarity of the titration curves of Figures 3 and 5A and from the spectral scans of Figure 6, which show the presence of trace bromine during the continuous titration. Immediately after the end point, the reaction summarized by eq 3 occurs (curve 2) followed by the reaction of eq 1 (curve
(3) Arcand, 0. M.; SwlR, E. H. Anal. Chem. 1856, 28, 440-443. (4) Kolthoff, I. M.; Stricks, W.; Morren, L. Analyst (London) 1853, 78, 405-409. ( 5 ) Laitinen, H. A.; Woerner, D. E. Anal. Chem. 1855, 27, 214-217. (6) Krlvls. A. F.: SUDD, . . G. R.: Gazda, E. S . Anal. Chem. 1963, 35, 2216-22 17. (7) Christian, G. D.; Knoblock, E. C.; Purdy, W. C. Anal. Chem. 1863, 35, 2217-2219. 18) Wu. A. H. 6.: Rotunno. T.: Malrnstadt, H. V. Am. Lab. (FalrfW, coin.), in press. .
I
.,
RECEIVED for review December 19, 1980. Accepted June 8, 1981. This project was supported in part by a research grant from NIH, GM21984.
