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ANALYTICAL CHEMISTRY, VOL. 51, NO. 7, JUNE 1979
approximately 1 day of exposure. The location of the peaks in the initial spectrum corresponds closely to the literature data ( 4 ) : eight peaks between 300 and 500 nm for the eight conjugated polyenes -(CH=CH),- from n = 4 to 12. The results are summarized in Table 111.
DISCUSSION Our method is essentially based on two assumptions: (1)The absorbance decrease during photooxidation is due to the disappearance of preexisting chromophores. This assumption is verified in the case of concentrations as low as 1 ppm (polyene unit per monomer unit)-the change in chemical structure is then too small to induce a significant change in sample morphology (and therefore in light diffusion) or in refractive index (and therefore in reflection losses). Only oxidizable pollutants, such as polyaromatics, can interfere in the dosage, but they can easily be detected by their strong absorption band in the medium UV or by fluorescence. Stable impurities such as colorant or pigment traces do not interfere, giving a relatively major advantage to the Raman resonance method. (2) The photooxidation is selective, i.e., the destruction of the conjugated sequences is favored relative to the HC1 zip photoelimination. As previously shown, this latter process requires a radiation of wavelength shorter than 330-340 nm ( 5 , 6). In our case, due to the active part of the lamp spectrum being filtered off by a glass plate, new conjugated sequences cannot appear a t a significant rate, so that the asymptotic value of the absorbance corresponds effectively to the total disappearance of the polyenes in the sample, and can therefore be used for their quantitative determination. The relative advantage of this method to the direct determination in films using their extinction coefficient lies essentially in the fact t h a t physical factors such as light diffusion, which depend strongly on the film morphology, and which are difficult to appreciate quantitatively, are not taken into account in the
measurement by selective photooxidation. For instance, in the tetraene dosage in two suspension polymers, the difference between the concentration values obtained from the film extinction coefficient and from the photooxidation experiment, i.e., 46 f 1 pmol/L for the two samples, can be attributed to the light diffusion by the films. The overestimation of the concentration caused by the use of the extinction coefficient is therefore very important (100 to 200% in our case); thus, the film turbidity, although low, cannot be neglected a t least in the determination of small polyene concentrations (lower than mol/L). With a routine spectrophotometer, it is possible to easily determine tetraene concentrations of mol/L, corresponding to approximately 4 units per IO7 monomer units, i.e., less than 1 unit per 2000 chains for the majority of the commercial polymers. The method is limited to tetraenes or larger polyenes. For the dienes and trienes, the accumulation of stable oxidation products such as a,P-unsaturated carbonyls or dienones, resulting from the oxidation of larger polyenes, can compensate a t least partially for the absorbance decrease due to the short sequence destruction. This is clearly shown in Figure 3 in which the absorption increases between 200 and 250 nm (dienes), whereas it decreases in the other parts of the spectrum.
LITERATURE CITED (1) R . Schlimper, flaste Kautsch., 13, 196 (1966). (2) G. Pritscher and W. Holtrup, Angew. Makroml. Chem., 47, 111 (1975).
(3) C. Bassez, Thesis, Lille, France, 1978. (4) F. Sondheimer, D. A. Ben Efrain, and R. Wolovsky, J . Am. Chem. Soc., 83, 1675 (1961). (5) M. A. Golub and J. A. Parker, Makromol. Chem., 85, 6 (1965). (6) R. F. Reinisch. H. R. Gloria, and G. M. Androes, in "Photochemistry of Macromolecules", R. F. Reinisch, Ed., Plenum Press, New York, 1970, p 185.
RECEIVED for review October 30, 1978. Accepted February 12, 1979.
Ozone Interference in the Determination of Nitrogen Dioxide by a Modified Manual Saltzman Method Eduard H. Adema' Central Laboratory, DSM, Geleen, The Netherlands
To determine whether ozone interference occurs in a modified manual Saltzman nitrogen dioxide procedure, several experiments were performed using dynamic mixtures of nitrogen dioxide and ozone. Two types of sample bottle were used in the examination. I n one of them no ozone interference was observed. Apparently the ozone was destroyed before it reached the Saitrman reagent. With the sample bottle of the second type, a negative interference was observed caused by the reaction of NO2and O3in ail probabillty in the aqueous solution of the reagent. However, a positive ozone interference could be observed if the Saltzman reagent contained impurities. No reaction of ozone with the Saitzman reagent or the azo dye agent occurred. The negative interference dependent on the ratio of ozone to nitrogen oxide as found by Baumgardner could not be confirmed. Present address, Agricultural University, Department of Environmental Sciences, Section Air Pollution, Wageningen, T h e Netherlands. 0003-2700/79/0351-1002$01.00/0
A method of manually determining nitrogen dioxide in ambient air was described in 1954 by Saltzman (1). The method consists of absorption of NOz by a solution of sulfanilic acid, N-(1-naphthy1)ethylenediamine dihydrochloride, and acetic acid. The solution forms a stable color which shows an absorbance maximum at 550 nm. According to Saltzman, ozone caused an orange to yellow-brown tint which can be regarded as a positive interference. With 30 pL of ozone, the coupling reagent was completely destroyed. It was also found that specially prepared manganese dioxide decomposes the ozone but at the same time accelerates the oxidation of the available nitrogen dioxide by the ozone. Another investigation on the ozone interference was carried out by Baumgardner et al. ( 2 ) ,who reported a negative interference, the magnitude of which was dependent on the ratio of ozone to nitrogen dioxide. In their study they made use of a Bendix monitor, working on the principle of chemiluminescence, and of a continuously operating Technicon instrument using a modified Saltzman procedure. The decrease of the response in the 1979 American Chemical Society
m C
h
1003
d
4
I
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 7, JUNE 1979
1 ;1
Id 5
7
?_ _
-gluss joint
h
I
nitrogen
I
Ill
rIQt
e
+
N 3 2t
air
f
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Schematic drawing of t h e mixing manifold: (a) rotameter, (b) mixing flask, (c) sampling point for NO,, (d) sampling point for ozone, Figure 1.
(e) sampling point for NOp in the ozone-NO, mixture, (f) permeation tube at constant temperature, (9) ozone generator, ( h ) needle valve Bendix monitor was approximately equivalent to the calculated NO3-concentration, while the decrease observed when using the Technicon instrument was considerably greater than could be accounted for by NO3. formation. The calculation of the NO3. concentration was based on Reaction 1 between NOz and O3 in the gas phase during the residence in the sampling system. T h e subject of the present report is a study of ozone interference in the manual Saltzman procedure modified by Lahmann e t al. ( 3 ) ,with two different types of absorption bottle.
EXPERIMENTAL A diagram of the mixing manifold is given in Figure 1. Dry and pure nitrogen is passed at constant temperature through a chamber containing a permeation tube releasing 200 yg/h NOz. Part of the stream can be split off and diluted with pure nitrogen. Purified air is led through a pen-ray tube producing ozone at the rate of 250 pg/h. Part of this stream also can be split off and diluted with pure nitrogen. In this way, the NOz and ozone concentrations can be varied over a range of Ck4000 yg/m3. Since, during the experiments, the gas flows containing NO2 and ozone are maintained in the order of 50 L/h, a Tee connection will suffice for adequate mixing. From the mixing point, A, the gas mixture is led through a 2-m long PTFE tube to point B, where it can be used or analyzed. The main parts of the manifold are made of glass, but where necessary PTFE connections are employed. The rotameters are used only for indication, the flows being controlled by valves and measured with soap bubble flow meters. The real component concentrations in the gas mixtures are determined by the wet colorimetric methods described below. In accordance with the draft standard for ISO/ 146, describing the determination of nitrogen dioxide in ambient air, a modified Saltzman reagent was used consisting of 4.0 g of sulfanilamide (Merck, Darmstadt), 10.0 g of tartaric acid (Merck, Darmstadt), 0.100 g of disodium ethylenediaminetetraacetate (Siegfried,Zofing, Switzerland), 0.100 g of N-(1-naphthy1)ethylenediammonium dihydrochloride (BDH, Pool, England), and 10 mL of acetone per L (3). The absorbance has to be measured at 540 nm. Calibration is performed with a standard sodium nitrite solution according to the above-mentioned standard, For the determination of ozone, use is made of a reagent containing 12.5 mg of sodium indigotin disulfonate (IDS) (Fluka, Buchs, Switzerland), 14.2 g of Na2HP04 (BDH), and 13.6 g of KH2PO4 (Baker) per L (pH 6.85). This blue-colored solution, showing maximum absorbance at 610 nm, was calibrated by the
porosity 2
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1
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id25
-
Absorption bottle for the determination of ozone by the IDS method (dimensions in mm)
Figure 2.
(dimensions in rnm)
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Figure 3. Absorption bottle for the determination of NO2 by the manual Saltzman method (dimensions in mm)
KI method ( 4 ) . In the analysis two types of sample bottle are used. Sample bottle I (Figure 2) is used to determine ozone and
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 7, JUNE 1979
'ooo
500
1500
Figure 5. Effect of ozone on the Saltzrnan reagent before and after the introduction of NO The increase of the absorbance of 540 nrn is expressed in h g / m 3 NO2
.
J
TrOP5mlttonCa
.15
Figure 4. Increase in analytical response in the Saltzman method as a function of the NO2 and ozone concentrations
Table I. Interference of Ozone in the Modified Saltzman NO, Method (Sample Bottle of Figure 2)d ~ g / m ~ calcd decrease b c
app increase of NO, concn
initial concn NO, 0,
obsd NO,
I.r 9 /
230 205 228 232 233 506 511 509 513 997 1093
254 253 247 246 244 539 554 534 511 1017 1109 1119 1009
+24 +48 +19 +14 +11 +33 +43 +25 -2 +20 +16 +11 -87
1108 1096
220 405 614 785 1565 220 385 802 1612 214 405 859 1565
m3
%
a 6
5
8
10
51 56 59 0 10 45 72 10 40 +1.0 6 0 -7.9 157
13 17 34 10
+10.4 +23.4 +8.3 +6.0 ~ 4 . 7 +6.5 +8.4 +4.9 -0.4 +2.0 +1.5
18 38 77 20 41 88 159
0.1 0.1 0.2 0.2 0.5 0.1 0.2 0.5 1.0 0.3 0.6 1.2 2.2
Decrease of the NO, concentration corrected for the absorbance due to reaction of ozone with the Saltzman reagent. calculated with the value k , = 1 . 2 x l o 9 cm3, mo1-l.s-I. Calculated with the value k 3 = 1.5 x 10' ~ m ~ ~ r n o l(Ref. - ~ ~5s) -. ~ The values of the NO, concentration are mean values out of 2-5 observations (concentration in pg/m3, 22-25 "C). a
the influence of the ozone on the Saltzman reagent. Sample bottle I1 (Figure 3) specially designed by Dr. H. de Graaf of the Food Inspection Service, Rotterdam, The Netherlands, is used to determine NO2. During use of this bottle, the Saltzman reagent is forced through the frit into the inner tube, whereas in bottle I the reagent remains in the outer tube. With respect to the determination of ozone the bottles are essentially different (see Results and Discussion). The colorimetric measurements were performed with a Vitratron colorimeter. The spectra were obtained by means of a Beckman ACTA M IV spectrophotometer. In all experiments 25 mL of Saltzman reagent was used, the sampling time was 20 min, and the sample intake rate was 20 L/h.
RESULTS AND DISCUSSION The nitrogen dioxide and ozone concentrations in the PTFE tube (after mixing of the components) can be calculated from the concentrations observed a t points c and d (see Figure 1) and the NOz and ozone flow rates. Whether and to what degree ozone will interfere with the nitrogen dioxide determination by the Saltzman method depends largely on the design of the sample bottle used. With the bottle of Figure 2 the results shown in Table I and Figure 4 were obtained.
500
400
600
7oc
Flgure 6. Absorption spectra of some compounds of the Saltzrnan reagent treated with ozone and of the Saltzman reagent treated with ozone and nitrogen dioxide: (1) Saltzman reagent treated with nRrogen N-(1-naphthy1)ethylenediimine dioxide and ozone, (2) sulfanilamide hydrochloride, (3) sulfanilamide, (4) N-(1-naphthy1)ethylenediarnine hydrochloride, (5) acetone N-(l-naphthyl)ethylenediarninehydrochloride
+
+
In all experiments the same bottle was used. Especially a t low ozone concentrations a strong relative increase of the absorbance was observed. This increase has been expressed as a percentage of the initial NO2 concentration. At higher concentrations the increase becomes smaller and eventually even a n absorbance decrease is noticed. The effect of ozone on the Saltzman reagent in the absence of NO2 is shown in Figure 5. I t is expressed here in terms of pseudo NOz concentrations, which can be calculated from the increase of the absorbance a t 540 nm. T h e same results were obtained by the use of a Saltzman reagent pretreated with NOPand consequently containing the so-called azo dye agent (see Figure 5). In both cases the absorbance increases up to an ozone concentration of about 500 pg/m3. If the concentration rises beyond this point, the effect remains constant. In both series of experiments t h e same bottle, of the type shown in Figure 2, was employed. Before the photometric determinations were performed, the reagents were left to stand for 1 h, after which time the absorbance of the solution did not change any more. T o gain further information, the solutions obtained in the above-mentioned experiments were examined spectrophotometrically. Curve 1 in Figure 6 represents the spectrum of the azo dye agent formed upon contact of the Saltzman reagent with nitrogen dioxide. A maximum was found at 545 nm. Curve 2 in Figure 6 relates to Saltzman reagent treated with ozone. The spectrum here shows a maximum at 470 nm; it is quite different from the other one and cannot be obtained
ANALYTICAL CHEMISTRY, VOL. 51, NO. 7, JUNE 1979
Table 11. Interference of Ozone in a Sample Bottle of the Type Shown in Figure 3 [NO,] with no. re1 increase [O,], of [ N O , ] , 0, found pg/m3 expts of response, % pg/m3 pg/m3 23 1 235 460 458 591 595 581 595 594 596 1112 1120 2006 1950
233 239 452 459 600 592 582 589 599-493 619-505 1098 1110 1998 1964
1507 1466 1492 1445 1613 1634 3110 3010 4865 4864 1433 1499 1447 1445
4 4 4 3 4 4 4 4 9 9 4 4 2 4
+1.0 +2.0 -1.7 +0.2 +1.5 -0.5 +0.1 -1.0 +0.8 t o - 1 7 + 3 . 9 to -15 -1.3 -0.9 -0.4 +0.7
by treating just one of the Saltzman reagent components, which were examined one by one. A few examples are given in Figure 6. For instance, when sulfanilamine or N - ( l naphthy1)ethylenediaminedihydrochloride was treated with ozone, yellow solutions were obtained which produced the spectra represented by curves 3, 4, and 5. However, after treatment with ozone of the combination of these two substances in the concentrations in which they occur in the Saltzman reagent, the spectrum represented by curve 2 was found, which is the same as that of the total Saltzman reagent treated with ozone. T h e picture of ozone interference is completely different when a n absorption bottle of the design shown in Figure 3 is used. As can be seen from Table 11, there is no interference of ozone in concentrations up to 3000 pg/m3 with the determination of nitrogen dioxide at several concentration levels. The observed deviations are within the error of the analytical procedure. However, with about 1 wg/rn3 ozone an unreproducible interference of up to -17% was found. T h e proposed Netherlands Standard for the manual determination of NO2in ambient air by means of the Saltzman reagent specifies the use of an absorption bottle as shown in Figure 3. Employing this type of bottle, and using the blue IDS solution for the determination of ozone, it was found that all the ozone was destroyed before it could reach the solution if the ozone concentration was 400 pg/m3, and that 75-100% of the ozone was destroyed if it occurred in a concentration of 3000 pg/m3. The decomposition of the ozone clearly takes place a t the wet surface of the glass of the bottle. However, if an absorption bottle as shown in Figure 2 is used, ozone can very well be determined by the IDS method. This method will give a perfectly linear calibration curve. In our first set of experiments, we found the Saltzman reagent to form a slightly colored solution in contact with ozone. With several ozone concentrations (see Table 11) a curve as shown in Figure 5 was obtained; this may be regarded as a colorimetric titration curve. The equivalence point is a t 0.6% with respect to NEDA and at 0.01% with respect to sulfanilamide. I n later experiments it became evident that coloration does not always appear; it depends on the purity of the chemicals used. Since the combination of these two Saltzman reagent components is necessary t o obtain the colored product upon contact with ozone, it must be concluded t h a t this product had been formed from an impurity in the NEDA with sulfanilamide or from an impurity in the sulfanilamide with NEDA. In accordance with the fact that ozone is destroyed in t h e sample bottle of Figure 3, no colored solution formed in this bottle. T h e stability of t h e azo dye agent in contact with ozone, already concluded from the experiments represented in Figure
1005
Table 111. Influence of Ozone on the Azo Dye Agent in the Saltzman Solution (Sample Bottle of Figure 2 ) run
absorb. of azo s o h
absorb. after treatment with 0,
concn, ~ d m ’
1 2 3 4
0.399 0.334 0.340 0.338
0.399 0.344 0.340 0.338
482 482 3445 3445
0,
Table IV. Determination of NO, Using Ozone-Treated Saltzman Reagent
run
increase in absorb.
1 2 3 4 5 6 7 8
0.327 0.347 0.348 0.348 0.345 0.312 0,316 0.323
response,
pg/m3 NO,
1244 1242 1233 1248 1237 1108 1120 1154
0
3
concn, a/m3 0.0 0.0 0.0
366 366 3400 3400 3400
5 , was once more demonstrated by another set of experiments, summarized in Table 111. T h e Saltzman reagent used here did not show coloration when contacted with ozone. A bottle of the type shown in Figure 2 was used. In these experiments the typical spectrum of the azo dye agent did not show a change (see Figure 6, curve 1). T h e applicability in NO2 determinations of Saltzman solutions contacted with ozone will not decrease, provided the ozone concentration is not too high. As can be seen from Table IV, the NOz response had decreased by about 10% when a gas with an ozone concentration of 3400 pg/m3 had been passed through the Saltzman reagent for 20 min, a t the rate of 20 L/h. I t might be suggested that the presence of nitrogen dioxide in the ozone as a contaminant could be responsible for the rise in absorbance which is sometimes noticed. However this cannot be so, for several reasons. In the first place, as the ozone has no influence on the azo dye compound in the Saltzman reagent, the increase of the absorbance in the experiment represented in Figure 5 must be proportional to the ozone concentration; secondly, there is a distinct difference between the spectra obtained with Saltzman reagent contacted with ozone and with this reagent contacted with nitrogen dioxide (see Figure 6, curves 2 and 1). T h e measurements referred to by Figure 4 and Table I, which were performed with the sample bottle of Figure 2, showed positive as well as negative ozone interferences, depending on the ozone and NO2 concentrations. T h e interference in this case is due to a t least two effects: (i) the formation of an unknown compound by ozone and a contaminant of the Saltzman reagent, which produces the spectrum shown in Figure 6 (curve 2), and (ii) oxidation of NO2. T h e latter effect can take place in the gas phase, on active surfaces (glass wall or frit of the sample bottle), or in the Saltzman reagent. To determine where the oxidation of NO2 takes place, the measurements were corrected for the effect mentioned under i above. For this purpose the values from Figure 5 were used. T h e corrected results were summarized in Table I (under column a, indicating only the effect of the decomposition of NO2). The decomposition of NOz by ozone can be represented by the following equations:
--
NO2 + 0 3 N03. + 0 2 (1) NOz + NO3- N,05 (2) it being assumed that the formation of NO3. will be followed by a fast reaction in which a second NOz molecule is involved.
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 7, JUNE 1979
T h e rate equation for this mechanism is:
(3) Using the most reliable value from the literature ( 5 ) ,hl = 1.5 X lo7 ~ m ~ . m o l - ~f - s8-% ~ a t 25 "C, and a residence time of 2 s in the sample tube, the decrease of the NO, concentration due to the gas phase Reactions 1 and 2 can be calculated for several runs. T h e results are compiled in Table I under column c. As can be seen from these calculations, the corrected interference effect under a cannot be explained with the simple gas-phase Reactions 1 and 2. With the use of a much higher value, h,' = 1.2 X lo9 cm3-mol-'.s-', the calculated decreases of the NOz concentration should come much closer to the corrected deviations. (See Table I under column 6). Therefore, it is suggested that Reactions 1 and 2 either are catalyzed by the glass wall or the frit of the bottle (and are therefore dependent on the type of bottle used), or take place in the reagent. From Table V it can be seen that no ozone interference is observed with the bottle of Figure 3. This actually agrees with the facts that with a bottle of this type no ozone can be measured by the IDS method and that in this bottle filled with
a few milliliters of water the ozone is decomposed in concentrations up to 3000 kg/m3. Consequently, it is probable that the ozone interference takes place in the Saltzman reagent if the bottle of Figure 2 is used. On the strength of these observations, we recommend the use of an absorption bottle as shown in Figure 3 for determining NOz by the Saltzman method in order to avoid ozone interference. ACKNOWLEDGMENT The author is indebted to Ir. C. Huygen of the Research Institute for Public Health Engineering T N O and to Th. IJpelaan of Hoogovens IJmuiden B.V. for valuable suggestions and discussions. LITERATURE CITED (1) 6.E. Saltzman, Anal. Chem., 28, 1949 (1954) (2) R. E. Baumgardner, T. A. Clark, J. A. Hcdgeson, and R. K. Stevens, Anal. Chem., 47, 515 (1975). (3) E. Lahmann, B. Seifert, H. van de Wiel, C. Huygen. R. W. Lanting, H. Hartkamp, and H. Gies, Atm. Environ., 10, 835 (1976). (4) Y . Dorta-Schaeppy and W. Tredwell, Helv. Chim. Acta, 32,356 (1949). (5) D. D. Davis, J. Prusazcyk, M. Dwyer, and P. Kim, J. Phys. Chem., 78, 1775 (1974).
RECEIVED for review June 12, 1978. Accepted February 6, 1979.
Photometric Ion-Pair Titrations in the Presence of an Immiscible Solvent and Their Application to Drug Analysis Hussain Y. Mohammed and Frederick F. Cantwell" Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
A recently described twophase photometric titration apparatus is used to continuously monitor the absorbance of the aqueous phase in a vigorously agitated chloroform-aqueous buffer mixture in which the cationic conjugate acids of amine drugs are titrated with picrate. Theoretical titration equations are derived and verified experimentally. The dependence of titration curve shape on the values of several relevant equilibrium constants and on pH is discussed. Accuracy and precision of this two-phase, ion-pair photometric titration are several parts per thousand at sample concentrations of 5 X 10-4 M.
Photometric titrations performed in an aqueous solution in the presence of an immiscible organic solvent have recently been described for acid-base titration reactions ( I ) . Another class of chemical reaction which has been used for two-phase photometric titrations is complex formation. For example, Galik has titrated metal ions with ligands such as dithizone. The titration is performed in an aqueous carbon tetrachloride medium and the resulting neutral metal-ligand complex extracts into the organic phase ( 2 ) . After the addition of each increment of titrant and agitation, the bulk phases are allowed to separate and the absorbance of the organic layer is measured. Titration equations have been derived for these titrations ( 3 ) and the subject has been reviewed ( 4 ) . T h e formation of extractable ion pairs is a type of complex-forming reaction which has found extensive use in the analysis of ionic surfactants by two-phase titration. High 0003-2700/79/0351-1006$01.00/0
molecular weight quaternary ammonium cations are used to titrate anionic surfactants in water in the presence of an immiscible organic solvent. The resulting ion pair is extracted into the organic phase. In the reverse process, a high molecular weight anionic surfactant is used as titrant for a cationic surfactant analyte (5-7). In these titrations, a cationic or anionic dye which has been added to the solution transfers from one phase into the other in the presence of a slight excess of titrant, making possible visual detection of the end point. Anionic surfactants have also been used to titrate the ammonium form of amine drugs in a two-phase system. Here again the resultant ion pair extracts from the aqueous into the organic solvent phase and a cationic dye undergoes a color change a t the end point when it forms an extractable ion pair with the first excess of titrant (8). In a related approach, anionic dyes such as bromthymol blue have been used as titrants for the ammonium form of drugs (9). The drug-dye ion pair extracts into the organic solvent phase and the end point is indicated by the appearance of the color of the first excess of titrant in the aqueous phase. Behrends ( I O ) has derived the photometric titration equation for a two-phase ion pair titration of a non-absorbing ion such as tetraphenylarsonium with an absorbing titrant such as permanganate, in which the absorbance of one of the phases is monitored. In the derivation, the titrant and analyte ions were assumed to undergo no side reactions which would change their charge. He has also derived the titration equation for the case in which neither titrant nor analyte absorbs, but an absorbing indicator ion is added which forms an extractable ion pair with the titrant. Several examples of the latter type C 1979 American Chemical Society
