Spectrophotometric determination of water by flow injection analysis

A general evaluation of the flow Injection method for the de- termination of water has been carried out using a large num- ber of organic solvents wit...
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Anal. Chem. 1085, 57, 2571-2575

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Spectrophotometric Determination of Water by Flow Injection Analysis Using Conventional and Pyridine-Free Two-Component Karl Fischer Reagents I n g r i d Nordin-Andersson* a n d Anders Cedergren

Department of Analytical Chemistry, University of Umea, S-901 87 Umea, Sweden

A general evaluation of the flow injection method for the determinatlon of water has been carried out using a large number of organic solvents with viscosities and refractive indexes in the ranges (0.3-2.3) x lo-' g cm-' s-' (20 "C) and 1.32-1.50 (20 "C), respectively. The mean deviation between FIA and coulometric values was 0.0014% (v/v) H,O. Triethylamine gave rise to increased Interference effect, whlle less influence from ketal forming samples was found for the two-component reagents. The latter effect was not noticed for Hydranai.

The Karl Fischer titration is the most widespread and most universally applicable titrimetric method for determining water content. This fact is underlined by its incorporation in the most important pharmacopoeias and by its adoption as an ASTM method. The original Karl Fischer reagent was prepared as a methanolic solution containing iodine, sulfur dioxide, and pyridine. The reaction rate with water is first order with respect to iodineltriiodide, to sulfur dioxide in the form of methyl sulfite (1,2),and to water (1-3).The rate for the reaction involving iodine is about loo00 times higher than that for triiodide. Since iodide is a reaction product, the overall reaction rate will decrease during a titration since the ratio of iodine to triiodide concentrations will be decreased with higher concentrations of iodide due to complexation (1, 2). As a consequence of this the overall reaction rate can differ by a factor of at least 10 between the initial and end periods of a conventional titration (1,2). Therefore, side reactions consuming iodine are favored in comparison to the main reaction at the end of the titration. I n o r d e r to be able to minimize side reactions, we have suggested the use of flow injection analysis (4-6) because by this technique the reaction time can be kept very short and reproducible. A similar approach was suggested by Koupparis et al. (7) who determined water using an automated stopped-flow analyzer with pyridine-free two-component Karl Fischer reagents. Good agreement was obtained with conventional titrations for a large number of solvents tested. One drawback with the method, however, was the relatively low sensitivity, 0.5 mg/mL. In several laboratories the conventional Karl Fischer method has nowadays been replaced by titration systems based on coulometry. By using the coulometric technique, much smaller amounts of water (micrograms or lower) can be determined, which means that only very small sample volumes are required. In this way, the concentration of interferents can be kept low and consequently the effects of side reactions can be minimized. One drawback with the coulometric method, however, which has not received much attention is the possibility of errors arising from electroactive compounds in the sample. The use of pyridine-free reagents was suggested by Verhoef (3)who showed that the role of pyridine in the Karl Fischer reaction is only that of a pH buffer. Several investigators have therefore proposed the replacement of pyridine by other and 0003-2700/85/0357-257 1$01.50/0

more harmless buffer substances like sodium acetate (1,8), diethanolamine (9,lo),imidazole (11),or urea (12).These bases will buffer the reaction medium at a pH value that ensures a convenient overall reaction rate and a stable end point. A number of two-component pyridine-free Karl Fischer reagents have been developed since the works of Verhoef et al. (12, 13). One of the components contains iodine in methanol and the other, called the solvent, consists of sulfur dioxide and a base dissolved in methanol. The main reason for splitting the reagent into two components is to obtain a long time stability since most of these new types of reagents decompose rather quickly. One of the main problems associated with the method based on flow injection analysis mentioned above, results from the rather large variation between the calibration curves for different types of samples (6). However, it was recently shown that this problem could be partly eliminated by using a specially constructed spectrophotometric detector (Beckman Model 160) in combination with peak area measurements. For example, the relative mean deviation for methanol, 2-propanol, isopropyl acetate, and propyl acetate samples containing 0.1% of water was as low as 2 70. The aim of the work presented in this paper was to make a more general evaluation of the flow injection method by extending the variety of sample types (organic solvents) to include a wider range of refractive indexes, viscosities, and densities as well as chemical properties. Since the flow injection analysis technique offers a convenient way to mix two-component reagents, we include three commonly used commercially available two-component reagents in this study. EXPERIMENTAL SECTION All organic liquids were of analytical grade. Samples. A 100-mL portion of each of the organic solvents listed in Table I was taken and used without any preparation. All solvents were from Merck except acetonitrile (Rutburn Chemicals, Ltd.) and diethyl ether (May and Baker, Ltd.). Standard Solutions. The standard solution of water in methanol and in isopropyl acetate were standardized coulometrically (14). Reagents. The conventional Karl Fischer reagent used for the coulometric titration contained 25.4 g of iodine (Riedel de Haen), 38.4 g of sulfur dioxide (Fluka), and 80 mL of pyridine (Merck), diluted to 1L with dried methanol (molecular sieves, 3 A, Union Carbide). Corresponding amounts of iodine and sulfur dioxide in the Karl Fischer reagent employed for FIA were 6 g and 50 g, respectively. Before use, the reagent was calibrated by amperometric titration. In the modified one-component Karl Fischer reagent for the coulometric determination of water in active carbonyl compounds, most of the methanol was replaced by formamide and pyridine. The composition was in accordance with that recommended by Bizot (15). Two-Component Pyridine-Free Karl Fischer Reagents. Merck. Titrant: The concentration of iodine in methanol was equivalent to 5 mg of water/mL. Solvent: an odorless base and sulfur dioxide dissolved in methanol. ReAquant (Baker Chemical B.V.). Titrant: The concentration of iodine in methanol corresponded to 3.5 mg of water/mL. Solvent: a methanolic solution which contained 0.25 M methyl 0 1985 American Chemlcal Society

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ANALYTICAL CHEMISTRY, VOL. 57, NO. 13, NOVEMBER 1985

Table I. Physical Properties of the Organic Solvents

"At

solvents

refractive index"

isopropyl acetate propyl acetate 2-propanol methanol ethanol benzene diethyl ether chloroform dichloromethane triethylamine acetonitrile acetic acid MIBK acetone cyclohexanone

1.375 1.384 1.375 1.324 1.360 1.5011 1.353 1.449 1.425 1.401 1.344 1.372 1.396 1.359 1.451

102 x viscosity, g cm-l s-l

102 x kinematic viscosity"

density,a g~rn-~

absorbanceb 0.000 -0.003 0.000 C

-0.001~ -0.006 0.002 -0.004 -0.004 -0.004 0.000 -0.002 -O.0Ol5 0.001

-0.003,

20 "C. bMeasuredat 546 nm with Unicam SP 1800 IJV spectrophotometer. Standard deviation = f0.0015. 'Reference.

'-w

R

Figure 1. The flow injection arrangements used for (a) one-component (R) and (b) two-component Karl Flscher reagent: S, solvent; T, titrant; P, pump; reactor, SBSR or coil (0.5 mm i.d. and coil diameter 15 mm); L = 200 cm; L = 25 cm with coil diameter 30 mm; L ,(d) = 200 cm with coil diameter 6 mm; L ,(a) = the same coil as in (a); Teflon tube i.d., (a) 0.5 mm, (b) 0.3 mm, (c) 1.0 mm, (d) 0.7 mm; D, spectrophotometer; W, waste. Details about the SBSR are glven in the text. Flow rates were 3.0 mL min-' with coil reactor and 2.3 mL min-' with SBSR.

sulfite and an acetate-acetic acid buffer with 0.26 M acetic acid. Hydranal (Riedel de Haen). Titrant: A methanolic iodine solution with a standardized titre of 5.00 0.02 mg water/mL. Solvent: diethanolamine and sulfur dioxide dissolved in methanol. Before use, each titrant solution was diluted about 5 times with dried methanol. Instrumentation. The flow injection manifolds used are outlined in Figure 1. The equipment comprised the following items: a four-channel peristaltic pump (Gilson, Minipuls 2) equipped with silicon rubber or Acid Flexible tubings (Elkay), a chromatography inlet slide valve for low pressure (Cheminert) with a volume of 10 pL, a coil reactor of Teflon with 0.5 mm i.d. or a single bead string reactor (SBSR), which consists of a Teflon tube of 0.8 mm i.d. filled with glass beads of 0.5 mm diameter, and a spectrophotometer (Beckman absorbance detector Model 166) supplied with a mercury lamp and a filter assembly for monitoring at 546 nm. The flow cell has a volume of 18.5 pL and a standard 10-mm optical path length. The cell has an internal configuration specially designed to minimize band spreading and refractive index sensitivity. To further minimize the latter effect, a lens is placed in front of the cell entrance (16). The integrators (HP 3388A) and C1-10 (LDC Milton Roy) were used for recording and evaluating the peaks. The performance of both integrators was generally similar, although the HP 3388A occasionally evaluated the peaks with an incorrect base line measurement. Consequently, results obtained with the (21-10 are usually reported. Teflon tubings of 0.5 mm i.d. were used for connection. To attain as uniform a mixture as possible between the titrant and the solvent, the experimental setup shown in Figure l b was used. The mixing point is situated on the draw side of the pump since the flow is pulse-free or nearly pulse-free at that side. To achieve as large and as stable a mixing ratio as possible, different combinations of Teflon tubes of various inner diameters but of the same length were tested. The largest mixing ratio was found by using Teflon tubes of 0.5 mm i.d. and 0.3 mm id. for the solvent and titrant flows, respectively. However, the mixing ratio ap-

*

.

-

peared to be very critically dependent on the performance of the pump tube. The reagent was then further mixed in the 200 cm long tightly coiled Teflon tube of 0.7 mm i.d. With this FIA arrangement,the silicon and Acid Flexible pump tubings only lasted for about 1 day. The advantage of Acid Flexible pump tubings as compared to silicon tubings is that they are less permeable to air. The consequence of this will be a lesser influence of air humidity on the concentration of water in the reagent. However, diffusion of water into the reagent stream does not constitute a problem providing that there is not pulsation in the flow.

RESULTS AND DISCUSSION In a previous paper (6),different combinations of flow injection arrangements and detector types were studied in order to find a system which minimized effects caused by differences in the physical properties of the samples. The following solvents/samples with a large variety of viscosities and refractive indexes were studied (see Table I): methanol, 2-propanol, isopropyl acetate, and propyl acetate. It was found that peak area evaluation gave much better results compared to peak height measurements. For peak area measurements only a slight improvement was obtained when using an SBSR reactor instead of a simple coil. Table I1 shows the results obtained with a conventional one-component reagent in combination with a simple SBSR or coil arrangement (see Figure l a ) using methanol as the solvent for the standard solutions. As can be seen, considering peak area results, a good overall agreement is obtained between the coulometric and flow injection analysis results, the best correlation between the methods being observed for the SBSR arrangement. With peak height measurements, the difference between the SBSR and the coil is more significant. These results are in very good agreement with those reported by us earlier (6) when methanol, 2-propanol, isopropyl acetate, and propyl acetate samples were studied. According to the literature (13, In, the large deviation seen for triethylamine can be explained by a chemical interference, as a consequence of the large pH change caused by the amine sample. I t was also considered appropriate to investigate whether isopropyl acetate was a better choice than methanol as the solvent for the standard solutions. The reason for this is that isopropyl acetate, with a similar viscosity to methanol, has a refractive index which better approximates most of the solvents than does methanol. The corresponding results with isopropyl acetate standard solutions are shown in Table 111. As can be seen, the overall mean deviations are somewhat larger in these cases. Only for peak height measurements in combination with a coil reactor is there an improvement using

ANALYTICAL CHEMISTRY, VOL. 57, NO. 13, NOVEMBER 1985

2573

I _ -

Table 11. Comparison of Coulometric Results with Those Obtained by FIA Using Different FIA Arrangements and 0.007 M Conventional Karl Fischer Reagentn % HzO (v/v)

-

. I I FIA

solvent 2-propanol diethyl ether benzene acetonitrile ethanol propyl acetate chloroform dichloromethane acetic acid isopropyl acetate triethylamine

SBSR

coulometric titration 0.0436 0.0233 0.0177 0.0090 0.0490 0.0545 0.0145 0.0030 0.072, 0.029" 0.0326

--

___-__

peak height coil

peak area coil

SBSR _

diffb 0.0382 0.0231 0.021~ 0.011, 0.0430 0.0542 0.0168 0,0046 0.064, 0.0297 0.0394

diffb

-0.0054 -0.0002 0.0038 0.0022 -0.006 -0.000~ 0.002~ 0.001~ -0.008 0.000, 0.0068

l

_

~

-

diffb

0.000, 0.001 O.0Ol8 0.0008 -0.002~ 0.002 0.001~ -0.001~ -0.0002 0.0019 0.0050

-0.002, 0.001, 0.0062 O.0Ol5 --0.0038

0.0411 0.0248 0.0239 0.0105 0.0457

0.001~

0.055, 0.01& 0.0037 0.0700 0.0293 0.0470

0.0038 0.00Oj --.0.002, 0.000~ 0.0144

0.0034 0.0023

0.0018 0.0014

0.0094 0.0067

0.0029 0.0026

mean deviation' mean deviation (I)d

0.0437 0.0244 0.0195 o.o0g8 0.0465 0.0565 0.0162 0.001, 0.0719 0.0309 0.0384

-0.0053 0.0062 0.0149 0.012 -0.005 0.0054 0.0043 0.0053 -0.0048 0.0043 0.0358

0.0383 0,0295 0.0326 0.0210 0.0441 0.0599 0.0188 0.0083 0.0673 0.0333 0.0684

i

diffb

"Solutions of water in methanol were used as FIA standards. Values obtained by coulometry, means of two determinations with a n = 5. bDifference between the results maximum difference of 0.0003% (v/v) H 2 0 FIA, standard deviation ~(0.0002-0.0007)% (v/v) H20; obtained by FIA and by coulometry. 'Calculated from the absolute value of each difference. dMean deviation (I), triethylamine excluded from the calculation. l_llll_llll

l__l_____

_ _ _ l _ p p l l l _ l l l l l

Table 111. Comparison of Coulometric Results with Those Obtained by FIA" 70

HzO (v/v) -_1__-__11__

_

_

~

~

~

l

l

l

l

l

_

~

FIA

peak height

peak area _ l l l

solvent 2-propanol diethyl ether benzene acetonitrile ethanol propyl acetate chloroform dichloromethane acetic acid methanol triethylamine mean deviationc mean deviation (I)d

SBSR

coulometric titration 0.0436 0.023s 0.0177 o.oo90 0.0490 0.0545 0.014, 0.0030 0.072, 0.0244 0.0326

coil

coil

SBSR

l l l l l _ l

diffb 0.037, 0.022, 0.0209 0.0108 0.0421 0.0531 0.0163 0.0043 0.0628 0.0234 0.0385

-0.0063

-o.ooo8 0.0032 O.0Ol8 -0.0069 -0.001~ 0.0018 0.001~ -0.0093 -0.001" 0.0059 0.0036 0.0034

diffb 0.0334 0.0249 0.0279 0.0167 0.0390 0.0542 0.0146 0.0046 0.061, 0.0196 0.0623

-o.0102 0.001~ o.0102 0.0077 -0.0100 -0.000~ 0.0001 0.001~ -0.0108 0.0048 0.0297 0.0079 0.0057

diffb 0.0415 0.023~ 0.0188 0.0097 0.0440 0.0534 0.0157 0.0020 0.0678 0.0232 0.0365

-0.002, 0.000, O.0Ol1 0.0007 -0.0050 0.001, O.0Ol2 0.0010 -0.0043 -O.0Ol2 0.0039 0.0020 0.0018

diffb 0.0408 0.0253 0.024, 0.011, 0.0453 0.0549 o.0191 0.0053 0.0683 0.024, 0.0465

-0.002, 0.002, 0.0067

0.0027 -0.003,) o.0oo4 0.004, 0.002, -o.oo38

o.ooo* 0.0139 0.0039 0.0029

"Experimental conditions were the same as those given in Table I1 except that solutions of water in isopropyl acetate were used as FIA standards. The same coulometric results and FIA standard deviations as those given in Table 11. bDifference between the results obtained by FIA and by coulometry. CCalculatedfrom the absolute value of each difference. dMean deviation (I),triethylamine excluded from the calculation. I

the isopropyl acetate standard solutions. However, It can be observed that with isopropyl acetate as the standard solvent, the largest deviations between the results obtained by coulometry and FIA were found for acetic acid, ethanol, and 2-propanol. These solvents have about the same refractive index as isopropyl acetate, but their viscosities are considerably higher (see Tables I and 111). The remaining sample solvents with viscosities similar to or somewhat lower than that for isopropyl acetate gave rise to smaller deviations despite the large differences in their refractive indexes. This indicates that differences in refractive indexes between sample and carrier streams affect the sample peak very little, especially with an SBSR on line. Of greater importance is the influence of the viscosity of the sample affecting peak height and the peak area values. The observation that methanol is a more appropriate standard solvent for samples with widely different viscosities may be due to a combination of the relatively low

refractive index of methanol and the apparently higher viscosity, which may result from hydrogen bonding interactions between the carrier stream and the methanol sample (18). One explanation to account for the fact that the so-called viscosity effect influences the magnitude of the peak area, especially when using an SBSR, could be that the registered dispersion of the sample zone is affected by a chemical reaction in addition to the pure physical action (18-20). Two-Component Reagents. Usually the developed twocomponent reagents are more basic than the conventional KF reagent. The higher the pH of the solvent, up to around pH 6, the more methyl sulfite is formed and therefore the less sulfur dioxide is left in the reagent. This is indicated by the color of the spent reagent, since the complex SOJ causes a yellow coloration (22);see Table IV. One advantage with the higher pH of the two-component solvents could be a suppression of water formation through the ketal reaction of

l

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ANALYTICAL CHEMISTRY, VOL. 57, NO. 13, NOVEMBER 1985

to achieve a smooth base line and reproducible results. As shown in Table V a greater spread in the results was obtained by using this FIA configuration. In Table VI, which includes the active carbonyl compounds and triethylamine, the results of coulometric determination of water in the ketones should be accurate, since almost all methanol was replaced in the modified Karl Fischer reagent used. Besides, corrections for small increases in the background current observed with these samples were also made. Obviously, an interfering reaction appears with the amine sample and this is more pronounced when the two-component reagents are employed, especially Hydranal. This is in line with the results published by Bos (22)who developed a more acidic reagent suitable for use with amine samples. The degree of kinetic discrimination between the main and side reaction occurring with the active carbonyl compounds depends of course on the rate of the side reaction. The rates of water formation (at zero time) in cyclohexanone, acetone, and methyl isobutyl ketone were evaluated in separate experiments and were found to be of the order of 0.05,0.03, and 0.002 mg of H 2 0 per minute per milliliter of solution and per percent (v/v) of the ketone in the titration volume, respectively. These values are in line with the results shown in Table VI. Under the conditions used, only the side reaction occurring in the MIBK was completely discriminated against, independent of the type of reagent used. The least interference effects for the remaining ketones were obtained using ReAquant. The large deviations, noticed for the Hydranal reagent, can be explained by formation of water through the

Table IV. Physical Properties of K a r l Fischer Reagents refractive indexn

reagent

pHb

color spent reagent

One Component conventional with pyridine

1.356

4.2-1.2

yellow

Two Component Pyridine Free Merck ReAquant Hydranal

1.364 1.349 1.350

5.0-2.1 5.6-2.6

pale yellow uncolored pale yellow

"At 20.5 "C. bPublished in ref 12. The low pH values correspond to spent reagents.

samples containing active carbonyl compounds. The reason for this is that ketals are usually only formed in an acidcatalyzed reaction. A disadvantage with these reagents is the higher decomposition rate which is moreover accelerated by amine samples (13,23),causing an incorrect determination of the water content in such samples. By use of the FIA arrangement shown in Figure lb, the determination of water in the various solvents included in this investigation and in the ketones methyl isobutyl ketone (MIBK), acetone, and cyclohexanone was performed and the results are given in Tables V and VI. In this FIA configuration an SBSR was not used because it caused an excessive pulsation in the flow. When a colored and uncolored stream is mixed, it is especially important to avoid pulsation in order

Table V. Comparison of Coulometric Results with Those Obtained by FIA, Using Conventional a n d Two-Component Karl Fischer Reagentsn % HzO (v/v)

conventional solvent 2-propanol diethyl ether benzene acetonitrile ethanol propyl acetate chloroform dichloromethane acetic acid isopropyl acetate

Merck

ReAquant

coulometry

FIAb

diff"

coulometry

FIA

diff

0.024 0.026 0.023 0.078 0.046 0.061 0.014 0.015 0.043 0.027

0.021 0.029 0.039 0.081 0.047 0.065 0.024 0.017 0.038 0.030

-0.003 0.003 0.016 0.003 0.001 0.004 0.010 0.003 -0.005 0.003

0.028 0.021 0.021 0.081 0.054 0.065 0.016 0.012 0.046 0.038

0.022 0.035 0.030 0.080 0.052 0.073 0.020 0.011 0.044 0.037

-0.006 0.014 0.009 -0.001 -0.002 0.008 0.004 -0.001 -0.002 -0.001

coulometry

FIA

diff

0.028 0.021 0.021 0.081 0.054 0.065

0.026 0.026 0.036 0.092 0.049 0.075

-0.002 0.005 0.015 0.011 -0.005 0.010

0.012 0.046 0.038

0.018 0.044 0.038

0.006 0.002 0.000

coulometry

FIA

diff

0.024 0.026 0.023 0.078 0.046 0.061 0.014 0.015 0.043 0.027

0.015 0.021 0.030 0.077 0.048 0.060 0.018 0.011 0.035 0.021

-0.009 -0.005 -0.007 -0.001 0.002 -0.001 0.004 -0.004 -0.008 -0.006

0.0062 1.2

0.0048 0.9

0.0051 0.6

mean deviationd baseline absorbance

Hydranal

0.0047 1.2

nCoulometry, std dev max f0.0007% (v/v) HzO, n = 3; FIA, std dev in % (v/v) H,O, n = 5; peak height max &0.0015; peak area max f0.003. bPeak area measurement with integrator H P 3388, methanol standards. 'Difference between the results obtained by FIA and by coulometry. Calculated from the absolute value of each difference. Table VI. Comparison of Coulometric Results with Those Obtained by FIA for Triethylamine a n d Some Active Carbonyl Compoundsa % HzO (v/v)

solvent MIBK acetone cyclohexanone triethylamine

conventional coulometryb FIAc diffd 0.027 0.053 0.041 0.025

0.029 0.068 0.086 0.037

0.002 0.015 0.045 0.012

Merck coulometryb 0.028 0.065 0.040 0.039

ReAquant

FIA"

diffd

coulometry*

0.028 0.077 0.070

0.000 0.012 0.030 0.060

0.028 0.065 0.040 0.039

0,099

Hydranal

FIAc

diffd

coulometry*

-0.036e 0.065 -0.055e -0.070e

-0.008 0.000 -0.015 -0.030

0.027 0.053 0.041 0.025

FIA'

diffd

0.021 >0.12'

-0.006 >0.07 >0.11

>O.ld g

nStandard deviations as those given in Table V. bKF reagent with methanol replaced by formamide. 'FIA: coil reactor, peak area measurement, methanol standards. dDifference between the results obtained by FIA and by coulometry. e Estimated value. The difference in refractive indexes between reagent and sample leads to an incorrect base line being taken by the integrator. fEstimated because the value was found outside the range of the calibration graph. gReagent almost decolored.

Anal. Chem. 1985, 57, 2575-2579

reaction between the amine component in Hydranal and the carbonyl compounds.

CONCLUSIONS The proposed FIA method is capable of reducing the influence of large variations in the physical properties of the samples on the results. However, care must be taken in the choice of the solvent for the standard solutions to keep the matrix effect low. A standard solvent with a viscosity as similar as possible to that of the sample solvents seems to be the most favorable. Although the method has not been optimized with respect to kinetic discrimination between the Karl Fischer reaction and interfering side reactions such as the ketal reaction, slower side reactions have no influence on the results. Besides, the FIA technique offers some general advantages when compared to conventional titrations, for instance, high sampling rate, over 250 samples per hour; low consumption of the reagent, about 0.5 mL per sample; low sample volumes, 2-10 pL; a closed system, which means minimum contact with the toxic reagent; good reproducibility, typical values for the standard deviation in percent (v/v) HzO were 0.0004 (coil reactor) and 0.0007 (SBSR),with peak area measurements; no need for calibration of the Karl Fischer reagent; no problems with the humidity of air in the reaction chamber. The main drawback with the FIA method results from the requirement for standards which have to be regularly determined with an alternative method. However, one possibility to minimize this disadvantage could be the use of electronic calibration according to Olsen et al. (23),where only one standard solution is needed. The linear range obtained for a conventional 7 mM Karl Fischer reagent is 0.001-0.100% (v/v) HzO, which is suitable for the determination of water in organic solvents. A change in the strength of the Karl Fischer solution with a high buffer capacity is all that is required for use in a different concentration range.

2575

Registry No. H20, 7732-18-5; 2-propanol, 67-63-0; diethyl ether, 60-29-7; benzene, 71-43-2; acetonitrile, 75-05-8; ethanol, 64-17-5; propyl acetate, 109-60-4;chloroform, 67-66-3;dichloromethane, 75-09-2;acetic acid, 64-19-7;isopropyl acetate, 108-21-4; triethylamine, 121-44-8.

LITERATURE CITED Verhoef, J. C. "Mechanism and Reaction Rate of the Karl Fischer Titration Reaction"; Dissertation, Amsterdam, 1977. Verhoef, J. C.J . Nectroanal. Chem. 1976, 77, 305. Cedergren, A. Talanta 1974, 21, 265. Kagevall, I.; Astrom, 0.; Cedergren, A. Anal. Chim. Acfa 1980, 7 14, 199. Kagevall, I.; Astrom, 0.; Cedergren, A. Anal. Chim. Acta 1981, 732,

215. Nordin-Andersson, I.; Astrom, 0.; Cedergren, A. Anal. Chim. Acta 1984, 762, 9. Koupparis, M. A.; Malmstadt, H. V. Anal. Chem. 1982, 54, 1914. Sherman, F. B.; Zabrokrickij, M. P.; Klimova, V. A. 2.Anal. Chim. 1973, 28, 1624. Scholz, E. Fresenius' 2.Anal. Chem. 1981, 306, 394. Schoiz, E. Fresenius' 2.Anal. Chem. 1981, 309, 30. Scholz, E. Fresenlus' 2.Anal. Chem. 1982, 372, 462. Bos, M. Talanta 1984, 31, 553. Scholz, E. "Karl Fischer Titration. Methoden zur Wasserbestimmung"; Springer Verlag: Berlin, 1984. Cedergren, A. Talanta 1974, 27,367. Bizot, J. Bull. SOC. Chim. Fr. 1967, 1, 151. Stewart, J. E. Anal. Chem. 1981, 5 3 , 1125. Mitchell, J., Jr.; Smith, D. M. "Aquametry, Part III", Wiley: New York,

1980. Betteridge, D.; Cheng. W. C.; Dagless, E. L.; David, P.; Goad, T. B.; Deans, D. R.; Newton, D. A.; Pierce, T. B. Analyst (London) 1983, 708, 17. Painton, C. C.; Mottoia, H. A. Anal. Chim. Acfa 1984, 758, 67. Betteridae, D.; Marczewski, C. Z.;Wade, A. P. Anal. Chim. Acta 1984, 135,227. Reijn, J. M.; Poppe, H.;van der Linden, W. E. Anal. Chem. 1984, 56,

943. Verhoef, J. C. Anal. Chim. Acta 1977, 9 4 , 395. Scholz, E. Fresenlus' 2.Anal. Chem. 1980, 303, 203. Oisen, S.; Ruzicka, J.; Hansen, E. H. Anal. Chim. Acta 1982, 736,

101.

RECEIVED for review April 9, 1985. Accepted July 9, 1985.

Monosegmented System for Continuous Flow Analysis. Spectrophotometric Determination of Chromium(VI), Ammonia, and Phosphorus Celio Pasquini* and Walace A. de Oliveira Instituto de Quimica, Universidade Estadual de Campinas, C.P. 6154, Campinas, Sao Paulo, Brazil A system for continuous flow analysls In which the sample is Introduced In the flow manlfold between two alr bubbles Is descrlbed. The characteristics of the system were evaluated In the absence of chemlcal reactlons and have shown that the proposed approach can be employed for determinations that requlre long residence tlmes. The system was tested in the spectrophotometrlc deterrnlnation of ammonla, phosphorus, and chromium(V1). The results demonstrate that the proposed system can replace the classical segmented continuous flow analysis or flow lnjectlon analysis In many determinations with advantages in sampllng rate, sensltlvlty, and reagent and sample consumptlon. The detectlon llmlts for determlnatlon of ammonia, phosphorus, and chromium(V1) are 5, 20, and 3 ngmL-', respectively at 99.7% confldence level. The precislon Is better than 1% for all determlnatlons and samples can be Introduced at rates of 120 per hour or more.

Automated analysis with continuous flow systems can be

classified into two major categories, one using segmented and the other using nonsegmented flowing streams. Skeggs' (I) classic work introduced the general technique of segmented continuous flow analysis (SCFA) with air-segmented streams. The function of air segmentation is to reduce longitudinal dispersion of sample along the flow path, which in turn decreases sample interaction and permits a long residence time for the sample, favoring sensitivity and enabling the use of relatively slow reactions. However, the air bubbles introduced into the fluid stream must be removed prior to measurement. The usual method employs aspiration of the bubbles before detection ( 2 ) . This operation always removes some fluid, causes a delay in the half-wash time, and also disturbs the concentration profile of the sample zone. In order to avoid these shortcomings, SCFA systems work with signals at 90%, or more, of the steady state. Some other approaches, such as electronic identification of the bubbles, have been proposed to overcome this interference (3-5). In addition, SCFA systems usually require more complex and expensive instrumentation.

0003-2700/85/0357-2575$01.50/0 0 1985 American Chemical Society