Anal. Chem. 1980, 52, 541-546
mined by vapor phase osmometry was 8068 and that calculated from the total number of end groups was 7645. The GPC trace indicated a symmetrical distribution with no high molecular weight tails. T h e results obtained from measurement of the number of amine groups in samples E, F. and G (Table 11) indicate that the termination reaction is fast and efficient, since the concentration of amino end groups is independent of the amount of buffer solution and of time of contact with the buffer solution.
541
on unsaturated polymer backbones and therefore of the number of polyether branches grown from a halogenated polymer after addition of silver salt.
ACKNOWLEDGMENT We thank F. Hutterer of Northeastern Ohio Universities College of Medicine for the use of the fluorescence spectrometer and M. P. Dreyfuss of the BFGoodrich Co. for providing the standard P T H F samples. LITERATURE CITED
CONCLUSIONS T h e fluorescamine method has the advantage over other analytical methods of determination of end groups on polymers in its high sensitivity even a t high molecular weights of the polymers. We have found that this method provided quantitative results in less time, with less sample, and with greater sensitivity than obtained by other analytical methods. In addition the method can be utilized in further investigation of the mechanism of initiation of P T H F polymerization and quantitative determination of oxonium ion concentration. Preliminary experiments show that this method is also applicable to the determination of the number of amine groups
( 1 ) Lee, K. I.; Dreyfuss, P. "Ring-Opening Polymerization", ACS Symp. Ser.
1977, 59, 24. (2) Lehmann, J.; Dreyfuss, P. "Multiphase Polymers", Adv. Chem. 1979, 776, 587. (3) Eckstein, Y.; Dreyfuss, P. J . Inorg. N m I . Chem., in press. (4) Saegusa, T.; Matsumoto, S. J . Po/ym. Sci., Par7 A - 7 1968, 6 , 1559. (5) Saegusa, T. Kyoto University, personal communication, 1976. (6) Schmidt, K.;Geckeler, K. Anal. Chim. Acta 1974, 77, 79. (7) Udenfriend, S.; Stein, S.; Bohlen, P.; Dairman, W.; Leimgruber, W.; Weigele, M. Science. 1972, 778, 871 (8) Hubin, A . J.; Smith, S. U.S. Patent 3 8 2 4 197, July 16, 1974.
RECEIVED for review March 15,1979. Accepted December 20, 1979. Work supported by the U.S.Army Research Office.
Improved Neutral Buffered Potassium Iodide Method for Ozone in Air Gijsbertus Bergshoeff
and Roelof W. Lanting
TNO Research Institute for Environmental Hygiene, Postbox 2 14, Delft, The Netherlands
Jozef M. G. Prop Central Laboratory of Dutch State Mines, Postbox 18, Geleen, The Netherlands
Hans F. R. Reynders National Institute of Public Health, Postbox 7, Bilthoven, The Netherlands
This paper describes a neutral buffered potassium iodidepotassium bromide-thiosulfate (KIBRT) method for ozone in air, proposed in The Netherlands as a standard reference procedure for calibration purposes. I n this new variant, ozone is absorbed in a 1 % potassium iodide solution containing a known amount of thiosulfate and 2 % potassium bromide. The thiosulfate prevents losses of iodine, resulting in a substantial improvement of the collection efficiency and sample tenability. After sampling, a known excess of iodine is added to the absorber and this excess is determined simply by photometry at 352 nm. Subtracting a blank (obtained with ozone-free air) eliminates errors by instability of the solutions and by the possible presence of reducing impurities in the reagents. The time between sampling and further treatment of the sample is not critical. The need for high purity water and careful conditioning of glassware is demonstrated. Owing to lowering of the phosphate concentration and addition of 2 YO potassium bromide to the absorbing solution, the influence of sampling flow rate and relative humidity on stoichiometry has been eliminated. Comparisons with gas phase titration (GPT) in an interlaboratory study yielded the relation: KIBRT = 0.96 ( & 0.02). GPT ( 9 5 % confidence interval).
T h e neutral buffered potassium iodide (NBKI) method is well known as a standard reference procedure for ozone measurements (1-9) and has been officially recommended by 0003-2700/80/0352-0541$0 1 O O / O
such agencies as the US.EPA ( I O , II), the WHO ( 1 2 ) ,and, in a modified version, by the VDI in the Federal Republic of Germany ( 2 3 , 2 0 ) . It is based on the assumption (1) that 1.0 mol of I, is liberated per mol of O3 absorbed. This 1:1 stoichiometry has indeed been reported by several investigators ( 4 , 5, 14-16) using quite different methods. However, with the introduction of absolute instrumental calibration techniques like ultraviolet photometry (UV), gas phase titration (GPT) and long path infrared spectrometry in the area of ozone calibration, i t was established that the stoichiometry of the NBKI method (10, 1 1 ) was dependent on relative humidity of the sample (6, 17). At the same time several other inconsistencies with regard to the method arose (5, 7 , 8, 17, 18). Nevertheless, we believe that the simple and cheap manual potassium iodide method will retain a place beside the instrumental calibration techniques, provided some of its drawbacks can be eliminated. We therefore started with detailed interlaboratory studies of the K I T (neutral buffered potassiium iodide thiosulfate) method proposed by Bergshoeff (14, 19) and obtained very good results pertaining to loss of iodine, repeatability, limit of detection, sample keepability, and collection efficiency. These results show the favorable effect of thiosulfate and are presented in the first parts of this paper. Since the stoichiometry of the KIT method was dependent on the relative humidity and the flow rate of the sample, we also studied the neutral buffered potassium iodide-bromide-thiosulfate variant. This KIBRT method combines the C 1980 American Chemical Soctetv
542
ANALYTICAL CHEMISTRY, VOL. 52,
NO.3,
MARCH 1980 PTFE
n i c r o - e q u va!ents
t S
I
-\
O-ring
outward diameter 8 //
inward diameter 5
L;4
S 1315
I
F
stopcock
N S 19/26
2001
-bottom
closed
Figure 1. Principle of the photometric variant (KIT and KIBRT methods),
illustrated by t h e determination of 7.2 pg ozone (e.g., 60 L of air containing 120 pg 03/m3). A s - A , = 0.609. See also Figure 3
outward d i a m e t e r 2 5
120
concepts of t h e Dutch KIT-method (14) a n d t h e German KIBR method (13,20) a n d has been briefly reported on by Lanting (21). Our interlaboratory results include comparisons of K I B R T with GPT and are presented mainly in the section on stoichiometry. PRINCIPLE T h e air is passed through a n absorber containing 10 m L of sodium thiosulfate solution (AC in Figure 1). This solution contains 17‘ potassium iodide and is buffered at p H 6.8. T h e following reaction is supposed t o take place: 0 3
+ 31- + 2H’
-
13-
+ 0 2 + H20
The iodine liberated (BC in Figure 1) reacts immediately with the thiosulfate. After sampling, the remaining thiosulfate (AB in Figure 1) is taken away by adding iodine (AD), !eaving BD to be measured photometrically. In t h e blank, exactly t h e same amount of iodine (AD) is added, leaving only CD t o be measured. Subtracting the blank, CD, from the sample, BD, results in t h e amount of ozone, BC. Thanks t o this subtracting, small changes in the titers have no influence on the result, provided sample and blank are prepared and measured almost simultaneously. EXPERIMENTAL Generation a n d Calibration of Known Constant Ozone Concentrations. Air streams with reproducible ozone content were generated by ultraviolet irradiation of an air (or oxygen) stream with a Pen Ray lamp and subsequent dynamic dilution with zero air. Zero air was obtained by filtering through active carbon with particle size 1.5 mm. All gases contained less than 1 ppb NO. For a stable ozone production, the lamp was thermostated and the power supply (15 kHz and 1500 V) was stabilized. A dry feeding gas was used. Subsequent dilution with humidified air was used to produce ozone air mixtures with known humidities ranging from 0--50%a t 20 “C. U’e compared our method with gas phase titration (GPT) with excess NO, using a Bendix 8101 NO, monitor. This GPT method is based on gravimetric determinations of NO2 from permeation tubes (22). Fluctuations in the ozone concentration were followed continuously with either a Bendix 8002 ozone monitor or a Dasibi 1003 AAS ozone monitor. Apparatus for Sampling a n d Analysis. The two sampling trains (one for the sample and one for the air blank) consisted of a short PTFE suction line, one or two midget impingers, a critical orifice with a protective membrane filter, and an air pump. Blanks were obtained by placing a tube (100 X 15 mm) with activated carbon (1.5-mm pellets) before the impinger. Samples should come into contact only with glass or PTFE. Pressure before
Figure 2. All-glass midget impinger with spherical joints (dimensions
in mm) and after the orifice was measured with two separate manometers. Sample flow rates were controlled by the critical orifices and calibrated with soapbubble meters or mercury-sealed pistons. All-glass midget impingers were used as shown in Figure 2. They should be calibrated accurately a t the 25-mL mark. Pipets and volumetric glassware should also be calibrated within 0.3%. An air pump capable of maintaining a pressure drop of more than 50 kPa over the critical orifice (0.1-2.0 L/min) is needed. Absorbances were measured a t 352 nm with a spectrophotometer equipped with 4.00-cm path length cells. Conditioning with Iodine and Ozone. To prevent adsorption (or desorption) of iodine at glass surfaces, conditioning is necessary. After cleaning, the glassware is soaked during one or two days in a 0.003 N iodine solution and then rinsed with absorbing solution and water. The thiosulfate in the absorbing solution removes traces of free iodine from the glass surfaces. This procedure should be tested as described for purified water. To prevent ozone losses, the suction lines and impingers were conditioned with ozone-containing air during 1 h. Improper conditioning with iodine or ozone can cause serious errors. Purification a n d Testing of Water. Water of sufficiently high purity was prepared by distillation of demineralized water after addition of barium hydroxide and potassium permanganate, or by using a mixed-bed ion-exchange column followed by activated carbon and a 0.2-ym pore size membrane filter. Water quality was tested by measuring the absorbance of a buffered 0.000010 N iodine solution containing 1% KI within 10 min as under “Procedure”. The absorbance should be about 0.5 and should not decrease more than 3% in 20 minutes. If not, the water purification and/or the conditioning of the glassware wm repeated. Reagents. All solutions were prepared with purified water and reagent grade chemicals. Stock solutions of 0.100 N iodine and 0.100N thiosulfate were prepared and stored in the dark. They remained stable for several weeks. The neutral buffered KI-KBr solution containing 5.44 g KH2P0, and 14.32 g Na2HP04.12H20together with 40 g KBr and 20 g KI per two liters was stored in the dark and remained stable for several weeks.
ANALYTICAL CHEMISTRY, VOL. 5 2 , NO. 3, MARCH 1980
The absorbing solution was prepared as follows: 5.00 mL of' 0.100 N thiosulfate solution was diluted to 100 mL with buffered KI-KBr solution. The resulting 0.0050 N thiosulfate solution is stable in the dark for more than one week. Exactly 2.00 mL of' this solution was diluted to 200 mL with buffered KI-KBr solution. The resulting solution (1%KI, 2 % KBr, 0.02 M KH,PO,, 0.02 M Na2HP04,and 0.000050 N NazSz03)is stable for at least one day if kept in the dark. The 0.00012 N iodine solution was prepared by diluting 3.00 mL of 0.100 N iodine solution to 100 mL with buffered KI-KBr solution. 10.00 mL of the resulting 0.0030 N iodine solution (which remains stable for several days) was diluted to 250 mL with buffered KI-KBr solution. The resulting 0.00012 N iodine solution is stable in the dark for only 1 h. The reagents for the KIT method contained no KBr and their phosphate concentrations were 0.1 M instead of 0.02 M. In all other respects, both methods were equal. Procedure. The well-conditioned impingers were filled with 10.00 mL of freshly prepared absorbing solution and connected to the sampling manifold of the ozone generator. The impingers were protected from direct sunlight. Usually, a sampling time of 30 min was used. Within 24 h after sampling, 5.00 mL of the freshly prepared 0.00012 N iodine solution was added to the impinger and made up with buffered KI(-KBr) solution to 25 mL. The absorbance As of the resulting solution was measured within 10 min at 352 nm vs. water. The same procedure was followed for the absorbance AB of the air blank. The ozone concentration c was calculated from: 24k(As - AB) x io3 V in which c = pg O3 per m3 at actual temperature and barometric pressure, k = calibration factor (microequivalents of iodine per 25 mL at net absorbance 1.000),and V = volume of sampled air in liters. For the air blank, zero air was sampled at about the same time and in the same amount as the sample itself. Subsequently iodine was added as in the sample itself and the absorbance AB vs. water was measured. The reagent blank AR was obtained in the same way as As and AB, but without air sampling. The concentrations obtained with the formula can be corrected for stoichiometry as indicated under Conclusions. Calibration Graph and Factor. Aliquots of a freshly prepared buffered 0.00012 N iodine solution were pipetted into well-conditioned 100-mL volumetric flasks. The solutions were made up with buffered KI(-KBr) solution and shaken. Within 10 min, the absorbances were measured against water. After subtracting the absorbance of a blank, the net absorbances were plotted as in Figure 3. The best line through the origin was calculated using linear regression. The calibration factor k was then calculated as the amount of iodine in microequivalents per 25 mL at net absorbance 1.000. This value should be between 0.486 and 0.500. Collection Efficiencies. Two methods were used. In the first method an ozone monitor with a low detection limit was connected behind the impinger. In the second method two impingers were connected in series. Assuming that the collection efficiency CE is independent of the ozone concentration and assuming that both impingers have the same efficiency CE, we have the two following relations: CE CE MI = - X M Mz = - ( M - M I )
Table I. Relative Standard Deviations in Percent for Both the KIT Method and the KIBRT Method ( H 6-10)O ~
100
100
RESULTS AND DISCUSSION Generation and Calibration of Known Constant Ozone Concentrations. At ambient temperature the ozone production decreased 570 when the relative humidity of the
sdmple flow rates
p g 0 :m3
in
~
0 5
16
1.0
L(min
___
1.5
1.9
52
31
2
28 55 60
2 3
1
5
850 100 122 143 200
D
8
2
3
3
3
2 3
3 3 2
23'ib
4750 500
3
617
3
1
i
a The results of b o t h methods did n o t differ significantly. Unless otherwise stated? the sampling time was 30 min. t 15 m i n . ~
Table 11. Relative Standard Deviation in Percent for the NBKI Method without Thiosulfate 1:n = 10)"
'
P B O im
- -
c =
in which M = pg ozone entering the first impinger, M , = pg ozone collected in the first impinger, and M 2 = pg ozone collected in the second impinger. After elimination of M we get:
543
.-_
21 55 250 t
,
1
sample flow rate in Limin -
0.5 39 10 10
_ _ 1 816 5
2"
Unless otherwise stated. the sampling time was 30 min = 1 5 min
feeding gas was increased from 5 to 80%. Therefore the use of a dry feeding gas is preferred. Humidification of the ozone-containing air in the range from 5 to 80% relative humidity showed no effect on the ozone concentration as indicated by a Dasibi model 1003 AAS ozone monitor. T h e independence of the Dasibi data with respect to relative humidity of the sample stream was also demonstrated by Pitts (6) and Demore (7). Effect of Conditioning with Iodine and Ozone. Losses of iodine cause high values for the factor k mentioned under Experimental. As shown in Figure 4. careful conditioning of impingers, optical cells, and other glassware, and the use of well purified water is necessary. We generally obtained iodine losses of less than 0.570in 3 h, even when only 0.2 wequiv of iodine were present in 25 mL. Strangely enough, only very few indications about the importance of conditioning are found in the literature. Naturally, in the usual KI methods without thiosulfate, iodine losses will be even more serious than in the KIT or KIBRT method described here. Replicate measurements of the same ozone concentration frequently gave very low results for the first samples. This effect was caused by losses of ozone and could be avoided by previous flushing of the sampling train with ozone-containing air during 1 h. Repeatability. The KIT method and the KIBRT method have about the same standard deviation. Table I shows the repeatability of these methods. T h e results obtained by the three institutes do not differ significantly. I t is found that the relative standard deviation remains about 2-5% even for ozone concentrations as low as 28 pg/ m3. However, a t lower concentrations a sharp rise of the relative standard deviation is observed. Table I1 shows the repeatability of the NBKI method without thiosulfate. At the lower sampling rates, the repeatability of the methods with thiosulfate appears to be
ANALYTICAL CHEMISTRY, VOL. 52, NO. 3, MARCH 1980
544
n i c r o g r i n szone in
2
absorbonce
5
L
8
25
absorbance
ml
(4.00c m )
12
10
0700
0 600
0 509
0 500 0 130
0 300 glass cell
c
p a t h length
L O C cm
wave e n g t h
352 nm
100
0
010
020
030
-
050
OLC
p i c - o - e q u v a l e r t s i o d i n e in
m l c r o - e q u i v a l e n t s i o d i n e in 2 5 r n l
25 m l
Figure 4. Effect of glassware condtioning (with iodine) on the calibration curve. The two dotted lines were obtained with carelessly conditioned glassware (2-h soaking in iodine solution). The third line was obtained with well conditioned glassware (2day soaking in iodine solution)
Figure 3. Example of a calibration graph found with standard iodine solutions. T h e upper abscissa indicates the corresponding amounts of ozone when 1 mol O3 gives 1 mol I p . (The indicated 7 . 2 pg ozone refers to Figure 1)
Time between Sampling and Measurement of Absorbance. On determining ozone with the neutral buffered 170 potassium iodide method (without thiosulfate), often a maximum absorbance is reached 20-30 min after sampling ( 2 , 3, 5 , 10, 11, 18). After this period, very often a decrease of the absorbance is observed, owing to loss of iodine ( 5 ) . However, in the methods with thiosulfate, time between sampling and measurement is not critical. This was demonstrated by starting with an absorbing solution through which ozone had been passed. From this solution, aliquots were taken after different time lapses and iodine solution was added as described under Procedure. After addition of iodine the absorbances were measured within 5-10 min. The results in Table I11 show t h a t there is no increase in iodine formation after sampling. Moreover, Table IV shows clearly t h a t the samples even remained nearly unchanged for a t least two days. Obviously the thiosulfate stabilizes the samples, resulting in much better keepability. These results were obtained with the K I T method, b u t there is no reason t o expect different results with the K I B R T method. Calibration Graph and Factor. A typical calibration graph is shown in Figure 3. Beer's law is obeyed u p to absorbances much higher than those in Figure 3. The factor k calculated from the graph was 0.493 ( s = 0.002) lequiv of iodine per 25 mL ( n = 10). Each of the three collaborating laboratories found values for k differing less than 0.5% from 0.493. This value remained the same after change-over from the KIT to the KIBRT method and is in excellent agreement with literature data ( 1 4 , 19, 2 4 ) . Higher values than 0.493 indicate losses of iodine by improperly conditioned glassware or insufficiently purified water or by reducing contaminants in the reagents. Comparison of Collection Efficiencies without and with Thiosulfate. Figure 5 gives the results obtained when
Table 111. Effect of Waiting Time between Sampling and Measurement for the KIT Method" timelapse,min absorbance, A s a
5
15
25
35
45
55
0.522 0.524 0.524 0.523 0.524 0.523
800 f i g 0 , / m 3 was sampled with 0.5 Limin during 1 0
min. better. Probably, the absence of free iodine in the samples results in smaller errors. Blanks and Limit of Detection. Figure 1 shows that the absorbance AB of the air blank (and the absorbance ARof the reagent blank) is about 0.20. As found by the three laboratories, the standard deviation in ARvaries from 0.004 to 0.006 and the standard deviation in AB varies from 0.004 to 0.008 (60 L of air in 30 min; KIT method). This standard deviation in AB can be attributed mainly to the reproducibility in pipetting 10.00 mL of absorbing solution and 5.00 mL of iodine solution. Therefore, the effect of bubbling of blank air through the solution is very small or nil. We define the limit of detection as the lowest concentration for which the relative standard deviation is not more than 5 70. This results in the relation sv'2 = 0.05 (As - AB),in which s represents the standard deviation in AB (23). Therefore, As - AB varies between 0.113 and 0.226 and the limit of detection for 60 L of air in different laboratories was found to range from 23 to 45 l g O3 per m3 (KIT method). Similar detection limits were found for the KIBRT method by plotting the standard deviation in wg O3 per m3 against the ozone concentration, and extrapolating to zero concentration. As can be expected from the repeatability, the method without thiosulfate will probably have a limit of detection higher than 45 pg 0 3 / m 3 .
-
__.____
Table IV. Sample Keepability for the KIT Methoda time lapse, h 0.0 0.6 2.8 absorbance, ''-1s A B a
0.493
0.495
0.514
~
3.2
5.2
23.7
25.2
47.4
49.7
0.509
0.502
0.496
0.494
0.488
0.502
370 pg 0 , i m 3 was sampled w i t h 0 . 5 L/min during 2 5 min. ._
_ _ I _
___-
ANALYTICAL CHEMISTRY, VOL. 52, NO. 3, MARCH 1980
-
_____.
545
.____-_______
Table V. Mean Ratios KIBRTiGPT (Stoichiometry) with 95% Confidence Intervalsa series
2
3
237 0.99 I 0 . 0 2 1.06 - 0 . 0 3
85 0.91= 0.06 0.87 i 0 . 1 0
1 GFT, p g 0 ~ R I V ratios TXO ratios n
-
' All
m 3
472 0.98 0.94 9
0.02 I 0.01
i
samples were taken a t 1.0L,'min during 15 min.
4
458 0.94 - 0 . 0 4 0.95 f 0 . 0 2
9
RH
-
the thiosulfate was omitted from the K I T method. Many points are t h e mean results of 6-10 determinations. At 2 L / m i n t h e collection efficiency (CE) was only 65-7070. Unfortunately, for low ozone concentrations a flow rate of 2.0 L / m i n is necessary, resulting in very low CE values. Even a t 0.5 L/min only 90% of 50-500 pg O3/m3 is collected. T h e low C E values are caused by volatilization of iodine. This was demonstrated by the following experiments. An air stream containing 285 pg 0 3 / m 3was sampled with 0.3 L/min. T h e air leaving t h e impinger with K I T solution was led through an electrochemical detector ( 2 5 ) ,sensitive t o both ozone and iodine vapors. The detection limit of this monitor was 10 pg 03/m3. No signal was observed till 12 pg of ozone was absorbed. Immediately after this moment, a gradually increasing signal was measured. When the ozone generator was switched off, t h e signal decreased slowly but did not disappear. However, addition of a small crystal of thiosulfate to the impinger caused an immediate drop of the signal to zero. Both phenomena indicate the formation of free iodine and its subsequent volatilization. As the absorbing solution contains only 0.5 pequiv of thiosulfate, less than 0.25 pmol ozone or 1 2 pg ozone should be sampled. These results are in accordance with previous experiments (14, 19). Hodgeson also reported serious losses by volatilization when sampling for 30 min a t 0.5 L / m i n ( 5 ) . Surprisingly enough. low collection efficiencies in K I without thiosulfate have long been ignored in the literature. Using the thiosulfate-containing absorbing solution of the K I T method, about 300 determinations of CE were carried out by three laboratories. T h e concentrations ranged from 16 to 1720 pg 0 3 / m 3and the flow rates from 0.3 to 2.0 (or 3.0) L / m i n . Both methods given under Experimental for the determination of collection efficiencies nearly always resulted in CE = 98-10070. Even at 3.0 L/min about 100% was found, provided less t h a n 12 pg O3 was absorbed. As the relative standard deviation in C E was 2 to l o % , CE did not differ significantly from 100% (see Figure 5 ) . T h e same applies to t h e K I B R T method. Stoichiometry. Many different sample streams were analyzed simultaneously with the K I T method and with gas phase titration (GPT). Our G P T method shows no significant difference with ultraviolet photometry and provides a precise a n d reliable standard reference method with stoichiometry about 1.00 (20, 21). We found the stoichiometry of the K I T method t o be dependent on the relative humidity as well as on t h e flow rate of the sample stream. As found by Lanting (21)in Delft in 1977, lowering of the phosphate concentration to 0.01 or 0.02 M and addition of 2 % KBr eliminated the flow effect as well as the humidity effect! This could be confirmed by Prop in Geleen in 1977 with air streams containing 269 p g O3/m3. He found no change in stoichiometry when varying R H from 0 to 50% or the flow rate from 0.5 to 1.25 L/min. This was also found by Reynders in Bilthoven in 1977 for R H varying from 0 to 50% and flow rates varying from 0.50 to 1.87 L / m i n (53-446 pg O3/m3). These remarkable effects, discovered by Lanting (21),are not easy to explain, but they clearly indicate that t h e KIBRT variant seems to be a reliable manual standard reference
--
2 3
13
311
.a!?
IP
I *'e5
, .. , .
i ?
T h
Figure 5. Collection efficiency in % in relation with the flow in L/min for the KIT method without and with thiosulfate. Concentrations are given in pg 03/m3
method for routine operation. T o check this and to study the stoichiometry in more detail, many experiments were carried out simultaneously by the National Institute of Public Health (RIV) in Bilthoven and by the T N O Research Institute for Environmental Hygiene in Delft in May 1978. Different sample streams were analyzed simultaneously with t h e K I B R T method (1% KI, 0.01 M Na2HP04,0.01 M KH2P04,5 X 10-5N Na2S203and 2% KBr) and with GPT. All sample streams had a relative humidity R H = 50% a t 22 "C. Four different ozone concentrations were sampled. These remained constant within 0.2% (series 4 within 0.5%) as shown by continuous measurements with a Bendix 8002 ozone monitor. At each concentration, nine or six K I B R T samples were taken a t 1.0 L/min during 15 min and the ratio K I B R T / G P T was calculated. This ratio remained the same when the phosphate concentration was 0.02 M instead of 0.01 M. The mean results (without elimination of possible outlying results) are given in Table V. I t seems from Table V t h a t there exists no significant ( a = 0.05) relation between ratio and ozone concentration. This conclusion agrees with the fact that the regression lines nearly pass through the origin and have correlation coefficients r close to 1.00:
RIV
( n = 33): KIBRT = 0.97(& 0.03). GPT 1.2(f 3.4)
r = 0.977
= 33): K I B R T = 0.94(*: 0.03). G P T 6.2 (* 4.3)
r = 0.995
~
TNO
(11
~
546
Anal. Chem. 1980. 52. 546-552
The mean ratio (with 95% confidence interval) for all series together is: 0.955 f 0.021 for RI\' ( n = 33) and 0.966 f 0.035 for TNO ( n = 33). Combining all ratios ( n = G G ) , the following mean ratio is obtained: 0.956 f 0.020 or 0.96 f 0.02.
CONCLUSIONS Addition of a known amount of thiosulfate to the absorbing solution greatly improves the collection efficiencies as well as t h e keepability of t h e sample solutions. Photometric determination of the excess of thiosulfate after addition of iodine is more sensitive than titration and simpler than coulometry. T h e amount of iodine liberated does not increase after sampling, making a waiting time between sampling and measurements superfluous. Subtracting an air blank eliminates errors by remaining instability of the solutions and by possible reducing impurities in the reagents ( 5 , 6, 17, 18). Careful conditioning of glassware with iodine and thiosulfate (or ozone) and the use of high purity water is necessary to prevent losses of iodine or ozone. I t should be noted that these facts have received very little attention in literature. The same applies to the low collection efficiencies of the usual NBKI method without thiosulfate. T h e absence of a flow effect and a humidity effect when applying a n absorbing solution containing KBr and having a low phosphate concentration, as observed by Lanting (211, was confirmed. T h e stoichiometry of the KIBRT method (mol I2 per mol 0,) is: 0.96 f 0.02 (95% confidence interval). Therefore the results obtained by t h e calculations indicated under "Procedure" should be multiplied by 1.04. This relatively simple KIBRT method can be used in those laboratories which cannot apply rather complicated instrumental methods such as UV or G P T (26). ACKNOWLEDGMENT T h e authors thank J. van Ham of the T N O Division of Technology for Society, Delft, for helpful discussions and
comments on the manuscript.
LITERATURE CITED (1) B. E. Saltzmann and N. Gilbert, Anal. Chem., 31, 1914 (1959). (2) Intersociety Committee, "Methods of Air Sampling and Analysis", American Public Health Association, Inc. Washington D.C., 1972, pp 351-355. (3) Natl. Bur. Stand. (U.S), Tech. Note585, 11-25 (1972). (4) R. N. Dietz, J. Pruzansky. and J. D. Smith, Anal. Chem., 45, 402 (1973). (5) J. B. Clements, "Summary report: workshop on ozone measurement by the potassium iodide method", Environ. R o t . Agency (U.S.), Rep. 65014-75-007 (1975). (6) J. N. Pitts, Jr., J. M. McAfee, W. D.Long, and A. M. Winer, Environ. Sci. Technol., 10, 787 (1976). (7) W. B. Demore, J. C. Romanovsky, M. Feldstein, W. J. Hamming, and P. K. Mueller in: "Calibration in air monitoring", Am. Soc. Test. Mater. Tech. Pub/.. 598, 131-143 (1976). (8) R. J. Paur, R. K. Stevens, and D. L. Flamm, "Status of calibration methods for ozone monitors", paper presented at Int. Conf. on Photochem. Oxidant Poll. and its Control, EPA, North Carolina, September
1976. (9) B. Seifert, H. M. Wagner, and H. Press, Gesund.-lng.. 97, 225 (1976). (IO) EPA reference method for the measurement of photochemical oxidants, Fed. Regist., 36 (228),22393 (1971). (1 1) Environmental Protection Agency, "Measurement of photochemical oxidants in the atmosphere", f e d . Regist., 41 (195),41049 (1976). (12) "Selected methods of measuring air pollutants", World Health Org., Geneva, 1976,pp 88-91. (13) Verein Deutscher Ingenieure, VDI 2468 Watt 1 (1978). (14) G. Bergshoeff, paper no. 147 presented at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, 1970. Publication No. 342, IMGTNO, Delft, The Netherlands. (15) J. A. Hodgeson, R. W. Baumgardner, B. E. Martin, and K. S. Rehme, Anal. Chem., 43, 1123 (1971). (16) S . L. Kopczynski and J. J. Bufalini, Anal. Chem., 43, 1126 (1971). (17) E. R. Stephens and A. M. Winer, Calif. Air Environ., 6(1),1-3 (1975/ 1976)
(18) D ~ L ~ ' F l a m m Environ. , Sci. Technol., 11. 978 (1977). (19) G. Bergshoeff, report H 70, IMGTNO, Delft, The Netherlands, 1967. (20) H. J. van de Wiel. H. F. R . Revnders. H. Gies. R . W. Lantina. and W. Rudolf, Atmos. Environ., 13, 555 (1979). (21) R. W. Lanting, Atmos. Environ., 13, 553 (1979). (22) F. Lindqvist and R. W. Lanting, Atmos. Environ.. 6, 943 (1972). (23) H. Kaiser, Fresenlus' 2. Anal. Chem., 209, 6 (1965). (24) J. J. Custer and S. Natelson, Anal. Chem., 21, 1005 (1949). (25) F. Lindqvist, Analyst (London). 97, 549 (1972). (26) I.Watanabe and E. R . Stephens, Anal. Chem., 51, 313 (1979). ,
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RECEIVED for review J u n e 15, 1979. Accepted October 12, 1979.
Measurement of Free Ion Concentrations by Spectrophotometry: The Sample Ion Increment Method Robert B. Fulton' and Byron Kratochvil" Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
The determination of unbound or "free" concentrations of species such as metal ions is generally done either potentiometrically using ion selective electrodes or spectrophotometrically using weak metal-indicator complexes. A modified spectrophotometric technique has been developed that supplements and extends these methods by overcoming a number of their limitations. I n this technique, a known quantity of indicator is added to the sample and the absorbance measured; then a known increment of the species to be determined is added and the absorbance remeasured. The second measurement determines the extent of "species buffering" in the orlginal sample and permits correction for complexation by the indicator. The technique is evaluated by using pnitrophenol to measure ionic [OH-] at about lo-' M in a series of solutions. The pOH measured by this technique agrees well with that measured potentiometrically. ' P r e s e n t address: D e p a r t m e n t o f Chemistry, St. .John's U n i v e r -
sity, Collegeville, Minn. 56321. 0003-2700/80/0352-0546so1 . O O / O
T h e need to measure free hydrogen ion concentrations led to the development of first colorimetric and, later, potentiometric methods. T h e p H meter and glass electrode combination, when carefully standardized, provides a precise, straightforward method for routine p H measurements. Corresponding routine, accurate methods for directly measuring free metal ion concentrations, pM, or free ion concentrations in general, p X , are far less common. Such methods are needed in industrial applications in metallurgy, electroplating, and food processing (I), in clinical measurements of physiologically important ionic calcium and magnesium in serum and urine (2-5), in biomedical investigations of mechanisms such as formation of kidney stones (6-9), in calcium binding t o casein in milk ( l o ) , and other calcium binding and transport studies (11-17); in studies of the role magnesium plays in biological systems (18-22); and in analyses of various ionic species in natural waters, particularly copper (23) and calcium and magnesium ( 2 4 ) in seawater. Although the value of knowing free, or ionic, concentrations C 1980 American Chemical Society
