ANALYTICAL CHEMISTRY, VOL. 50, NO. 6, MAY 1978
po4tion very close t o the sample holder ( 1 4 ) . I n a configuration, where the sample is outside the feedback loop, a calibration of the instrument using a standard substance can be a delicate problem. As follows from Equations 13 and 14 the measured signal is inf'luenced by both the heatiiig rate b and the time constant of the sample 7,. So it is not enoiigh t o use only ihe same heating rate during calihration and measuring runs but also to have the same time coilstants of the samples of both reference and investigated s u bst a n ce. T h e heat exchange between the sample and the surroundings cannot be fully discussed in this paper. To do this, one should more carefully analyze the problem of reference temperature T H in scanning calorimeters. But even if we assume that the direct heat exchange between the sample and surroundings is negligible, the problem of the time constant of the sample does not disappear if the sample is outside the compensation feedback loop. An analysis of ihe ecluivalent problenis connected with a differential mounting in coiiipensated scanning calorimetry !rill be Lreated later.
Ll?'lEHATUHE CTlED (1,
F t. KaidSZ, li. E. B a r , arid J M O'Reilly, J . Po/ymer Sci., Part, 6 , 1141 (1968).
707
S Strella and P. F. Erhardt. J . Appl. Polym. Sci., 13, 1373 (1969) M. J . Richardson and N . G. Savill, Thermochim. Acta, 12, 213 (1975). W . P. Brennan, B. Miller. and J . C. Whitwell, "Analytical Calorimefry", Vol. 2 , R. S. Porter and J. F. Johnson, Ed., Plenum Press, New York, N . Y , . 1970, p 441. M. J. O'Neill, Anal. Chem., 36, 1241 (1964). A. P. Gray, "Anatykal Calorimetry", Vol. 1, R. S. Porter and J. F. Johnson. Ed.. Plenum Press New York, N.Y., 1968, p 209. J. H. Flynn, "Anawical Calorimetry", Vol. 3, R. S. Porter and J. F. Johnson, Ed., Plenum Press, New York, N.Y., 1973, p 17. C. M. Guttman and J. H. Flynn. Anal. Chem., 45, 408 (1973). H. M. Heuvel and K. C. J. B. Lind, Anal. Chem., 42, 1044 (1970). W. P. Brennan, 8. Miller, and J . C. Whitwell. rnd. Eng. Chem., Fundam., 8, 314 (1969). M. J. O'Neill and A. P. say, "Thermal Analysis", Vol. 1, H. G. Wiedemann, Ed., Birkhauser Verlag, Basel and Stuttgart, 1972, p 279 J . H. Flynn, "Thermal Analysis", Vol. 1. H. G. Wiedemann, Ed., Birkhauser Verlag. Basel and Stuttgart, 1972, p 127. J . H. Flynn, Thermochim. Acta, 8, 69 (1974) T Ozawa, Netsusokutei, 4, 45 (1977). S. Randzio and M. Lewandowski, "Fundamentals of Electronic Temperature Control in Diathermic Calorimetry", Ed. Polish Academy of Sciences, Institute of Physical Chemistry, 1973. E. Margas, A. Tabaka, and W. Zielenkiewicz, Bull. Acad. Pol. Sci., Ser. Sci. Chim., 20. 329 (1972). M. J . O'Neill, Anal. Chem., 4 7 , 630 (1975). S. Randzio and M. Lewandowski. 1977, unpublished results
RECEIVED for review October 3, 1977 Accepted January 16. 1978 This paper was presented at the 5th Internationa! Conference on Chemical Thermod>namics, 22-27 August, 1977, Ronneby, Sweden.
Determination of Phosphates in Natural and Waste Waters after Photochemical Decomposition and Acid Hydrolysis of Organic Phosphorus Compounds J. T. H. Goossen and J. G. Kloosterboer* Philips Research L abordtories, Eindhoven, The Netherlands
Total dissolved and suspended phosphate in water samples may be determined after photochemical decomposition of organic phosphorus compounds and thermal hydrolysis of acid-hydrolyzable phosphates, followed by conventional spectrophotometric determination of the liberated orthophosphate as molybdenum blue. With the procedure described, a 75-W medium pressure Zn-Cd-Hg lamp is used for photolysis and hydrolysis. The combined action of UV radiation and heat from the lamp enables the simultaneous conversion of organic phosphates and acid-hydrolyzable phosphates to orthophosphate. I f a thin aluminum sheet is placed between the lamp and the acidified sample solution, only hydrolysis occurs. I n this way ortho, ortho -!- acid-hydrolyzable, and total phosphate may be determined. The method avoids complicated and time-consuming chemical pretreatment and may easily be automated.
I n the analysis of natural and waste water samples, it is of'{en necessary to discriminate between the various forms of
phtrsphorns present. T h e element occiirs almost solely in the form of inorganic and organic phosphate compounds ( 1 ) . Phosphates are commonly classified into orthophosphate, acid-hydrolyzable phosphates, and organically bound phos-
Table I. Simplified Classification of Phosphate Determinations Form of phosphate
Method of analysis
Totala orthophosphate
N o digestion, direct spectrophotometric determination of PO, 'Mild acid hydrolysis followed by spectrophotometric determination of PO,'. Wet chemical digestion followed by spectrophotometric determination of PO,'.
Totala ortho + acid - h y dr o 1y z a b le phosphate Totala ortho + acid-hydrolyzable + organic phosphate, i.e. to tal phosphate a
Total means suspended and dissolved
-____~___
phates. These categories may be subdivided into filtrable or dissolved and particulate phosphates. The latter distinction can be easily introduced in an analysis by the insertion uf a filtration step in the procedure (0.45 pm membrane filter) and will not be considered here. A simplified scheme of ar,alysis is shown in Table I. Acid-hydrolyzable phosphate and organic phosphate are usually determined by subtraction I I ) . T h e determination of total and organic phosphate requires preliminary digestion, for which several standard methods are being used: perchloric
708
ANALYTICAL CHEMISTRY, VOL 50, NO 6, MAY 1978
acid digestion, sulfuric acid-nitric acid digestion or digestion with persulfate ( I ) . Alternatively, the Schoniger flask oxidation (2) or the sulfuric acid-hydrogen peroxide digestion (3)may be used. We prefer the latter method since it is rather simple. For orthophosphate, identical calibration curves were found with and without digestion. For the sulfuric acid-nitric acid method, t h e preparation of a separate calibration curve is advised ( 1 ) We sometimes found low values with this method if measurements were made immediately after vigorous fuming-off. This may have been caused by the temporal formation of small quantities of condensed phosphates by dehydration of phosphoric acid in t h e hot concentrated sulfuric acid since correct values were found after dilution and after the solutions had stood for 90 minutes. Since all digestion methods are rather time-consuming and not very suitable for automation, alternative methods for t h e determination of total phosphate have received attention. Photochemical destruction of organic material combined with thermal hydrolysis of acid-hydrolyzable phosphates is particularly attractive since it is a method which can easily be automated and does not require special reagents, thus avoiding the risk of contamination. Several authors have reported on photochemical decomposition of organic phosphorus compounds (4-8). Generally, however, long irradiation times (several hours) and the use of high intensity light sources (requiring several hundred watts of power) have been found necessary. Since it has proved very useful to employ light sources of low or medium pressure and low power in the form of spectral lamps for the destruction of organomercury compounds (9), we have adapted the same method to the decomposition of organic phosphorus compounds. As expected from our previous results (9), a combined Zn-Cd-Hg lamp (medium pressure, 75 W) proved to be an excellent light source. Contrary t o the low-pressure single element lamps (Zn, Cd, or Hg, 12-15 W), the combined lamp in our setup produces just enough heat to bring the sample solution to its boiling point in 6 min. If carried out in acid solution, complete hydrolysis of the condensed phosphates occurs in about 20-25 min. Therefore irradiation with the combined Zn-Cd-Hg lamp may be used in the determination of total phosphorus in aqueous samples. By inserting a thin metal screen between the light source and the sample cell, the heat is still transferred to the solution but the light is cut off. This enables a separate determination of ortho- and acid-hydrolyzable phosphate to be made. Since orthophosphate can be determined directly without irradiation and/or heating. the amounts of organic and acid-hydrolyzable phosphate can be found by subtraction.
EXPERIMENTAL Chemicals. All chemicals were reagent grade (Merck) except sodium-hexametaphosphate (Hopkin and Williams, general purpose reagent), glycerol 2-phosphate (Baker, practical grade), disodium riboflavine phosphate (Sigma, 95-97 70,commercial grade), adenosine 5’-mOno-, di- and triphosphate (Boehringer, 98%) (AMP, ADP, and .4TP), adenosine 5’-tetraphosphate (Sigma, 95%, Grade 11) (ADTP),methyltriphenyl phosphonium bromide (Aldrich, 98%) and tris(3-propionic)acid phosphonium chloride which was a laboratory preparation. All chemicals were used without prior purification Reagent and Standard Solutions. The following reagent solutions were used for the determination of phosphate. Solution A: 28 mL of concentrated H,S04 (sp. gr. 1.84) diluted to 100 mL with de-ionized water. Solution B: 2.4 g ammonium molybdate, (NH,)6M07024.4Hz0dissolved in de-ionized water and diluted to 100 mL. Solution C: 5 g ascorbic acid, 55 mg potassium antimonyltartrate, KSb0.C4H40s.’/zH20,25 mg disodium salt of ethylenedinitrilotetraacetic acid (EDTA) and 0.5 mL formic acid dissolved in de-ionized water and diluted to 100 mL. Solution C is stable for at least 5 months when stored in a dark bottle. An aqueous solution of potassium dihydrogen phosphate
Cd Hg (931L61
1369
1I
1 ZL5 -
1120 0996 -
t 0871 I 07L7 I E
::;l”L 0622 .
2 OL98
0 12L
180
220
260
-
300 3L0 h.(nml
380
L2[
Figure 1. Spectral power distribution of the Zn-Cd-Hg in W l 5 nm
lamp. Energy
which contained 8 fig P/mL was used as a standard solution for the preparation of a calibration curve for the orthophosphate determination. The same slope was obtained with and without the treatment used for hydrolysis and/or photolysis. Apparatus. Photolysis and hydrolysis of phosphorus compounds were carried out with a 75-W medium pressure Zn-Cd-Hg lamp (Philips spectral lamp 93146). Alternatively low pressure lamps (12-13 W) were used. These lamps emit either the Zn, Cd, or Hg spectrum with important lines at 214, 229, and 254 nm, respectively (Philips spectral lamps 93106,93107,and 93109). The lamps are connected to the line voltage (110/125 or 220 V ac) via an auto-leak transformer which limits the current to 0.9 A. Spectral power distributions of these lamps were measured between 180 and 440 nm. The results for the Zn-Cd-Hg lamp are shown in Figure 1. Spectra of the other light sources can be obtained from the authors. As can be seen from Figure 1, the Zn-Cd-Hg lamp has a rather favorable emission in the 200-250 nm region. This is of importance since the molar absorptivities of a large number of organic phosphorus compounds increase strongly with decreasing wavelength in this region. The use of the Zn-Cd-Hg source is advantageous over the use of the more conventional Hg or Xe high-pressure lamps since the emission of the latter sources strongly decreases below 250 nm. We used toroidal silica irradiation cells which can be placed around the lamp (Figure 2). The outer walls of the cells were coated with aluminum, which in turn was protected by a layer of varnish. Since heating of the solution had to be avoided for some experiments, the cell was provided with a water jacket. If only hydrolysis was to be carried out, a piece of thin-walled (0.5 mm) aluminum tubing had to be inserted between the lamp and the inner wall of the irradiation cell. Pyrex tubing was not sufficient for complete exclusion of photolysis. The dimensions of the cell were as follows: height, 40 mm; internal diameter, 32 mm; thickness of sample layer, 6 mm; and outer diameter of the water jacket, 80 mm. The sample volume was approximately 30 mL. Sampling. Unpreserved, unfiltered natural and waste water samples were analyzed within 2 h after their collection. The addition of Hg2+has been recommended as a preservative for phosphate samples ( I ) to prevent the interconversion of various forms of phosphates. However, Tillman and Syers have reported an interference from the mercury owing to reaction with the ammonium molybdate, not with ascorbic acid (IO). Contrary to their observation, we have noted that the increase in absorbance is nonspecific. It is caused by reduction of Hg2+t o Hgo by the ascorbic acid. The addition of excess chloride prevents the reduction. Since our natural water samples contained only small quantities of organic phosphorus compounds, no significant difference between samples with and without added mercuric ions could be observed and, therefore, the addition of Hg2+was omitted. Method of Analysis. Throughout the investigation, orthophosphate was determined by the procedure of Murphy and Riley
ANALYTICAL CHEMISTRY, VOL. 50, NO. 6, MAY 1978
709
Table 11. Determination of Acid-Hydrolyzable Phosphate in Various Compoundsd Compound Sodium pyrophosphate, Na,P, 0 ; 1OH20
Sodium tripolyphosphate, Na,P,O,, Sodium hexametaphosphate, (NaPO, )&
Adenosine Adenosine Adenosine Adenosine
5'-monophosphoric acid 5'-diphosphate disodium salt 5'-triphosphate disodium salt 5'-tetraphosphate trisodium salt
Time, min
Method
10 15 20 30 30 10 15 20 10 15 20 25 20 20 20 20
a
40 Flavine-mononucleotide disodium salt Glycerol 2-phosphate Tris(p-propionic acid)phosphonium chloride Methyl triphenyl phosphonium bromide
30 30 30 30
Per cent conversion'
a
80 91
a
100
b b
95 100 85 92 101 79
a a a a a a
94
a
98 99
b b b b b b b b b
51 67 71 76 7 3 0.8 0.1
1.9
a The sample solution is heated by means of the U V lamp; a sheet of aluminum foil wrapped around the lamp prevents photolysis. A 50-mL volumetric flask containing the sample solution is placed in boiling water. Reference method: wet chemical destruction with H,SO,-H,O,. The samples contained approximately 8 gg P in 30 mL sample solution, n
Figure 2.
Irradiation
n
cell with light source
and
reflux condensers
( 1 2 ) . This method is based on the reaction of ammonium molybdate and potassium antimonyl tartrate with dilute solutions of orthophosphate in an acid medium to form phosphomolybdic acid which is reduced to the intensely colored molybdenum blue by ascorbic acid. In the original procedure, all reagents were combined to one solution. However, the one-solution method had to be discarded since we preferred to use the same reagents for determinations with and without digestion and/or hydrolysis. Irradiation of the ascorbic acid/ammonium molybdate solution caused photoreduction of the latter. Therefore the sulfuric acid (solution A) was added before irradiation and/or hydrolysis and all other reagents were added afterward. Procedure. (i) Direct determination of orthophosphate in natural water. Transfer up to 40 mL water to a 50-mL volumetric flask. Add 2 mL of solution A, 2 mL of solution B, and 1 mL of solution C; dilute to volume with de-ionized water and mix. Wait
for 5 min and measure the absorbance at 880 nm in a cell with an optical path of 40 mm. The reference solution should contain the same volume of sample and 2 mL of solution 4 in o r d r r t o correct for color and turbidity of the sample. The phosphate concentration is read from a previously made calibration curve. (ii) Direct determination of orthophosphate plus acidhydrolyzable phosphate in water; indirect determination 0 1 acid-hydrolyzable phosphate in water. Transfer 2 mL of' solution A and a known amount (530 mL, containing up to 15 p g P, 0.054.5 ppm) of natural water in the sample compartment of t h e irradiation cell, make up to the glass joints with de-ionized water. and mount a small reflux condenser on each joint. Place the cell for 25 min around the Zn-Cd-Hg light source> which is surrounded by a piece of thin-walled aluminum tilling. Transfer the solution after hydrolysis to a SO-mL volumetric flask, cool to room temperature, and add 2 mL of solution €3, 1 mI, o f solution C, and proceed as for orthophosphate. The concentration of acid-hydrolyzable phosphate is found by subtraction of the concentration of orthophosphate. (iii) Direct determination of total phosphate in water; indirect determination of organic phosphate in water. The procedure is the same as for acid-hydrolyzable phosphate, except thal t h e aluminum hood is removed from the lamp and that the irradiation time is 30 min. The concentration of organic phosphate is found by subtraction of the concentration of ortho plus acid-hydrnl;vzahle phosphate. Note: With synthetic mixtures but not with unfiltered samples of natural water, bumping of the lamp-heated solutions occurred. This can be avoided by passing a small stream of air through thr solution with a thin Teflon tubing which is introduced through one of the reflux condensers.
RESULTS AND DISCUSSION Choice of Model Compounds. Since phosphorus occurs in natural waters and in waste waters almost solely in the form of phosphates ( I ) , we have mainly investigated the hehavior of a number of organic phosphates--adeno$ine 5'-phosphatri. riboflavine phosphate, and glycerol 2-phosphate. which might be expected from the decomposition of biological mat>erial. and inorganic phosphates-pyrophosphate. trimetaphosphate. and hexapolyphosphate, which can be expected from detergents. In addition two compounds that do not contain P - 0 bonds but only P-C bonds were investigated-tris (&propionic acid)phosphonium chloride and methyltriphenylphosphoni~irn bromide.
710
ANALYTICAL CHEMISTRY, VOL. 50, NO. 6, MAY 1978
Table 111. Determination of Total Phosphate in Various Compounds a , d Per cent conTime, vermin sionb
Compound Disodium phenyl phosphate Adenosine 5'-monophosphoric acid (AMP) Adenosine 5'-diphosphate disodium salt (ADP) Adenosine 5'-triphosphate disodium salt (ATP) Adenosine 5'-tetraphosphate trisodium salt (ADTP) Flavine-mononucleo tide disodium salt Glycerol 2-phosphate Tris(p-propionic acid)phosphonium chloride Methyltriphenylphosphonium
5 30
100 100
30
95
30
96
30
97
30
98
30 30
98 96
30
3OC
bromide Solutions were irradiated and heated with the Zn-CdHg lamp for 30 min. Wet chemical destruction with Wet chemical HISO,-H,O, as reference method. destruction with H,SO,-"0, as reference method. All samples contained approximately 8 p g P in 30 mL, sample solution. I -
__ --
Table IV. Duplicate Analysis of a Synthetic Mixture of Potassium Dihydrogen Phosphate, Sodium Pyrophosphate, and Adenosine- 5'-monophosphoric Acid Ortho, mgP/L 0.080 0.081 0.081
Hydrolyzable, mgP/L 0.084 0.084 0.084
Organic, mgP/IA 0.076 0.075 0.076
Total, mgPIL 0.240 0.240
Figure 3. Hydrolysis of 10'' M sodium pyrophosphate solutions containing 0.3 M H,SO, at 100 O C . (0)heated on steam bath (x) heated by means of the UV lamp surrounded by an aluminum hood. Percentage hydrolyzed is given with respect to orthophosphate found after wetchemical oxidation
the rate of hydrolysis (xi the other side of the ino1ec:ule since prolonged hydrolysis, u p t o 120 min did not sigiiificaritly 0,241 increase the yield of orthophusphate (12). T h e phu3phate group of AMP clearly requires a more drastic treatnient for its separation from the adenosine. Hydrolysis. T h e usefulness of the Zn--Cd-Hg lamp as a Photolysis. Figure 4 5hoWS decomposition uf hhll' a b a heat source for the hydrolysis of Na,P207 as compaied with filnction of time for various light sources. It is observed that treatment on a steam bath is shown in Figure 3 . Initially, under our coiiditions the combined Zn -C'd--Hg lamp is by far t h e heating on the steam bath seems to be somewhat more t,he most efficient source. An irradiation time of 30 min is efficient than heating by the lamp but in the end the lamp sufficient for the complete conversion of AMP into orthoproves to be the most efficient heat source. This may arise from the fact that the temperature of the solution on the steam phosphate. The molecule is prohably decompoaed to a large extent since the ultraviolet absorption hand at 258 nm, which bath reaches 98 "C whereas the lamp-heated solution reaches i:, also observed in the spectrum of adenine. disappears its boiling point. Similar results were obtained for other completely upon irradiation. Results similhr to thobe i h AMP compounds. Table 11 shows t h e results of hydrolysis when were obtained for other compounds. Table 111 ahowti the ultraviolet irradiation is excluded. The adenosine phosphates results of I!V decompusition of a number ui organic I J ~ O S behave partly as acid-hydrolyzable phosphate, partly as orphorus compounds, Except for ADP: A T P and ADTP. ganic phosphate. The results suggest that AMP, ADP, AIP. hydrolysis of the other organic compounds studied (esters as and ADTP (adenosine 5'-tetraphosphate) lose zero. one. tivo well as compounds containing C-P bonds) was negligible under and three phosphate groups, respectively, upon hydrolysis. our experimental conditiuns (Table 11). Of t h e corripoutlcls T h e presence of the adenosine moiety on one side of the molecule does not seem to have a serious retarding effect. on studied. niethyltriphenylphosphonium bromide naa the most Table V. Duplicate Analysis or Orthophosphate, Ortho t Hydrolyzable Phosphate, and Total Phosphate in River Water Added Found
I _ _ -
Ortho hydrolyzable phosphate, mg PIL 0.080 0.082 3.20 3.23 0.87 0.87 +
Sample Dommel River, upstream, sewage works Dommel River, downstream, sewage works Waal River a
35 minutes o n a steam bath at 100 "C.
Orthophosphate, mg PIL 0.071 0.070
3.07 3.06 0.79 0.80
ltefereiice mg PIL 0,083 0.082 3.24
.,.
0.87
...
Wet chemical digestion with €I,SO,-H,O,.
Total phosphorus, mg PIL 0.094 0.095 3.54 3.60 0.94 0.95
Reference method,b mg PIL 0.092 0.095 3.68 3.66 0.96 0.95
ANALYTICAL CHEMISTRY, VOL. 50, NO. 6, M A Y 1978
dissolve and desorb suspended phosphates. T h e photolysis of ADP, ATP, and ADTP in neutral solution yielded only a small quantity of orthophosphate in addition to condensed phosphate, whereas AMP was quantitatively converted to orthophosphate. Combination of photolysis and acid hydrolysis in one experiment yielded the theoretical amount of orthophosphate for ADP, A T P and A D T P (Table 111). Synthetic and Natural Water Samples. I n Table I V some results obtained on a synthetic mixture are given. Table V presents results on real samples (natural and waste water). T h e photochemical method appears to be quite useful. In heavily polluted waters, however, strong absorption of light by nonphosphorus compounds may interfere with t h e photodecomposition of phosphorus compounds. Owing to the high phosphorus content usually found in heavily polluted natural waters, these samples require considerable dilution, thereby reducing the interfering absorbance to the same extent. Thanks to the construction of the cell, turbidity of the sample does not prohibit adequate photolysis: the scattered light may still be absorbed by dissolved species since it is reflected a t the outer wall. T h e simplicity of the method facilitates its incorporation in monitoring systems constructed for "unattended" operation.
ZnHg Cd
0
10
20
30 LO time ( m i n )
50
711
60
70
80
90
Figure 4. Photodecomposition of M 5'-AMP solution containing 0.3 M H,SO, at 25 OC by means of various light sources. Percentage decomposed is given with respect to orthophosphate found after wet-chemical oxidation
stable. This compound was only 30% decomposed under conditions of simultaneous heating and UV irradiation (Table 111). However, the normal treatment with H 2 S 0 4 / H 2 0 2also yielded only 4% of the expected value. More drastic methods were required to decompose this compound: fuming-off with HNO,-HClO, followed by t h e normal treatment with H2S04-H202yielded 92.5% of the theoretical amount, vigorous fuming-off with H2S04-HK03yielded 95% of the theoretical amount. Our method offers, at least in principle, the possibility of a separate determination of ortho and organic phosphate by performing photolysis on a neutral solution which is being kept a t room temperature. This method, however, is not practical for t h e analysis of natural water since acid is required t o
ACKNOWLEDGMENT The assistance received from the Central Laboratory of the Philips Lighting Division consisting in the measurements of the spectral power distributions of our light sources is gratefully acknowledged. LITERATURE CITED (1) "Standard Methods for the Examination of Water and Waste Water", 13th ed., American Public Health Association, Washington, D.C.. 1971, p 518. (2) L. E. Cohen and F. W. Czech, Chem. .4nal., 47, 86 (1958). (3) Scandinavian Pulp, Paper and Board Testing Committee SCAN-W 8:73. Sven. Papperstidn., 1973, 654. (4) F. A. J. Armstrong, P. M. Williams, and J. D. H. Strickland, Nature(London), 211, 481 (1966). (5) F. A. J. Armstrong and S. Tibbitts, J , Mar. Bioi. Assoc. U . K . , 48, 143 (1968). (6) K. Grasshoff, Fresenius' Z . Anal. Chem.. 220, 89 (1966). (7) Q. W. Osburn, D. E. Lemmel, and R. L. Downey, Environ. Sci. Techno/.. 8, 363 (1974). (8) S. V. Lyutsarev, V. V. Sapozhnikov, and Ye. P. Selifonova, Okeanologiya, 13, 903 (1973); (Eng. transl., Oceanology, 13, 746 (1973)). (9) A. M. Kiemeneij and J. G. Kloosterboer, Anal. Chem., 48, 575 (1976). (10) R . W. Tillman and J. K. Syers, Analyst(London) 100, 322 (1975). (11) J. Murphy and J. P. Riley, Anal. Chim. Acta, 27, 31 (1962). (12) J. T. H. Goossen and J. G. Kloosterboer, Phofochem. fhofobiol., in press.
RECEIVED for review August 24, 1977. Accepted January 19, 1978.
