A Versatile System for Liquid-Phase Quantum Yield Measurements D. J. TRECKER and J. P. HENRY Research & Development Department, Union Carbide Chemicals Co ., South Charleston 3, W. Va.
b A specially designed system has been constructed and tested for the purposes of accurately measuring quantum yields over a range of wavelengths and readily determining the effect of wavelength on photochemical reactions. Consisting of five concentric tubes connected b y ring seals and standard taper joints, the apparatus enables {he introduction of four solutions: two chemical filter solutions, one of which can b e circulated as a coolant; a reaction mixture; and actinometer soluiion. The overall reactor represents a new and unusually versatile laboratory scale photochemical system. A number of filter solutions have been screened spectrometrically and found suitable for use in the reactor. Mercury lines a t 31 3, 405, 436,546, and 578 mp have been isolated b y various filter combinations.
centric equivalence of all surfaces should help minimize errors due to these effects. The apparatus is pictured in Figure 1. It consists of five borosilicate glass cylinders of lOO-mm., 80-mm., 60-mm., 39-mm., and 28-mm. o.d., joined together by appropriate ring seals and standard taper joints, as indicated. The two interior tubes (filter jackets No. 1 and No. 2), with cell spaces of 3 mm. and 8 mm., respectively, can accommodate chemical filter solutions of predetermined transmittance properties. A pumping system, shown in Figure 2, provides effective cooling for both the lamp and the two exterior cells. Filter solution 2 is continuously circulated through a reservoir of brine-cooled acetone. Combined with a n air jet
Table
T
HE NEED FOR A VERSATILE REACTOR
which 11ould allow the isolation of various regions of the ultraviolet and visible spectrum prompted the design and construction of a new type of photochemical apparatus. This system may be discussed most readily from the standpoint of design, filtering capabilities, and quantum yield determinations. DESIGN
The designs of most quantum yield systems utilizing chemical actinometry (6, 6) have previously relied on the use of parallel light passing at right angles through optically parallel surfaces. Collimation of nonparallel light is important in such designs, as is the necessity of catching all of the incident light by sufficiently large flasks. The quantum yield system described herein was designed with the thought of alleviating some of these difficulties. The apparatus is based on the radial equivalence of sealed concentric tubes and, thus, does away with the problems of obtaining and measuring perfectly parallel light. Moreover, since light is emitted radially from moyt highpressure mercury arcs, all of the available light from such sources is utilized here. Though reflection and scattering of light nil1 be more pronounced in a circular apparatus of this type, the con1882
ANALYTICAL CHEMISTRY
I.
Solution in 3-mm. jacket I. NiSO4.6H2O (10 g. per 30 ml. water) 11. Iodine (2.25 g . z e r 100 ml. C 14) 111. S a X O z
( i 5 g. per 100 ml. water)
IC'. CoClz.6HzO
( 1 . 0 g. per 100
ml. acetone) J-. C11804.5HzO (33.3 g. per 100 nd. water)
Transmittance Characteristics of Filter Solutions
mp
I
313
1.456
366
405 436 540
0.620
Transmittance niax., mu 330
Solution in 8-mm. jacket COSO~. 7Hz0 (12.5 g. per 30 nil. water) CU(NOQ)Z.~HZO (saturated aqueous soln) CUSO~. 5H2O ( 4 . 0 g. in 30 ml. ",OH per 70 d. water) CuClz .2H2O (1 ;ns5hg..i 100 nil. KaCr207
Width Light of band transa t halfmitted, % height, max. my 61
59
403
44
38
430
59
48
524
42
55
587
46
60
(saturated aqueous soln)
Table II.
Wavelength,
directed over the lamp, the cooling system has been able to maintain a constant temperature of 38" + 2" C. in the reaction cell and 25" i 1" C. in the actinometer jacket during photolysis with a 200-watt mercury arc. The reaction jacket (Figure I) has a capacity of 425 ml. to the lower ring seal. An inlet tube equipped with a tapered capillary leading to the bottom of the reaction cell provides for a continuous flush of nitrogen through the solution. Aliquots may be removed from the reaction mixture without interrupting the photolysis with a 12-inch, 22-gauge needle passed through the inlet seal. A brine-cooled reflux condenser is attached, via a ground glass joint, to the second arm of the reaction cell. The exterior jacket, designed to hold
Distribution of Transmitted Light"
Watts transmittedb I1 111 IV
Transmitted light,b %
v
I
I1
I11
IV
v
62.8
0,704
0,039 0,091
0.140 1.46
26.7 1.7 0.87
0.09
88.6 11.4
9 .0 91 .o
86.5
5.8
13.5 94.2 8.8 0.14 1.46 578 0,204 a Values calculated are f o r a Hanovia 654.4-36 200-watt high-pressure mercury arc utilizing the transmittance rurves described in Table 1. NuInerals correspond to filter solution combinations listed in Table I.
DIRECTED AIR
-AM FROM
\
WMP
APPARATUS
L W P JACKET
QUANTUM YIELD APWATTVS
FILTER SOLN
UIANTUM nm APPARATUS
J
REACTION JPCKET
Figure 2.
ACTINOMETER’ &%ET
Pumping and cooling system for photoreactor
7AGNETIC S T I a N G BAR
Figure 1.
Schematic diagram of quantum yield apparatus
the actinometer solution, has a capacity of 720 nil. TTO13-1nm. outlet tubes are provided for periodic removal of aliquots of the solution. FILTER SYSTEM
A borosilicate glass ssectrophotometric cell with compartments equivalent in depth to those of the reactor jacket (3 mm. and 8 mm.) was used to determine transmittance curves for each filter solution. Sumerom two-component irradiation filters habe been compiled, (1, 4 ) and may be used to isolate various regions of the mercury line spectrum. A number of such chemical filter combinations can be suitably adapted for use in this sjstem. The transmittance curves attainable from four such combinatioiis are summarized in Table I and pertinent dimensioiis of the curve> are indicated. The distribution of transmitted light front a 200-watt highpressure mercury arc (Hanoria 654A36) through these filter solutions is compiled in Table 11. With such filter coribinations, photochemical reactions m:ty be investigated over a wide range of wavelengths and the effect of wavelergth on a process studied. Moreover, quantum yields can be determined in any region of the ipectrurri. The use o ’ borosilicate glas, cyliiiders ill the rractor does, honevcr, rehtrirt soinen 1i:tt the range of ultrabiolct light transmi1 tecl, since borosilicate g1:tss iiorniallj abiorbs all light of wavelengths below 290 mp. Aging of filter solutions, a problem
often encountered in photochemical systems, was found to be a t least partially alleviated by the circulation of one of the filter components (see Figure 2). Spectroscopic examination of the nickel sulfate- cobaIt sulfate solutions after 1400 hours of use showed only minor changes in the transmittance curve (Figure 3). Q U A N T U M YIELDS
Quantum output rates for a 6‘ wen lamp and filter combination are determined in the following manner. Actinometer solution is placed in both the reaction jacket and the actinometer jacket and irradiated for 5 seconds. An aliquot is withdrawn from the reaction jacket and the moles reacted in
fl \
I
I I
I
I I
I
I \
\ \
\
X I LL.P4l
IMLLMCEINLI
Figure 3. Filter combination I (-before use; -after 1400 hours)
--
time 2 are determined by a suitable means [oxidative titration, (3) ; spectrometry (2, 7 )]. The quantum output is calculated from the following equation, where = moles of actinometer reacted in x ieconds, AT = Avogadro’s number, and 9’ = quantum yield of actinometer a t the wavelength employed. This procedure is repeated
qeveral times, and the outputs obtained are averaged. Average deviation should be less than 6%. Finally, a n aliquot is withdran n from the actinometer jacket, and the moles reacted over the entire period are determined. The output derived from this value is then added to the average output recorded for the reaction jacket to afford the total quantum output of the system. qi
Total
= qi
Inner jacket qi
+
Outer jacket
(2)
In the determination of the quantum yield, the reaction mixture replaces the actinometer solution in the reaction jacket. Continuous nitrogen ebullition maintains adequate mixing. Actinometer solution is placed in the outer jacket, and the system is irradiated for y seconds, under conditions identical to those wed in the output determinations. At the end of this time, a sample IS withdrawn from the inner cell to determine the extent of reaction. Analysis of a n aliquot from the actinometer jacket wtahli-he. the quanta of light not absorbed I)y thcl reaction mixture : this slioultl not e x e e d 2 to 5% of the total ouput. The quantum yield, expressed in terms of niolecules of product obtained (or starting material reacted) VOL. 35, NO. 12, NOVEMBER 1963
1883
per quantum absorbed, is then calculated from the following expression, where iM‘ = moles of product resulting in y seconds and N = Avogadro’s number. a= (M‘) ( N ) (y seconds) (4 output - + actinometer) (3) Four to six determinations are usually required to obtain a reliable quantum yield. I n most instances, average deviations of less than 10% should be expected. The choice of the chemical actinometer depends largely on the absorption characteristics of the photochemical reagents under study. Uranyl oxalate (3, 7), potassium ferrioxalate ( Z ) , and benzophenone-isopropyl alcohol (6) represent three of the most widely-used systems. Each has its limitations, When visible light is used, an inconveniently thick cell of uranyl oxalate is needed to measure the output accurately. This limits the range of reactions to those run in the region of 254 to 366 mp. Benzophenone-isopropyl alcohol obviously can be employed only with wavelengths of light which are absorbed by benzophenone.
Potassium ferrioxalate is particularly useful in the visible portion of the spectrum and with light sources of low intensities; it is, however, a very sensitive actinometer and subject to some error at lower wavelengths. So the newly devised concentric tube system could be used with confidence, a quantum yield study was undertaken on the well established isopropyl alcohol-benzophenone reaction. The formation of acetone, which was shown by Pitts and coworkers (6) to be independent of oxygen concentration, was followed by gas chromatographic analysis. Benzene was used as an internal standard. Determinations a t varying concentrations of acetone and benzene established the relationship of the integrated peak areas to molar ratios. The nickel sulfate-cobalt sulfate filter combination (I) was used to isolate the region of maximum n - x * absorption of benzophenone (310 to 360 mp). Quantum output rates to the two outer jackets from a Hanovia 200watt high pressure mercury arc (654A36) were measured, utilizing uranyl oxalate actinometry (3). Reliably reproducible output rates of 3.0 X 10‘8 quanta per second were recorded. Under these conditions the quantum
yield of 0.99 previously reported by Pitts (6) for acetone formation was reproduced with an accuracy of lt0.02. ACKNOWLEDGMENT
The authors are indebted to R. A. Patrick for assistance in much of the experimental work. LITERATURE CITED
(1) Bowen, E. J., “The Chemical Aspects of Light,” 177-80, Clarendon Press,
Oxford, England 1942.
6..
(2) Hatchard, C. Parker, C. A.. Proc. ‘ Roy. SOC. London A235,520 (1956). (3) Leighton, W. G., Forbes, G. S., J . Am. Chem. SOC.52, 3139 (1930).
(4) Masson, C. R:, Boekelheide, V., Noyee, W. A., Jr., in “The Technique of Organic Chemistry,” 2nd ed., 281-3, Vol. 11, Interscience, S e w York, 1956. (5) Moore, W. M., Hammond, G. S., FOBS, R.P., J . Am. Chem. SOC.83, 2789 I l S 6 l,~. \ - - _ -
(6) Pitts, J. N., Jr., Letsinger, R. L., Taylor, R. P., Patterson, J. M., Recktenwald, G., Martin, R. B., Ibid., 81, 1058 (1959):
(7) Pitts, J. N., Jr., Margerum, 0. P., Taylor, R. P., Brim, W., Ibid., 77,5499 (1955). RECEIVEDfor review April 18, 1963. Accepted July 30, 1963.
DifFerential Dielectric Apparatus for Determining Water Added to Solvents WINTHROP C. WOLFE Nationol Bureau o f Standards, Washington 25, D .
b A simple and rapid technique i s described for determining small changes in water concentration in p-dioxone or bis(2-ethoxyethyl) ether (diethylene glycol diethyl ether). The moisture content of solids can be determined by extraction, followed by this procedure. Dual matched dielecttic measuring cells are filled with liquids to be compared and capacitance differences are measured with a , commercial Q-meter. The cells are calibrated with the solvent to be used and solutions containing known concentrations of water in the same solvent. Values for change in dielectric constant with change in water concentration (weight per cent), dc/dC, are calculated from experimental data and from literature values. The temperature coefficient of de/dC is sufficiently small so that precise temperature control is unnecessary. Calculations of dipole moment from the data support previous observations that water behaves in a nonassociated manner in dilute solutions in p-dioxane.
1884
ANALYTICAL
CHEMISTRY
T
C.
HERE has been some interest in determining moisture in agricult u r d and manufactured products by extraction with a solvent and then determination of the change in a physical property of the solvent, such as density (2, 4, l o ) , conductance of an added electrolyte (8), or dielectric constant. Since p-dioxane is completely miscible with water and has a low dielectric constant, it has been used to extract water from various products and the extract is then analyzed for water by measuring the change in dielectric constant (12, IS). Commercial moisture meters have been developed on this basis, one of which uses bis(2ethoxyethyl) ether, known commercially as diethylene glycol diethyl ether. I n applying this method to cereal grains, it is necessary to measure low concentrations of water in p-dioxane or bis(Zethoxyethy1) ether as indicated by extraction methods previously reported in which other solvents were used. For example, work on methanol extraction, in which Karl Fischer reagent was
used to determine water (9), indicated that it was necessary to use 200 ml. of methanol to extract 10 grams of grain. Since cereal grain usually contains 10 to 20% moisture, the water content of the solvent would increase 0.5 to l.O%, assuming complete extraction. While the most efficient extraction of water would be expected from a perfectly anhydrous solvent, it is not practicable to prepare and store solvent absolutely free from water. Moisture determinations using solvents invariably require a blank determination of the water in the solvent. Even under the most favorable conditions, the solvent is likely to contain several tenths of a per cent of water. Hence the property to be measured is de/dC, the change in dielectric constant with change in water concentration. There are no available data on tho dielectric constants of mixtures of water and bis(2-ethoxyethyl) ether. Data on the dielectric constants of dioxanewater mixtures reported in the literature ( 1 , 6, 12, 13, 15, 17) are inadequate for the purpose. The low range of con-
