Galvanic ,Monitoring of Ozone in Air PAUL HERSCH and RUDOLF DEURINGER Beckman Instruments, lnc., Fullerton, Calif.
b A new method for monitoring ozone in air down to air pollution concentrations uses a galvcinic cell with a platinum cathode, a carbon anode, and a neutral buffered solution of halide as the electrolyte. The air sample stream, aspirated into the cell, causes the electroly,ie to circulate, continuously supplying the platinum with halogen as the latter i s liberated by the ozone. N o external electromotive force i s applied, and there i s no need for continual, or intermittent, renewal of electrolytt!, the net effect being oxidation of the carbon surface. The factor converting the ozone concentration into the current i s determined solely by Faraday’s law, not b y geometry or temperature. Therefore there is no need for calibration and a standardized source! of ozone. The principle i s applicatde to the determination, continuous or batchwise, of many other species which react with ozone, or produce or consume free halogen, and to halogen itself.
T
importance of ozone in meteorology, air pollutior , industrial hygiene, and organic reactions is reflected by the large number of methods that ha\ e been designed for the determination of this species. The best known techniques USE ultraviolet absorptiometry or iodo netric titration. The readily achieved quantitative liberation b y ozone of iodine from aqueous iodide ha5 been exploited in continuous amperometric monitor3 (1, 4, 5 ) and was also the basis of a continuous coulometric analyzer (i’,3 ) . I n a recent commercial version of t h i j device a pump feeds iodide solution to a platinum wire heli.;, keeping it wetted by a moving film. Ozone ’rom the sample gas stream diwolves in the film, forming iodine, which is continuously reduced back to iodide through application of a baie potential to the platinum. The current flowing betn-een the helical cathode and a wire loop provided as anode measure- the ozclne. Using an electrolvte with bromide as its principal constituent, we have made a fuel cell for the continuous measurement of ozone in a wide range of concentration.. The cell is a1.o couiometric, but u\e. no applied potential, and re-u\e the electrolyte iridrfinitely. This avoid< a reagent-diqpeving pump and HE
Air From Scrubber
To Flowmeter And Aspirator A
Figure 1 .
Construction of cell
reagent storage. The anode of the galvanic cell is made of active carbon, which by incorporating hydroxyl groups or oxygen atoms from water in its surface can supply electrons to the outer circuit. The reaction taking place may be written
+ 2 Br+ 0-2 + Br, 2 e - + Brp 2 Br. . .C + 0-2 . . .CO + 2e-
O3
--
+0 2
where.. . CO represents an oxygenchemisorbate. The net effect of these reactions is the oxidation of carbon b y one of the oxygen atoms of the ozone molecule. Diatomic oxygen does not interfere, though in air it may be 10,000,000 times more abundant than ozone. Since the current signal is proportionately related to the rate of supply of ozone b y Faraday’s constant, temperature changes and the cell geometry have no effect, and no calibration with a standardized source of ozone is needed. APPARATUS AND PROCEDURE
Cell Construction. Figure 1 shows t h e construction of t h e cell. T h e air carrying t h e ozone enters through a jetlike port and breaks u p into a moving chain of gas bubbles a n d short slugs of liquid, as i n airlift pumps. A turbulent gas-liquid interface for a fast uptake of ozone is t h u s maintained. T h e dissolved ozone reacts instantaneously with t h e bromide. Only “free bromine” (or, rather, the tribromide ion, Bra-) but
no ozone reaches the plat’inum cathode. This is important: D k c t exposure of I)l:itinum to gaseous or diswlvcd ozone \\-auld rrqiilt in the loss of W I W ozonc because of the catalytic clecor~il)c,~ition: 2 os-. 3 0 7 . The cathode is a screen of platinime.g., 20-mesh per lineal cm.-lwnt to a scroll. X platinum wire attachrd, not soldered, to the screen leads upward to a spcond platinum \\.ire, which i.; fused into t,he glass tuhc near the air ??tit. The electrolyte is made 111) of 3.11 sodium bromide, a brace 10.001df) of sodium iodide, 0.1.11 sodium dihydrogen phosphate, and 0.1.11 disodium hydrogen phosphate. As the anode, a bed of activated coconut charcoal was mostly used, n-ith a small piece of platinum scrwn ( 2 X 4 em.) or a larger piece of graphite cloth buried in i t for a better distribution of the current. The carbon vias first made up into a paste with electrolyte, Some other forms of carbon were also found usable. Circuit and Calculation. T h e platinum cathode a n d carbon anode are connected t o a galvanometer, which in t h e interest of a speedy response should have less t h a n 1000-ohm impedance. For recording, t h e potential drop along a fixed-load resistor is monitored. From Faraday’s law i t can be calculat’ed t h a t a t a sampling rate of F ml. of air per minute (measured a t 20’ C. and 1 atm.) a concent’ration of .r volumes of ozone per million volumes of air (v.p.m.) develops a galvanic current i = 0.1338 Fx pa.
With a load resist’ance of R ohms, the potential drop fed to the recorder is E = 1.338 10-4 FRz mv.
F , R, or both may be adjusted to suit the scale of the instrument available. A galvnnometer with 20-pa. full scale deflection will cover 0 to 1 v.p.m. of O3 T i F = 149 ml. per minute; at a sampling rate F = 200 ml. per minute a I-mv. recorder covers the range 0 to 0.2 v.p.m. of 03 (0 to 20 v.p.h.m.) if R = 187 ohms. Extension of Range. T h e range of concentrations measurable with t h e cell can be extended much beyond 150 v.p.m. of 03 b y splitting t h e sample stream, F mi. per minute, into a minor and major branch, f and F - f ml. per minute, respectively, a n d passing t h e latter through latex or charcoal before i t joins t h e cell. I n this way t h e cell receives t h e total carrier stream, F , b u t only t h e fraction f / F of its ozone. F o r dividing t h e flow VOL. 35 NO. 7, JUNE 1963
897
glass capillaries may be used, but surfaces other than smooth, acid-washed glass should be avoided in all ducts that carry ozone, especially at low concentrations. Metal parts must not be tolerated. Connections should be glass-to-glass, with Tygon sleeves. For a ratio f :(F-f) = 1:49 a 50-fold extension of range from 150 v.p.m. to 0.75 volume % can be achieved. Air Moistening. I n continuous operation t h e evaporation losses from t h e cell must be prevented or made up for. il stream of 100 ml. per minute of dry air a t 20" C. would remove nearly 0.1 ml. of water per hour. D r y air tends t o cause a salt crust near t h e entrance jet. Such a crust is capable of absorbing halogen a n d thereby retarding t h e response. The remedy for this is a small gaswash bottle with dilute acid (0.05111 HzS04) preceding the cell. The solution may also contain 0.15 mole of KMn04 per liter for the retention of SOz. The washing with the acid permanganate solution increased the background level by a fraction of 1 la., possibly from ozone formed via manganese heptoxide. However, the current increment on introducing ozone (1 and 6 v.p.m.) was not noticeably affected. Surprisingly, the sludge of manganese dioxide hydrate that gradually clouds the permanganate solution has no effect on ozone, although a column of dry manganese dioxide completely destroys ozone. Interferents. Sulfur dioxide interferes in t h e cell, b u t with t h e permanganate scrubber preceding t h e cell t h e introduction of 1 volume of SO2 per million caused a decline of t h e signal given by 0.5 v.p.m. of ozone, which was not larger t h a n t h e margin of error from other sources. Without the permanganate the same concentration of SO2 suppressed the ozone signal entirely. Air with 10 v.p.m. of SO2
Table 1.
13,A
J J
100 vol p m 03 Pt Br Ag
I ~
1
I
Air flow, ml./min. Potentiometric titration Thiosulfate used, pmoles Time to end point, min. 0 3 concentration, v.p.m. Galvanic response Load, ohms Background, pa. Deflection from 08, pa. Defln. predicted from titration, pa. Coulombic yield Rise time to 90%, sec. Fall time to 90%. sec. Noise pa.
v.p.m.
ANALYTICAL CHEMISTRY
,
I
125,.A
Figure 2. Recorded response to ozone
-
7 IlinA
137mA =3%>titration
I
1
i produced currents in the direction opposite to that of ozone when permanganate was not used. These currents continued for some time afterwards, even in a stream of pure air. Nitric oxide does not interfere and nitrogen dioxide interferes only slightly. One volume per million of NOz simulates less than 0.03 v.p.m. of 03. The removal of YOz is required only when this gas is in considerable excess over ozone. The same scrubbing solution as for SOz may then be used-in a deep-wash bottle with a sintered-glass sparger reaching some 30 cm. below the level of the permanganate solution-to ensure complete conversion of the nitrogen oxides to nitric acid. I n the presence of nitric oxide, a n inefficient scrubber might create more NOz than i t destroys. As to the retention of olefins, a simple bubbler with mercuric perchlorate and perchloric acid (1 mole 4 moles of HClO4 HzQ of HgO made up to 1 liter), with a trace of permanganate added before use, removed all limonene from a n air stream, without removing a n y ozone. The air was bubbled through a 0.OliM solu-
+
+
Details of Ozone Determinations of Figure 2
Anode
898
' 1 -
3 min
Low level Activated carbon Darco G 60 100 0.20 16.5 1.45
High level Silver 100 10.0 12.17 98.5
400 1.3 20.0 19.5 1.026 40 45
2000 50 1370 1320 1.038 32 40
