A TREATISE OF ELECTRO-CHEMISTRY. EDITED by BERTRAM BLOUNT, F.I.O, ETC.
O Z O N E
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A TREATISE OF ELECTRO-CHEMISTRY. EDITED by BERTRAM BLOUNT, F.I.O, ETC.
O Z O N E
A TREATISE OF ELECTRO-CHEMISTRY. Edited by BERTRAM: BLOUNT, F.I.C.
THE MANUFACTURE OF CHEMICALS BY ELECTROLYSIS. HAUS> B.SC,
OZONE.
By
ARTHUR * J.
F.1C
By E. K. RIDEAL, M.B.E., M.A.,
Other volumes in preparation.
A TREATISE OF ELECTRO-CHEMISTRY, EDITED by BERTRAM BLOUNT, F.I.C., ETC.
O
E.
K.
Z
O
N
E
BY R I D E A L , M.B.E., M.A. (CANTAB.), P H . D . PROFESSOR OF PHYSICAL CHEMISTRY, UNIVERSITY OF ILLINOIS
LONDON CONSTABLE & COMPANY^ L T D . ORAUGE STREET, 1920
EDITOK'S
PREFACE.
T H E idea of a series of books on Electro-Chemistry emanated n o t from me, but from Messrs. Constable. Some years back I wrote for them a book called " Practical Electro-Chemistry," i n t e n d e d to cover a great part of the ground of knowledge t h e n extant. Fortuiiately, knowledge has a habit of growing a n d of propagating its kind, and my book, in consequence of t h i s , became a " back number ". The subject of Electro-Chemistry is so ramified and specialised that it was impossible for one man to make a s u r v e y of the whole field. This fact is the genesis of the p r e s e n t series in which those who have accurate and intimate k n o w l e d g e of the various branches of electro-chemistry have "undertaken the work for which they are particularly qualified. I t will be readily understood that, as the series of books was s t a r t e d at an early period of the war, many contributors were e n g a g e d in work of national and primary importance, and • w e r e unable, however willing, to apply themselves at the xxtoment to exacting literary work. But this difficulty was g r a d u a l l y overcome, as some prospect of a period to the s t r a g g l e came within view, with the result which the reader "will judge with consideration for the onerous conditions "cmcier which my contributors have wrought. The monographs resulting from their labours speak for themselves, and if the educational advantages which 1 have o b t a i n e d from reading them during their passage through t h e press is shared by the public, I believe that the thorough a n d modern work of my friends and collaborators will be appreciated, and such faults as there be will be attributed to t t i e person ultimately responsible—the Editor.
AUTHOR'S
PREFACE.
E V E R since the time of its discovery Ozone has attracted the attention of chemists, physicists, and industrialists alike. To t h e former it presented the first example of a gaseous allotrope of an element, differing from oxygen in many remarkable ways. The physicist frequently came in contact with the substance in his investigations on the conduction of electricity through air, whilst the industrialist was not slow t o avail himself of an oxidising agent, unsurpassed in strength, leaving no objectionable material in its wake, and at t h e same time easy, if indeed somewhat expensive, to manufacture. T h e angle from which Ozone and its modes of preparation was regarded by these three different sets of investigators naturally "varied, and an endeavour has been made in the following pages to summarise and correlate the many different references which are to be found scattered over a wide field of literature. The merest survey, however, was sufficient to indicate that our knowledge of Ozone, its properties and modes of formation, is exceedingly scanty. The industrialist is ever at hand with extravagant claims as to the utility of "electrified oxygen"; the evidence as to the chemical behaviotu: and properties of ozone is somewhat meagre and frequently conflicting, for example, the existence of t h e ozonates and of oxozone still awaits confirmation; whilst the hypotheses advanced to explain the mechanism of £to fom&Mon, either chemical, thermal, electrolytic, or photo-chemical, are purely speculative. Ozone is generally produced by means of the silent electric discharge, the
viii
AUTHOIt'S VUEVM It
Aladdin's lamp of synthetic chfiniMry, tor wh I v* *?* factory "modus oporandi" han hw n »iuv« tui *•
Cniickshank in 1801 likewise d r e w at-
tention to the fact that the oxygen gas produced by t h e electrolytic decomposition of dilute acids under certain conditions was possessed of a similar odour. These two investigators merely chronicled the r e s u l t s of their experiments, and did not pursue their inquiries t o elucidate the origin of the odoriferous substance.
Schonbein,
in a memoir presented to the Academy at Munich i n 1840, recognised fchat the smell noted in air subjected to t h e spark discharge, and in the oxygen generated by electrolysis, was due to the presence of a new gas, to which lie ga-ve t h e name tl
ozone" (#£©—to smell), he also showed that o z o n e was
formed in certain processes of autoxid&tion, notably by t h e action of air on phosphorus, but failed to establish t h e exact nature or composition of this new substance. W e shall have cause to observe, when discussing t h e processes of autoxidation, the development of Schonbein's hypothesis in that ozone or &ctrve oxygen is produced w i t h its
2
OZONE
electrical isomer " antozone" by the disruption of the neutral oxygen molecule— + O2 -> 0 ' ozone + 0 antozone. This hypothesis naturally led to the division of peroxides into two groups, the ozonides and the antozonides, and to an extended search for the two active electrically charged forms of the oxygen atoms. Various other speculative hypotheses were made as to the composition of ozone, all unsupported by experimental evidence, thus, Williamson suggested that it might be gaseous hydrogen peroxide, and Baumert considered ozone to be an oxidised form of hydrogen peroxide, i.e. H 2 O 3 . , Becquerel and Preny first showed that oxygen could be completely transformed into ozone, thus proving that ozone was an allotropic modification of this element. These experimenters effected the conversion of oxygen into ozone by the passage of a stream of electric sparks through the gas, the ozone formed being continuously removed by means of a solution of potassium iodide.
In this
way all the oxygen originally in the tube ultimately disappeared. Andrews, Tait and Soret (" C.B.," 1876) took up the investigation at this stage, and by the following experiments proved that the allotrope was actually a condensed form of oxygen:— A. tube of volume V connected to a sulphuric acid manometer and containing oxygen gas was submitted to the action of the spark discharge when a contraction in volume v was recorded on the manometer.
On heating up the tube to
OZONE 270° C. the ozone was destroyed and the gaseous mixture then occupied its original volume V. Soret showed that no change in the volume of the ozonised oxygen (V - v) took place when the gas was exposed to potassium iodide or metallic silver, nevertheless the ozone was destroyed. When, however, the gas mixture was exposed to turpentine a further contraction in volume was observed, the final
c Sulphuric Acid> In Manometer.
FIG.
1.
v o l u m e b e i n g V - 3v w h e r e v w a s t h e v o l u m e c o n t r a c t i o n o n ozonisation. A s a r e s u l t of t h e e x p e r i m e n t s S o r e t c a m e t o t h e c o n c l u s i o n t h a t t h e m o l e c u l e of o z o n e c o n s i s t e d of t h r e e a t o m s of o x y g e n , t h r e e v o l u m e s c o m b i n i n g t o g i v e t w o v o l u m e s of o z o n e — 3O2 -
2O3,
since t h e v o l u m e contraction v o n ozonisation is clearly equal t o o n e - t h i r d of t h e o x y g e n c o n v e r t e d i n t o o z o n e (or o n e - h a l f of t h e r e s u l t i n g o z o n e ) , w h i c h i s s u b s e q u e n t l y a b s o r b e d the turpentine.
Further, that
when
ozone
reacted
by
with
4
OZONE
potassium iodide or metallic silver it liberated an equal volume of oxygen :— O3 + 2Ag - Ag 2 O + O 2 . Soret ascribed the structural formula O—0 to the tri0 atomic allotrope of oxygen, and confirmed the existence of ozone by a determination of its density.
The theoretical
density of ozone at N.T.P. should be equal to one and a half times that of oxygen, and this value was obtained by Soret and Otto by several different methods, which may be briefly described :— A glass globe of about one litre was filled with pure dry oxygen at a determined temperature and pressure; and subsequently weighed; the oxygen was then displaced by ozonised oxygen reweighed, and the weight of ozone in the flask determined by titration with iodine and sodium thiosulphate. If V be the volume of the globe, containing w grams of oxygen of density J , and w + w' be the weight of the ozonised oxygen in the globe, where 8 is the density of ozone, and v and 10" the actual volume and weight of ozone in the globe, then (i)
w + w = Sv + (V -
(ii)
v)A,
w" = v$,
(iii)
w = AY.
v(B - A) = w + %of - AY « w\
From (i) hence :—
$K
-> = to"
or \w" - w
From two determinations Oto ihtiun^l thr \ tJu. * f d «! ratio ^ or the density of o/.om* wan practfisUly «*IM* un«l a h *lf urn* that of oxygen, I)KNsriY ItV \HV\ I *.n»N. Koret showed that the r.ifcr of trun**jnMtif»n
ihr »»i/l« JI
small aperture of the puivst CI/M»I* wh»« li h* r M*IM «*}fti$ii «»i^ intermediate heiwwn the va!u« H nbUtncd f«*r rhJt or of those substances when charged.
T h u s Stark (" 3?b.ys.
Zeit.," 14, 720, 1913) has shown the existence of two d i s t i n c t arc spectra of oxygen attributable to the substances O 3 & n d 6r The line in the visible red of the spectrum n o t i c e d Ladenburg and Lehmann (loc. cit) is possibly not due to
by
ozone
but to another allotrope of oxygen, viz. oxozone, O 4 ; w h i l s t the existence of a band in the infra red or thermal r e g i o n at X = 1040 [i^i has been claimed for ozone but has not r e c e i v e d confirmation. The lines of the oxygen spectrum at the negative e l e c t r o d e of a discharge tube were examined by Schuster, S t e u b i n g a n d F. Croze ("C.K.," 153, 680, 1916) who gives the f o l l o w i n g : X = 685*3 pp., 662*5, 603*2, 564*6, 529*6 and 498.
Schuster's
two negative bands X = 570 - 584 fi^ and X = 601 - 5 9 6 IAJA could not be resolved. The lines of atomic oxygen O are found in the e x a m i n ation of water vapour as well as in oxygen s u b m i t t e d
to
intense electrical discharges. Fowler and Brooksbank ( " l i o y . Astron. S o c , " 77, 511,1917) have likewise shown t h e
pres-
ence of lines of this series, the third line spectrum of o x y g e n X = 559*2 fip, and 39618 in stars of the /3 type as w e l l an in Wolf Eayet stars. The ultra-violet spectra of oxygen and its a l l o t r o p e s are of special significance in the consideration of their p h o t o chemical interconversion (see p. 70). That of ozone has been examined by L e n a r d (" A n n . der Physik," 1, 480, 1900), Goldstein ( " B e r . , " 3d, 304, 1 9 0 8 ) , and more especially Eegener (" Ann. der Physik," 2 0 , 1 0 8 3 , 1906).
0201STB
11
The 0 2 molecule gives short -wave length bands resolvable into lines between the region X200 and X 188 /^//-corresponding to the ultra-violet fluorescence of oxygen.
Steubing
noticed five "bands between A = 183*1, and 191*1 ftp, whilst L. and E. Bloch ("C.R.," 158, 1161, 1914) isolated two new ones conforming to the Delandies formula at X = 192*3 to 193*6, and 194*6 to 195*7 pp.
The ultra-violet oxygen atom
0 band in the region A = 230 /JL/UL and *X = 340 pp is observed in the positive column in pure ratified oxygen, and in the decomposition of dissociation of many oxygen-containing compounds.
The strongest band (see Meyerheim, Grebe,
Holtz and Itowler, " Proc. Eoy. Soc.," 94, 472,1918) is found at X, = 306*4 fifj,, a,nd is usually attributed to water vapour. Investigations on the carriers of positive electricity by Sir J. J. Thomson and Iris co-workers (" The Carriers of Positive Electricity") have revealed the presence of a great number of allotropes of oxygen which give rise to their respective band spectra.
E. Horton (" Phil. Mag.," 22, 24,1911)
ha,s sliown the existence of carriers of positive electricity in oxygen of electric atomic -weights, 8,16, 32, 48 and 96. Becq[uerel has shown that the magnetic susceptibility of ozone exceeds that of oxygen, and that the ratio of the specific magnetic susceptibilities exceeds that of the ratio of their densities. CHEMICAL PHOPEBTIES.
Chemically, ozone is a strong oxidising a,gent, capable of effecting the oxidaution of all the elements with the exception of gold a n d some of the metals of the platinum group. I t liberates iodine from potassium iodide and brings about
the oxidation of numerous substances such as lead sulphide, manganous salts and ferrocyanides, reactions which form the basis of its qualitative and quantitative detection and estimation. The general reaction may be expressed by the equation :— M + O8 - MO + O2. In some cases, however, oxygen is not liberated, but the whole 1
of the ozone reacts and no free oxygen is evolved.
Thus
sulphur dioxide is oxidised to sulphuric anhydride by ozone according to the reaction :— 3SO 2 + O3 = 3SO 8 (see Brodie, "Phil. Mag./' 1894, and Eiesenfeld, " Z e i t . Elektrochem.," 17, 634, 1911).
I n the combustion of the organic
matter in water during the process of sterilisation by ozonised air this reactivity of the ozone molecule as a whole is likewise noted. Eiesenfeld (" Zeit. Anorg. Chem.," 85, 217,1914) observed a similar series of reactions in the action of ozone on sulphur compounds.
Three atoms of oxygen in the ozone molecule
react with sodium hydrogen sulphite, whilst with neutral sulphites and alkaline thiosulphates only two atoms react, the third being liberated as oxygen gas. With certain peroxides, such as hydrogen peroxide, it undergoes decomposition as follows:— O3 + H 2 O 2 = H 2 O + 2O 2 , Eothmund ("Monatsh.," 38, 295, 1917) showed that the reaction was unimolecular in excess of hydrogen peroxide, but in dilute solutions the ozone underwent catalytic decomposition.
OZCXSTE
13
It has found many uses industrially as an oxidising agent, which will be detailed in a subsequent section of this volume. Eeference, however, m a y b e made to the deodorising of air, the conversion of manganates into permanganates, of chlorates into perchlorates, and the " d r y i n g " of oils in the preparation of linoleum and varnishes. At suitable temperatures selective oxidation of undesirable substances which give an objectionable colour or odour to many fats and waxes may be obtained, and such processes of bleaching are receiving extended application.
Attempts have
also been made to accelerate the ageing of spirits and wine "by fractional oxidation with ozone. Ozone is a powerful germicide, as was first indicated by Frohlich.
I t s high germieidal activity is doubtless due to its
oxidising power, and as a dual agent of this character it has been fairly extensively employed for the sterilisation of public water supplies, for the treatment of wounds in hospitals, and for various purposes of sterilisation and preservation in industries, such as hide preservation, cold meat storage and t h e like.
-Although ozone in high concentrations will effect
t h e sterilisation of air, yet such concentrations as are necessary (ca. '05 per cent.) are not eatable of respiration without damage to the tissues, consequently its chief function is as a deodoriser and "freshener "for air in confined and crowded spaces. In the realm of organic chemistry ozone has received application in two directions, firstly as an oxidising agent of great strength "which introduces no foreign matter, and secondly as a reagent for the ethylene linkage - C = C - . A.s an oxidising agent it is employed for the preparation
14
OZONE
of vanillin on an extremely large scale.
The
production
of
other substances, such as heliotropine, p i p e r o n a J , a n d a n i m a l n < » hyde, can also be accomplished with its a i d
C s e $ c h a . p - I2w-
Apart from its powerful oxidising properties, ozone with certain substances in two definite a n d
will x r e a c t
character! niic
ways to form ozonates and ozonides. T H E OZONATES.
Baeyer and Villiger ("Ber.," 35, 3038, 1 9 0 8 ) s t a t e
that
strong ozonised air fumes in moist air c o l o u r s b l u e l i red, and causes an increase in the c o n d u c t i v i t y of water when passed through it.
They t h e r e f o r e
regcurrlcsd
ozone as the anhydride of an unstable o z o n i c a c i d , According to these authors, if due p r e c a u t i o n s a r e highly coloured ozonates may be prepared b y t h e of ozone and moist solid alkali hydroxides o r
U^Of. ta/keti,
interaction concentrated
solutions of the same at low temperatures. The ozonates are usually orange or b r o w n .
If o z o n o
passed into a cold ammonia solution, it a c q u i r e s a d a r k colour attributed by these investigators t o t h e ammonium ozonate, N H 4 H 0 4 .
formation
Lithium o z o n a t c w a s
bo vot\ of
found
to be least, and that salt of caesium most s t a b l e . A white granular precipitate of calcium p e r o x i d e is f o r m e d on the passage of ozonised air into cold l i m e w a i t e r . According to W . Manchot ("Ber.," 4 1 , 4 7 , 1 9 0 8 ) , B a e y o r and Villager's results are to be attributed t o
the
presence
of small quantities of oxides of nitrogen i n t h e i r
ozonised
air, since he found that ozone possessed n o
acidic
OZOETE
15
T H E OZONIDES.
The ozonides are formed by the interaction of ozone with organic compounds containing unsaturated ethylene linkages according to the general equation :— —C !l + 0 3 _C
->
— C—(X I >O — C—(T
Discovered by Harries (" AJIH.," 343. 311, 1905 ; "Ber.," 38, 1195, 1905), this reaction was successfully employed by him to elucidate the composition of rubber (see p. 170), and has of recent years been frequently utilised to identify the presence of ethylene linkages. Where
compounds containing
ethylene linkages are
treated with strongly ozonised oxygen (ca. 40 per cent. O3) the ozonides thus formed on analysis yield more oxygen than is to be expected by the assumption of simple saturation of t h e ethylene linkage according to the equation:— _C II + O3 -»
—C—O x I >0
_G
—C—O—0
_c _ c—or According to Harries, oxozonides are formed by interaction of the organic compound with oxozone present in the gas:— _c —c—0—0 11 + o4 -> 1 1 More recent experiments, however (see p. 184), have failed to establish the existence of oxozone in ozonised air or oxygen and consequently some other explanation for the formation of oxozcnides must be advanced.
CHArTEB II. THE NATURAL OCCURRENCK OF OZONK.
THE occurrence of ozone in ordinary atmospheric air ha* long been a matter of dispute. C. Bchonboin (" J. f. Prziht' Chemie," 73, 99, 1858), as early as 1858, showed that wtarrh iodide paper, when exposed to the air, slowly turnrri t#It**\ thus demonstrating the existence of some oxidising aff«*m\v. He noted that the rate of liberation of iodino varied from tiny to day and attributed this to an alteration in tho. ozono
o^finiwi}
waii*r ; In* HXAU'H Unit tlit* arid with-ru of Hn^noitr, M«mt«4 r^ highly iontM
f« it
u
H
ir/,"
1
4, 345, IWKM Unit rh« drMJ ' of lin . i d n «n«J (< imd that tin* tum*'nitT AtljiiHt«r C* H«*r..M 35, J324, and 2902, JiK)*2|t whinh is (1IA^UU'!Iuhl btuh to an «' |uiJifti ititu amount of 1*5 \n*r ct*nt. by w* it'ht at 'iojs c1, Utiiining an arc in lujuid air a mixture «»f u/um 4 and iu(ri>gen pcroxidi* wan obfautwi whifh fi^jti«*T*tly **x|>l««l**i \%h«n attamptn wort* madr to w pnrat« Uu* r< Huhiui o x y p 11 Tho maxsmuin yi^W t»( u/^yir ohtAtnt «I by mt»im fif a glowing Ncrnst rUaiucnt in li«jutd tmyn^n WII^ 10 i?4i:ii4^ in twonty-fiv« minuivH with a currnit ruiiHiaiitptioti nf t) 25 atnpcrus at IOC) voltrt I*IJII:U in n, JJIUI rif I! ft i^rufii^ *4 o/**n»* per kilowatt hour. Kinchor having thiiM di*mon«tri*i«v4 tl»: thermal
prmlurtum
of oxomj with thu aid of h*juid air f {iroc«*t«cirtl t« t ^ t r n d ClomontH* exprrimeiiti on thf* producttun of 07,01m by pit^im^ air at a high flow mU* over a glowing Nirnut filni?ii*iit,
Wn
hava already noted that CitmiimU wa» not *!•!«• t
bnj'ht
pUtinum,
gold or platinum iriilnuu wwe found inn f MntuMe, sjnei* other Hod rude m n t i r m l s sneh ;*•• *il\er, bhek
|»" itintini,
f
or oxide anodes, nurh ;t? I* uH, irun or ruunj an* ^\ rxwt a vrry considcTablr activity in th«* rutalytir d* rnnjjin»,it}on ? ,ltf r
in TITHIS if 1 $ , ten*
HM
n
If, v:
11.^04 + o, HBO/ -> (HSO4)ff
In 18S9 Nernst, by tho introduction of the conception of electrode solution pressure, pointed out the method of determining the influence of the anode potential oa the discharge
of anions without having to tnk0" + Jt log CO").
T HT. CO" ,. log
Further, since in aquoouH Hulutions yuO" + 2/tII tr
t/
HT.
which giv^K an c*xprckHHtc>n for the variation of the oxygen electrode potential, with alteration in the hydrion concentration of the solution and the prcmnure of the oxygen gas. The value of V» in approximately - 1*35 volts, whence this value of the cathode1 potential for a hydrogen electrode in normal hydrion Holution under a pressure of one atmosphere in 4 i'OB - KW *- - 0*27 volts. If an oxygen eioctrode ha set up against an ozone electrode, the difference ia potential between the two electrodes COD he calculated in a similar manner and found equal t o : —
Luther and Ingli* ("Zoit. I Phyaik. Chem.," 43> 208, 1903) first attempted to obtain an approximate value for V / by
C)4 measurement of the* potential diitVrenri! h«-tw
Salmi'ijtifnt invi*»»tijL*-ttorH hn\r
Uminl ctm-
Hidorahly low«*r values: X«*riiht {" Zrit. Kli*ktr»^rhi'iu.,M o» HiJ, 190*i) ohtmnvil volts.
t)w
valut? V -
0 5 7 , aii«! F J ^ ' I H T AIK!
I t would apjM'ar from tin*«-xpfritta ntn of th*M* l:itt«*r
obscrvcrn on the thermal i*r|nthbriittu, that thf l*nvir valm% viz. - 0'5() voltn in probably mart* com*ft.
ft in pm^iblf?
that the* higher valwn obtain«'d by the t-arliKT i-xpr-ninrntrrH were ocramomd by t h r priwriftMtf ox«>£«*n«* O# in thr round th« ricctroch*, ami 11 rrinvf*itij{ati«in of thin < iu reaction in fi Mcl#i'oci*ti r'^nltH t#y a the artifice of watf*r-wotity, \t\
in* fli«*4 «f *ut]*\uyn$
mul, ur r4«+tir pt4»t*h or
a, an tin* t»Ii«firnlytf* In thifi way, llw Hirtmlyt** wan $JJV# n »nlJi 7
»>
'{»y nr^n«'lii^s*#iiH w» r«* IJI-4 Hilirdy corrrd.
K. Wurtntrg ( " A n n
*Ur l'li\«nk/' i j * Ull, 11HJI)
allowed that provitlwl that th« jx^^tits'*} «ii?l« r* ur»* upphni wa» stafficient t^> uusmtjiin a iinif^riii ;;low «l lit* pnml at thu air gap, ihi* yioW of ri^4t ;»** ninj.t?*4.' I M t^i tint manintu«b* of tin* •'l«Tlroi»' < tx:*rtHf-*i i i f w r i i l * * ! %%i!li th«* frjr« mation rjf a nifih*rtt!f *4 ^ / ^ m * , hut $t %%ill h< * ^i^l* tit fh?ii th»* niftximum yi**!*! f'*r tit* m m m u m on tb
, f?» VMVJ,, nnA t\
Utrhurr
u
( Z « i l . Klrktrurht'iii./' 17, II t, l u l l ; a t . aoti, JUi:i;. r
r h r following in u hrirf
T h r immr
yi« Id jHri'Mtilnuj!* n v . ni fir^i J ijndly with tin*
|H*riodirity *?f thf* Jtltrriiatin;* r i n i r u i :ind Ui« i« nftx r niojv hlnwly.
l » p t o r # u u |t* n««ls linvr JM tustliv l«st n 1 inph*}r4
ti*chrut*al inHtalliifioiiM.
in
r
Il j * i\id**nt fr^ia i r u n id* uUion nf
thi* fr^riu of flu- «uir IMIIV* t)i it *%u m* wvtttii*
j?« nr thi* ml** «*f »H« r.u^iit ^1 th*< n n j « nt A hi)j[h j«Tic*dnitv lik« wr«n J^miin
tin- ii$iriiiiniii4 }^t« n t m !
tu pjodtMv n mini! ih»«rh iri-t* ;%*'t***
M
upj»* r l i m i t , *i
lir>*«* f l u
t»rM4july
Th«* 4t'»iu i «
ing
Cmiceniratiofi* in Um*. per Cubic Metre.
10 Ota*
tJil
1*40 1-40 %i»*ti) ft •Oft
2-20 4-M
10,(MI lO/JOO 17,600 10»B00 13,WO {1 480 12'»00 9,S40 12,100
50 60 50 50 50 50 100 100 510 510
0-182 0*102 0*193 0*160 O'1B2 0*1 «39 0-30H 0-280 1*5B
0*185 88-8 4MI 0-8H 0-24H 65-i 0-415 6 M 60-1 7B'4 5S-B 02*4 0-451 75*7 81-4 0-447 54-0 m-o 57*1 06*0 0*704
i
6S'O
45-5 50-8 00*1 82*0 6VH 88*2 6ii*0 7li) 74-7
50-2 81-2 40'5 20*2 01*0 W-H 18'8 11-4
110
030NK
is
J5
/s
io
Cubic Mttrts p*r
$0
h 20 to
7B
35
f*§ ptt H0y With II,
w ft*it 40
N BY SILENT ELECTRIC DISCHARGE
111
It will be noticed that the conditions most favourable for the economic production of ozone obtain with low concentrations? of ozone or relatively high flow rate of air.
High air
flow rates likewise nerve to keep the ozonisor cool, an importtant connideration since the catalytic decomposition of ozone in con»idorably accelerated by high temperatures. For technical operations the air-flow rates are accordingly adjusted as to give the minimum concentration of ozone which will prove effective for the process under consideration ; for under thene conditions, although more energy must be expended for pumping air, yet a very considerable economy in effected in the ozone production. The concentration*) of ozone and the yields obtainable per kw. hr. arc higher in oxygen than in air, but the employment of oxygen instead of air does not prove to be economical in practice^ although concentrations up to 150 gms. per cubic metre, or nearly three times the maximum concentration attainable with air, can be carried out. The yield, however, doeB not increase indefinitely with the oxygen pressure, thug, If. von Wartenberg and L, Max ("Zoifc. f. Klcktrochom." 14, 879, 191H), operating with an cmmiHuv constructed to withstand high pressures with an intcr£>oiar apaeuof from 2 to 6 mm* and a current of 1 milliampnrc at 2ft,000 volt* and 50 periods, obtained the maximum ozone concentration and ozone yield per watt-socond at 0 5 to 1 atmosphere.
Pressures up to 5 atmospheres were reached
during the course of their experiments. It will be noted that 00 gms, of O$ in air, and 180 gms. of omttB in oxygen per kw. hr. represent t h e bent results yet obtained with ozoaisors operating under the optimum
112
OBONT.
conditions.
Taking 94,000 calories as f ho heat of formation
of ozono, this represents a theoretical yield of \"i kgm». per kw. hr., or imluBtrial ozoni«ei\s have an efficiency of only 5 per cent in air or 1/i per rent, in oxygen. Air suitable for ozoninaiion should be free* ftmn
dust,
which favours the paH«age of sparks, and from certain gaseous impurities such as oxides of nitrogen, chlorine, and mojv especially water vapour.
Allihret* p i *e * rt]ip< ar ! i xerf a
distinct inhibiting effect on the* formation «>f (Mi**U of immn which in only fit) to 70 j«r cent, of the air when d r y ; in the* prcinence of tuomttir** likewwe th«^ Un million t>t oxidas of nitrogen, dun to \Ua tin ruial fff»*rtH «»f ^j»i*rkin^ im wall an the pusKibiii intemcticm of f^om$ with hr#in#* forrtt of active nitrogen produced in the Hpnrk fli^b»rK*% w ii5*tiiilly occs^ioncsd.
T. Lowry ("J.G.K," iot, 1152, Hll'2), in an in-
teresting rmearch on tlio rffcict «^f this Hil««itt and xpark discharges on nir, Hhowecl that in dry air oxic!«*n of nilrogf.n won* not formed under this cxprritfiimUl condittonit by |m«ing«i through the ozoniftar or the; iij?irk dinchftrge g ^ p .
Whan
PRODUCTION BY SILENT KLECTKIC IHSCIIAEOTi
113
passed, however, through both in series, or when the air currents subjected to each discharge were mixed, oxides of nitrogen were produced. Lowry came to the conclusion that in the spark discharge an active* variety of nitrogen was formed which was easily oxidised by ozone. DIELECTRIC
MATNIUAL.
W e havo already noted that the yield of ozone per kw. hr. at a definite concentration increases with the increase in capacity of the ozoniser, but that t h e limiting yield decreases. In addition it must be remembered that extreme variation in the size of the air gap or interpolar free space is not permissible, since too small a gap will permit the passage of sparks and possible arcing with minute variations in the applied voltage, whilst with a large interpolar free space the luminous discharge, on which the formation of ozone appears to he largely dependent, will not fill or " s a t u r a t e " the field. Tt is evident that the use of dielectric material other than air, by which alterations in the capacity and interpolar distances can readily be made, offers the designers of ozonisers a very considerable latitude in these factors. For tho purposes of calculation we may take a Bimple plate form of ozoniser and considor the effect of inserting a plata of dielectric material in the air space betweea the two metallic electrodes.
Fio. is. 8
OZONE If the two plates are charged with a quantityof vU-rtru-ily of surface density ( )' the interpolar this means
distance
w e have
of
air
space
augmented
the
r e d u c e d
t o
c a p a c i t y
a n d
t h e interpolar distance of t h e o z o n i s e r , a n d efficiency;
at
the
same
time the
t e n d e n c y
t h u n t o
a
inert
h
d««eri»»M«J a^ad
s p a r k i n g
r a p t u r e of t h e g a p b e t w e e n t h e e l e c t r o d e s i s diiuinirthirtj, the
mechanical
force per
unit area
is
l i k e w i s e
J5y
r e d u c e d
it« a m ! sin
nincr tt
PRODUCTION BY SILENT ELECTBIC DISCHARGE
115
t h e possibility of the flow of currents of high densities naturally excluded. T h e following are the approximate values of the specific inductive capacities of the more common dielectric materials, dry air being taken as unity :— Mate) icil. Paraffin wax Rosin Ebonite Sulphur Glass
K. 2-3 2*6 3-2 3-8 6 to 7
T h e choice of dielectric material is naturally limited^ since _ it has to withstand both high temperatures and the destruc-^ tive oxidising action of the ozone.
Amongst those which
have been suggested may be mentioned:. shellac, mica, quartz, glass, and artificial insulators formed by condensation of phenol and formaldehyde, the so-called Baekelites; glass, however, is"the onl£ material whichJhas received extended" techy ical .application. T h e effect of insertion of a solid dielectric in the interpolar space is, however, not so simple as indicated by the above considerations, since like other materials, not only are they imperfect insulators, but many of t h e m possess the interesting property of acquiring residual charges.
W e may
regard the dielectric medium to consist of a number of conducting particles embedded in an insulating material, the fraction being smaller in the case of the more perfect insulators.
If this fraction be denoted by u> then the specific in-
ductive capacity can be calculated approximately from the relationship:—
110
OZOXK K
" v~4ir
w being determined from the molecular specific volume .. When a Htrip of dielectric material in charged up to a high potential, after discharge it will be. found to acquire a Hinall charge on standing, which in often Htiffieiently great to raine, tho potential of the dielectric up to *i(H> voltn.
Thin
property of acquiring a renidual charge in only poHKCHM'd by thoHo bodies which poswmH the* property of exhibiting clastic after-effects, it in never shown by nimple nubhtanctm, but only by mixtures such as the glanse.H; ihiiB xyirnu and paraffin oil alone do not nhow thin effect, but on mixing f hi* two, the residual charge in apparent.
One of tho eomtituentB muni
alno poKBOBS a certain umouut of electrical conductivity. I t is interesting to note that KioHcnfttid (" Zoit. Klcktrochem.,M 725,1911) failed to obtain a brutth dineharge with puro quartz glaBH, althr>wgh nneh di»chargc»B urit ri^iidily obtained with all forms of glasn which rimy contain quit*' large percentage** of mliea, attribufablo to the xituht i*tcutricni conductivity of tho glatis, and itn |K)»«c?HHifm of a r«»id«nl charge. Two other important proportion of dielectric tnak*rial« must be briefly alluded to, namely, the u l t o m t b n in conductivity with elevation of the temperature, and thti i which the dielectric undergoes* whan iiubjccttsd to ul stress. It is well known that the, conductivity of glaiut at civtin slightly alevatad temperatures rapidly mereancm.
Xfond and
Langcr and Huber hava actually umtd holiA ghmm electrolytes at temperatures between 200" and 50G* € ,
tm
PKOJDUOTION BY SILENT ELECTRIC DISCIIAIUJK
117
o v e r h e a t i n g a t o n e p o i n t i n t h e d i e l e c t r i c , d u e t o a s l i g h t irregularity in the current
flow,
w i l l t h u s c a u s e a n i n c r e a s e of
conductivity at this point, with a corresponding a u g m e n t a t i o n of t h e c u r r e n t . results.
F u s i o n and
finally
p e r f o r a t i o n of t h e g l a s s
I t i s for t h i s r e a s o n t h a t p o r c e l a i n s , w h i c h p o s s e s s
t e m p e r a t u r e c o e f f i c i e n t s e v e n h i g h e r t h a n t h o s e of t h e g l a s s e s , a r e u n s u i t a b l e for d i e l e c t r i c m a t e r i a l i n o z o n i s e r s . K e r r n o t e d t h a t t h e o p t i c a l p r o p e r t i e s of d i e l e c t r i c s w o r e c o n s i d e r a b l y m o d i f i e d b y t h e a p p l i c a t i o n of e l e c t r i c s t r e s s e s . T h e s e m o d i f i c a t i o n s a r e i n f l u e n c e d b y t h e p e r i o d of t i m e for w h i c h t h e s t r e s s haft b o o n a p p l i e d , t h u s F l e m i n g ( s e e " A m o r . B u p p L , " 4 5 , 1 9 1 2 ) s h o w e d t h a t t h e c o n d u c t i v i t y of a d i e l e c tric varied
with
the
frequency
of
the
applied
alternating
current, and L i p p ( " H o c h s p a n n i n g T c c h n i k " ) found
that
t h e a p p l i e d voltages* n e c e s s a r y for p e r f o r a t i o n of t h i n s h e e t s of dielectric material varied w i t h t h e period to w h i c h
the
dielectric material had b e e n subjected to t h e electrical stress. T h e p t T f o r a t i n g p o t e n t i a l s for t h e usual d i e l e c t r i c
materials
a r e a p p r o x i m a t e l y an f o l l o w s : — Perforating Potmtuih \ in KUoiZu j * r Cm. 000 to 750
MICA
Micanito Forc«Iaiicj ttlami
400 „ 500 100 75 „ 300
!*a4g!*si»
1000
Air (" Atnftduxati H. Cinuuita," 3 , 5i f 1012)— 7000 volti p«r 1*5 am. 07,000
t,
„ 13-6 „
100,000
„
„ 24
„
T h e p o t e n t i a l d i f f e r e n c e n a c e s n a r y for d i s c h a r g e b e t w e e n t w o c o n d u c t o r s in w
v a r i e s w i t h t h e nme a n d s h a p e o f
UR
OZOMK
conductor, point discharge taking place* much more r* adily than discharge across plane surf act IH.
T h e following fjjjjurei
(Abraham and Villard, " Physical Constants," HUOj incliniti* the potential difference required to cause a «M) mm, sp:irk to Rtrikc between two spherical electrodes of varying radius :- liattimin ~*ttn. a (plant) ino
0 (point)
1'uttntitl Inffmnuv. K:*,7i*l «I,JU>
.10/**)
The rulaiionship hetwren the jjofential dilfereure :uj«i tlie striking distance, in also not a Hunpli> one, an inriicatcd from the experimentally derived figures for j*pheri^al eltttiraden 1 cm. in radius - Distant e in (linn, 0-1 i) 0*40
ht/«»t lf», ii r\ n
For very »mall iniorpolar distaneen, say J to 50 ^
the
ag(? necessary is independent of th«? dintiuico and i rjmil to about 350 volts (K. Willianw, " Phyn. Chern,; f J I , 210, HflO). O. Hoveda (" Phyn. Hcv./' 34, 2.1}, PJP2) «iviw thu follow ing relationship for point to plane,
where a, b9 cf are conntantM, and I) is tint interpolitr The um of minute points corrugated or roughened metal electrodes in industrial o&rmiHcrH m very frequent, Kinc*«% an we hava seen, the presence of points faei!ital'*H tlir*
PRODUCTION BY SILENT ELECTRIC DISCIIABCKE
119
L . Becombe (" Jour, do Physique," 2, 181, 1912) has attrmptcd to calculate the energy dissipated in a condenser when connected to an alternating current source; ho shows that the energy absorbed, i.e. VSq (where V is the applied E,M.£\ and q the charge on the condenser), can be expressed in the form :— VBnv + R«SV, where m in the polarisation and Ro a constant, this is equivalent to the dissipated energy:—
or the dissipated energy is proportional to the square of the polarisation
current and
independent of tho periodicity.
V. Khrlich and V\ HUBS (" Zoifc. Elektrochom.," 19, 330,1913), as a result of an investigation on tho measurement of tho electrical quantities in a Siemens ozone lube, showed that tho ionisation or polarisation current and applied potential were always in phase, and that the potential
difference
acroHtt thu gas gap wag a direct measure of the energy. OhaHBy ("Jour, do Physique/' 2, 876, 3912), on tho other hand, showed that the energy absorbed per second by a gas under alternating fields, varied as tho charge Q and not as the square of the charge, as in metallic conductors. An increase in conductivity of the solid diolectric is also to bo expected from its exposure to tho ultra-violet light generated by tho brush discharge in tho interpolar air gap. A. Goldmann ( " A n n . der Phyeik/' 36, 3584, 1911) has shown that solid dielectrics exhibit both an increase in conductivity and a negative discharge of electrons similar to the effect in metals whan subjected to ultra-violet
J20
OZONK
irradiation; the conductivity of Kulphur in Miid to inert 1500 times when thus illuminated. of Industrial
O'uniMf^.
Industrial ozoniHcra may lx» grouped info t u » dixtiuct types: thoHo in which the nilrnt discharge p a w n HCTCISH thu air gap without the interposition of any Holid diH«u:tri protlutu* m\ without any dielectric and although !arjj«» unitn on varti>UH KyBtoins have Ixtcn conntructed from iiniu tw tiitti*, tlwir efficiency has usually been cxtr^mc'ly low and at tin* pri'hf-nt time all industrial o//>ntHorH contain dit*la«* din*
charging surface connisted ul a uhret of platinum gati/.if to provide a great number of juimll pointa HOim: 90 mm. fmtii the other nurfaco formed of perforat«*d mistal nhfrt, !hi* air current being forced through th« porforatiutin in the hhi*««t. T h e two electrcnleB w