Fermentable Sugars from Unfermented Reducing Substances in Cane Molasses %
C-ARL EKB .iXD F. W. ZEKB.4X .\ew I b r k Sugar Trade Laboratory, .\hw
In a stud? of the acid h?drol?sis of the unfernientable reducing substances in cane iiiolasse- it ha5 been found that the hydroljsis proceeds siiiiultaneousl~ with the destruction of the reducing sugars formed bj the reaction. The net result is an increase in reducing power to a maxinium, followed b y a decrease. The hydrolytic effect of the acid and its destructiie effect both increase with time, temperature, and acidity, hut the former decreases with rising concentralion of solids, \+ hile the latter increases. The formation of fermentable sugars by the hydrolysis of the unfernientable reducing substances has been dertionstrated by the higher alcohol Field obtainable.
S
.ITTI,ER and Zerban (8) have s h o w i that the greater part of the uiiferment,able reducing substances in cane niolasses is ,icrivcd from the reducing sugars in the cane juice. -4t the temperature prevailing during the manufacturing process these sug.irs are initidly converted into anhydrides, so-called reversion produrts, and also condense with the amino acids present in the juice. It was found t h a t fructose yields anhydrides more readily than glucosc. Both glucose and fructose react n-ith aspartic acid, glutamic acid, asparagine, and glutamine, which are the principal amino compounds in sugar cane. Upon prolonged hc,ating, the anhydrides and nitrogenous condensation products undergo further changes by progresiire condensation and polymerization, with the further loss o f water and, in the final stages, of carbon dioxide. The resulting dsrk-colored substances, termed "car:tmels" when obtairied from s u p a ~alone ~ and "melanoidim" when formed from sugars arid amino compounds, represcmt complex mixtures of products of varying degree of anhydi,idization and polymerization (11 I . They reduce Fehling solution, but their reducing power is smaller than that of glucose anti fruct ose. Kohl (9) discovered that high solids concentration favors t h e lorm:ttion of rrversioii products from reducing sugars, but, t h a t at low concentration the reversion products are partially hydrolyzed by acid to the original sugars. Later Reindel and Frey (51rcportcd that the uiifcrmented residue from cane niolasscs after hydrolj-sis wi?h acid has a higher reducing power than befor(,, and that a. second ferinrnt>:Lrion dtcr porrer. They ascribed these results t o the presence of a .sugar irhicli is diffkultly fermental)lc because of the high concent,rstion of ash and 0thC.r inhibiting subttziicrq. I t t e m p t s t o identify this hiigar failed. S:tttltTr and Zerbnn (8)pointed uut the similarity of the hytlrul>.sis curves of the unfernien re.iduc from cane iiio1:tsse.j irith those obt:tinetl upon hvdr of the unfrmicntable substaiiceh produced by hea.ting f i e and glucose solutions in nce or a1,serire of asparagine. L*pon prolonged heating ivitli acid the reducing p o w r of the hydrolysis products tended t o u w i l n ni:iriniiini, and i r i some c the curve descended after litiviiig re:achecl the mariniuni: this indicated destruction o f products. The prwclnt investigation TTRS undertiilioll t,o jtudy the effwt of temperature, acidity, and time OII t iic' murse of tho rcwction, t,o tlctermine under what, conditions
I-ork, ,\-.I-.
t l i u m:t.;iini.uii reducing poit-er ib r c x h e d , and to iiriti w l i c ~ t h c r d rctlucing power is act,ually due t,o the fcirni:iiifiii of iugitrs as disclosed by increased alcohol product ion. jiriiilar hydrolysis experiments have previously been 111:tde with p)l>-ixchavidesoccurring in nature. Saeman ( 7 ) sIi~i\r-ed that t hi, acid hydrolysis of cellulose and the decomposition of
the glucose formr~dfrom it are essentially first-order reaet ioiis. The reducing values give a curve TThieh ascends to a maximurn and tlicii descends again. The net sugar yield can be cxleulatcd from the velocity constants of the two reactions. K i t h cotton cellulnqe and wood cellulose freed from hemicklluloses, the observed sugar )-iclds agreed quite well with those calculatcd, but crudr cellulose slioned a marked difference betn-een the rate of hydrolysis of the portion that was easy, and t h a t of the portion that v a s difficult, to hydrolyze. The hydrolysis of starch and the reversion of the glucosc formed from it h a w been stxilied extensively, and it has been found that t h e owr-:ill renction is complex (3). Even if corrections arc :ipplic,d for secondary reactions, the rate of hydrolysis is a t the lifgiiiiiing gre:ttcr than, then equals, and finally becomes sniallcr than that calculated on the basis of a first-order reaction, a w i n i irig a strictly random hydrolysis of equivalent 1inkagc.s iri the, starch molccule. 1-arious t,heoretical and empirical cqu:ttioiis have b w n formulated for the coursc of the reaction, but' either t h r theoretical equations have not been verifitcd expcriniciitally or the c,nipirical equations arc valid only over a limited r m g e . SndovyI ( 6 ) observed that when starch is hydrolyzed unriw givm conditions the amount, cf dextrose destroyed is very much greater than when pure dextrose is treated n i t h acid under the b m i r 8o found that, at high inilia1 concwtr:ttion tion of the glricwst, formed is mucli g w i t ( l r coiiccritration : this confirms the> c~:ii~livr ohvi,vations of Kohl. of inulin, Jackson, Silsbc)cb. : t i i d 111 a study of the hydrol: Profitt ( 2 ) found that the rcactiori is approsimatc~lymo11onloleeular, although there n-eresmall, erratic variations in th(4 vctlority coiistnnt. Lawr ILathews and Jackson (4)investigatc.tl t h c stahilit>-of fructose under varying conditions of tcniperaturc, and aciditv. The results 6hon-ed that the destruction of this sugar i r ~ l l o t~hsc first-order. equation until the reducing p o ~ e has r EdleIi t o about 8 0 7 of the original. If t h a t rate weri: m a i n t a i n d , the cvluilibrium w l u c would bc r w c h t d a t a reducing pon.er of alwut 7iC, of the initial value. This equilibrium figure is practically constant b e t n w n pH -2.6 and +1, falls t o ahout 73"; :it 1111 3, a n d tht.11 ht~giiist o drop more rapidly. The actual C O U I ' ~of~ thc' reaction :iftrr the initial decrease of reducing power t o 80% i 1 vuluc WLS not investigated because tlie sulutioni; . tlari; as a i.esult of secondary reactions. rnientetl reducing substances in mo R highly crimpier niiiture of reaction products t,wo sugars, glucose ant1 fructose, and possibly mannose arid allulose, and a t lrast four amino compounds, aspamgirie, glutamine, aqpartic acid, arid glutamic acid, and possilily others; consequently i t cannot be cxpected that, the course of t h e reaction upon treatment with acid can be expressed by a simple formula. For this reaimi any conclusions dran-n from this investigation 1597
1598
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 39, No. 12
The hydrolysis experiments were carried out wit,h solutions of three samples a t four temperatures--40°, 60°, SO", and 100" C.-and at three acidities-pH 0.2, 1, and 2. The heating was continued beyond the maximum hydrolysis point until the reducing power reached again approximately the initial value through destruction of the reducing substances formed. An amount of material containing the desired quaritity of solids was weighed into a 500ml. volunietric flask, about 300 ml. of hot water were added, arid the flask was shaken mechanically until all the soluble material had gone int,o solution. The volume was completed a t 20°, and the solution 'filtered from the small amount of insoluble residue, FilterCel being used as a filter aid. One hundred milliliters of the filtrate were pipetted into a 250-ml. flask, and 20 ml. of hydrochloric acid rvere added of such strength as t o give t h e desired pH. The necessary acid concentration was determined by preliminary experiments. The flasks were then placed in a constant temperature bath for increasing lengths of time. For the 100" temperature a briskly boilBO , 1 ing ryater bath was used, which had a basket 5s ! l i 1 1 SAMPLE A of 0.5-inch wire screen inserted in it t o ensure i 1 HYDROLYSIS AT p H l l . t,hat t,he flasks were sl\yays immersed t o the 60 ' -{opt -eo'c same depth. The bath was provided with a 60°C --40°C siphon arrangement t o keep the v a t e r at a constant level. Four Bunsen burners were placed under the bath for the purpo-*e of uniform heat,ing. The experiments a t 60" and 80" were carried out in a water thermostat, and those at 40' in an incubator, which vias also used in some cases a t 60'. The time of heating Kas measured from the moment when the contents of the flask reached the desired temperature. The weight of t h e flasks was checked from time t o time, and enough water added t'o make up for evaporaI tion. At the completion of t'he heating period -__ - 4-6 1 the flask rvas rapidly cooled, and the free acid -10 1 , 1 1 1 1 I neutralized by careful addition of the prede2 4 6 8 IO 1 2 14 l e 1 0 2 0 2 0 2 4 2 6 2 6 3 0 3 2 1 4 3 6 3 s 4 0 4 2 4 4 4 6 4 8 . termined quantity of sodium hydroxide solution. Lop Time i n Minuter The solution was then made up t.o volume a t 20", Figure 2 clarified by the addition of dry potassium oxalate t o remove the calcium salts, and filtered. No lead acet,ate was added because it is known t h a t some of the reducing substances are precipitated by this reagent. will have t o be based on the experimental results themselves. The reducing power was determined, a t least in duplicate, in A theoretical int,erpretation must await the study of simpler 50-ml. aliquots of the filtrate by t h e Xlunson and Kalker method, aystems. which is most. generally used in this country for this purpose. Collaborative experiments under the auspices of the Association of Official Agricultural Chemists (IO)have shownthat this method, EFFECT OF ACIDITY, TEMPERATURE, AND TIiME ON when applied t o unfermentable reducing substances, leaves much HYDROLYSIS AND DESTRUCTION t o be desired if carried out by different analysts, but when the Samples of distillery residue were secured from three different analyses are made under carefully controlled conditions by the distilleries whose cooperation is gratefully acknowledged. The same operator, closely checking results are obtained in replicate determinations. first ( A ) was a dried slop with 96.91% solids, marketed by U. S. I n this series of experiments each 100 ml. of the original soluIndustrial Chemicals, Inc., as BG-CLT and supplied by F. 11. tion contained 5.000 grams (4.846 grams solids) of dried slop -4, Hildebrandt. The ot,her tFo, furnished by G. T . Reich of the 2.500 grams solids of slop B, or 2.500 grams solids of slop C. At Pennsylvania Sugar C,ompany ( B ) and by C. L. Gabriel of Pubthelower temperature, especially 40°, and a t the higher pH levels the complete destruction of the reducing substances formed by licker Commercial Alcohol Co. (C), respectively, were slops concentrated by evaporation a t low temperature and containing 73 65 and 520170 r solids, respectively. All had a pH around 5 . 0 The apparent invert sugar content on dry 3 K substance, calculated from the reducing poner, c was 12.587, in sample A , 12.457, in sample B, and 13.36y0 in sample C. These : values are not a measure of the total quantity -c of hydrolyzablc sugar derivatives present or fl of the potential reducing substances obtainable from them by hydrolysis, because the reducing power of the individual components of the mixture before hydrolysis varI -10 1 2 4 6 8 IO I 2 I 4 l B 1 8 2 0 2 2 2 4 2 6 2 8 3 0 3 2 3 4 3 6 3 8 40 c12 4 4 4 6 4 8 ies greatly, and some of them, such as difructopyranose anhydride, do not reduce Loq l i m e In Minutes Fehling solution. Figure 3 _t____-___-
0
8
8
December 1947
INDUSTRIAL AND ENGINEERING CHEMISTRY
hydrolysis required such a long time t h a t the heating had t o be discontinued before this time was reached, in order t o make the equipment available for other runs. For t,he same reason, and partly also for lack of available mat,erial, some of the experiments a t the lower temperatures and lower aeldities were omitted.
60
I
1
'
1
1599 I
I
SAMPLE B
55
SO A -
I
'
HYDROLYSIS AT pH 0 2 7
I I _ _ ~ _ -
-1oo*c
-8O'C
The experimental results have been tabulated'. The data ohtairied are shon-ii'graphically in Figures 1 to '3. Thc. percentage increase in apparent invert sugar over that found in the original material, not treatcdlvith acid, was plotted against the logarithm of the time, because this provides a clearer picture thaii if the time as such had been used as abscissa. In the laitcr case the curves for Ion- pH and high temperature i r o u l d have been unduly steep, and t,hosc% for high p€I and l o x temperature excessively 2 4 6 a I O 1 2 1 4 16 l e u) 22 2 4 28 2 a 3 0 3 2 3 4 3 6 311 4 0 4 2 4 4 4 6 4 a extended in the horizontal direction. Log Time in Minuies The curves were fitted as closely as possible t o t h e Figure 4 esperiment a1 values, but there was a certain amount of scattering which was attributable to manipulative difficul&s. It' was impossible t o shake the flasks constantly during the longer periods, and iniHYDROLYSIS AT pH1.08 perfect, heat distributionKithin the solution, caused 45 -A by convection currents, probably affected the results. The rate of heating from room temperature 4 and the cooling back to 20" could not be closely con3 trolled with changing atmospheric conditions. Loa cal dest,ruction of reducing substances during the E6 neutrnlization of the acid solutions with sodium E hydroxide is another source of error to be considered. Severthelcss, the general shape of the curves permits the drawing of valid conclusions. The results confirm previous findings that a t any given pH or temperature thereducingpowr first in_-___ -5 ( 1 ' ; cre:Lse:, reschcs a maximum, and then decreases -,o , I , ' I again t o the original value and beyond. The curve 2 4 6 5 i o 1 2 14 16 I 0 20 2 2 2 4 2 6 2 8 3 0 3 2 3 4 3.6 3 8 40 4 2 4 4 4 6 4 8 Log T i m e i n M i n u t i s is evidently the net result of two opposed resctions procecding at the same time Figure 5 original reducing substances, these arid of the products form Thc rate of hydrolysis, the maximum reducing po\~er.,and thp same manner). Solutions containing 16.88 grrtms of this morate of destruction are all t'he grcuter thc higher t'he acidity and lasses (9.09 grams of total invert sugar) in 100 ml. and acidified the higher the temperature. This is in line n-ith the observations to p H 0.2 with 20 ml. of hydrochloric acid were heated a t different of Saernnn ( 7 ) on the acid hydrolysis of cellulose. At any pH or temperatures for varying lengths of time, and the! reducing power temperature the rate of destruction k much sloxyer than the rate mas determined, The results are shown in Table I. of hydrolysis. I n three cases the hydrolysis curves are apparThere is a small increase in reducing power, followed at the ently S-shaped, but this is probably due to esperimental error. higher temperatures by a decrease after the mavimum has been T h w e art: differences in the hydrolysis curves of the three reached; this confirms the results obtained with the distillery samples coinpared to one another, but direct comparison can be slops. The acid a t high temperature undoubtedly attacks the made only between samples B and C, the solutious of which had sugars present in the molasses and decreases thf: net amount of the same dry substance concentration. Sample C shows a higher additional sugar formed by llydrolysis of the urifer~nentahle reducing substances hydrolysis maximum than sample B for all three acidities at equal temperature and for both temperatures used on the two saniples at equal acidity. Thi. is to be ascribed to differences EFFECT O F CONCENTRATION ON HYDROLYSIS AND DESTRUCTION in t composition of the mixture of uniermentable reducing su!xstances in the two samples, as might be expected. H o m v e r , I n the study of this question 100-ml. solutions of sample A , the rate of destruction in the later stages is, under equal condi2, 5, 7.5, and l5 grams, and of samy,le . tioiis of temperature and aciditv. - . essentially the same in both cases; this indicates that the same substances. presumably glucose and fructose, are formed upon hydrolysis. TABLE I. HYDROLYSIS OF BLACKSTRAP MOLASSESAT p H 0.2 Attcmpts have been made to interpret ?he esperimental results 30' C. 60' C. 80' C. 1000 c. on the basis of reaction rates, but so far n-ithout success, probably Invert Invert Invert' Invert Time, sugar, Time, sugar, Time, sugar, Time, sugar, because of the coniples nature of the mixtures and the effect of hr. % min. min. % rmn. % secondary reactions. 0 57.27 0 57.27 0 57.27 0 57.27 Sonic experiments vere also run v i t h a blackstrap molasses 6 57.61 5 j7.58 5 57.70 3 57.30 24 57.56 10 57.58 10 57.61 5 57.67 containing 57.27% of total sugars (expressed as invert sugar) and 57.58 15' 57.90 10 56.55 48 57.70 20 30 57.64 20 57.53 15 56.07 4.07yc of unfermentable reducing substance.? (espressed in the 1
~
1
c,
I
.
The coinplete tahles ill be s e n t ripon reqliest t o onyone interested in the
details.
45 60
S7.61 57.64
30 45
57.30 56.98
20
55.06
1600
Vol. 39, No. 12
INDUSTRIAL AND ENGINEERING CHEMISTRY
It is found that diflcrent fermentation reaidues hehare differently even under comparable conditions. .it all eonccmtratinns but the lowest, ,m.iiiple C was less readily hydrolyzed and suffered greater destruction of the reducing sugar than sample A . It is mident that the nature m t l proportion of t lie vai,ioi.is hy I r o l p h l i . rompiiiients v a i k froin case t o cape.
treatnicnt xi-as determined as clescri!icd preriou-ly. The results, expressed again as the pcrceiit L I W inci ap;jnrerit iiir-erL sugar over that in t h e unliytl:.~~lyze~l unntciial. h a x bet.!\ tabulated'. The plotted curw.: a w all oi t h e s a m ' pattern a i the previous ones shon-ing a rnpitl ri.zc 1 11 :I ni:iuimiitu and then it slnrc-cr descent. The important critc,ri:i l o r judging the efiect of coilcwitrntion are the iricr e in qqxirciit irive1.l sugar a1 the point of maxiiiiurn hydi,ol s and th:ii nt't:,r tlw e1:tpsc of the longest hesting period. Tile i'oriiwr :it'c slio\vu iit Table I1 nrid tiic latter in Table 111. The figures for iiiaiiniuin hydrolyiis (T:i!j!e I1 , iriilicatc a don-nn-ard trend with eliminate the effect of four series h a r e been slio\v a steady decrease, ucts. The decrease beeoiiies smaller Kith each ai1ili.d unit of dry substance. The results given ill Table I11 showing tlic destruction u l reducing substances, are less consistent than ihosc for maximuin hydrolysis. The decreases in the apparcnt itivc.rt, sugar found experimentally per unit of dry substance are quite irregular but on the average the first two units of added dry suljiiauce decreased the apparent invert sugar by 4.76 per unit and the next three by 2.24 per unit. Table IV s h o m a comparison bct,weeii the behavior of samples A and C upon hydrolysis under comparable conditions. The results are again expressed as the increase in apparent invert sugar over that found in the original samples.
1
40 33
3
42 O? 39 6 i 38.41
44.86
4
6
Relative Concn. 1 '2
3 4 6
4.'
30 01 JO.81 !%
2i.71 26.08
41.G4 41 6 1
2S.74
30.84
43.52 44.31 35.Oi 34 R i
29.81
26 GQ
39.14
4 i 54 4II,'J8
.Zv.Increase a t X\Iax. Hydrolysis, -800 Q l U 0 0 A C 33.45 .53.45 51.15 40 38 50.00 47 15
c,-
60.00 49.35
4,i.38
44.43
51 7 0
41.40
1.1%
8 3'1 0.9Y
2.87
.%v. Incrcn-o a t E n d of Heating Period. 80' & 100' C . A C
43.94 43.67 42.13 40.BG 31.64
47
in
43.03 20.78 29.28 26.84
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1947 60
1
It/
I
50 -.
i
sult converted into grams of' alcohol. For coinparison wit8ht h e hydrolyzed portions, n solution
I
I
1
~
'
1601
_____ -_ ___
of the original material \vas fermented a i d distilled in the same manner, but a quantity of SOdium chloride equivalent t o the hydrochloric acid used was added in order t o obtain co~iip:ti~able vnlucs. The results of t,he crpcrirncmrs with saniple d are given i n T
HYDROLYSIS AT pH I O
The oriyiiinl iiiatcrial sholr-ed that it siill con sugar. The at1tIitioii:il drolysis for 30, 45, and t\-as22.0, 28.4, and 23.8 f;)rmcd, contrasted t o PaLrtcuy'a idc1:iI yii,lti oi .1.12 i 7;. .\ similar celculatioii can bi: nuilcx o i l i;,!, 11:wi:
,
- o L _ -
2
4
6
8
10
L
_____14
I5
IE
-__
2 0 22 2 4
26
2-
30
_______-___ 35
3234
28
45
42
44
46
4a
L o g Ttme In Minutes
Figure 8 -~ig:;i iric~ntablesugar. Iieiire, the it~rincnr:iI)lr~ tei,ial gare :LII ;ili.iiIii~i i n the oiigii ~f : ~ l ~ o ht ) l c~s li l (,> i ( ' t l \-i(>lilof 1.7313.38 = I)IYWIIT
/
tillery slop fiiriiislicci I)? 1,oiiILnng of tlic S.1tiowl S u g x Ri>fi:i-
Ir?mm.- j
I. D.
n o t hydrolyzed &I1 cated 1.5 m i n . IIeateil 30 inin. Hcated 4.j min. pII 1.0, h i n t e d 2 lir
'>
20
5'04 3 40 4.64
4.28
o.84 1 20 2.44 2.08
2.80 3 68 4.1, 3.84
...
I
12mm
I. D.
sugar) in the di!. s Lll) e t a 11r c . I 1 >.drolysis a t pII 0.3 for 30 minutes increased thr invci,t sugar t o 22.12c,. The coryspontlirlg increase In alcohol yielil was 1.47;, or
... 0.88 1.32 1.04
...
iiiciit:iblc reduciiicr
Figure 10.
Sattler Fermentation Trap
s u g q R little lcss than that olrtnincd
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
1602
under similar conditions from the concentrated distillery residues. Some fermentation tests were also set up wit'h unhydrolyzed and hydrolyzed blackstrap, b u t because of the small increases in t h e invert sugar upon hydrolysis the additional alcohol yields were erratic. Experiments of this nature would have t o be made on a much larger scale t o furnish reliable results.
Vol. 39, No. 12
5 . K h e n blackstrap. molasses is treated with acid under the same conditions as distillery residue, the reducing power also increases to a maximum and then decreases again, but the increases are small because t,he acid exerts a destructive effect on the sugars in the molasses. For this reason a marked increase in alcohol yield upon hydroll-sis cannot be expected. Whether alcohol can be produced economically from hydrolyzed distillery residues would have t o be decided by large scale experiments.
CONCLUSIONS
1. When the reducing substances in the unfermented residue from blackstrap molassm are heated xyith acid, the reducing power first increases t o a maximum because of tmheformation of fermentable sugars by t h e hydrolysis of the sugar anhydrides and sugar-amino acid condensation products, and then decreases again t'hrough the destruction of the fermentable sugars. 2. Within the experimental limits of pH 0.2 to 2 and temperatures hctrveen 40" and 100" C. the maximum reducing power increases with the acidity a t constant t,emperature, and with the temperature a t constant acidity. The rate of destruction follows the same rule. 3. Increase in the concentration decreases the rate of hycirolysis and increases the rate of destruction. 4. Upon fermentation with yeast the sugars formed b y hydrolysis yield alcohol, b u t the alcohol recovery is much smaller than the theoretical because fermentation inhibitors are formed from the sugars b y theeffect of acid at high temperature.
LITERATURE CITED
(1) Davis, Intern. SugarJ., 40, 233 (1938). (2) Jackson, Silsbee, and Proffitt, Bur. Standards Sei. Paper S o . 519 (1926). (3) Kerr, "Chemistry and Industry of Starch," pp. 264-88, 1944. (4) Mathews and Jackson, Bur. Standards J . Research, 11, 619 (1933); R. P. 611. (5) Reindel and Frey, Z . Spiritusind., 57, 237 (1934). (6) Sadovyi, T r u d y Voronezh Khim.-Tekhnoi. I n s t . , 3-4, S9 (19301. (7) Saeman. IND. ESG. CHEM.,37, 43 (1943). (8) Sattler and Zerban, I t i d . , 37, 1133 (1945). (9) Wohl, Ber., 23, 2054 (1890). (10) Zerban, J . Assoc. O f i c i a l A g r . Chem.,24, 656 (1941). i l l ) Zerban, Sugar Research Foundation (X. Y.1, Technol. Xept Ser., 2 (1947). RBCEITED February 11, 1947. Report of work carried o u t under grant from the Sugar Research Foundation, Inc.. S e w Tork, S . 1 ' .
Anodic Reactions of Aluminum and Its Alloys in Sulfuric and Oxalic Akd Electrolytes '
RALPH B. RMSON AND CHARLES J. SLUNDERl .Jluminum Research Laboratories, S e w Kensington, Pu. T h i s article describes factors, such as temperature, concentration and agitation of electrolj te, time of treatment, current density, and type of alloy treated, which affect the formation of aluminum oxide produced by the anodic treatment of aluminum in sulfuric and oxalic acid electrol) tes. Electrolq tes used at elelated temperatures or in a more concentrated form tend to produce oxide coatings of lighter weight. The electrollte must be sufficiently agitated to remoie the heat deieloped at the aluminum anode; otherwise, $he oxide coating is dissolied. Long time treatment in the electrolJte brings about a decrease in the efficienq of oxide coating formation. Lilicitise, a decrease in the current demit) results in the formation of a lighter weight oxide coating. The purit? of the aluminum anode has a marked effect upon the efficiency of oxide coating formation, the highest efficiency beiug obtained itith pure aluminum.
T
HE anodic oxidation of aluminum is now a well established coat'ing process, and has found extensive use for the decora-
tion and protection of aluminum surfaces ( 4 ) . The value of anodic coatings for specific applications is usually determined by measurement of one or more of the specific properties required. Properties such as hardness, abrasion resistance, thickness, adsorptivc capacity, or resistance to corrosion may tie varied widely by changing the coating conditions. T h e development of suitable t e i t niethods and determinntion of the effect of the operating variablcs on the properties of the coatings have been the subject of considerable research. Some 1
Present address, Battelle Memorial Institute, Columbus, Ohio.
of the reports (2, 3,4,7 ) on this work have indicated the accuracy of these test methods arid their field of application. iilthougli these methods have been found satisfactory, the information they supply on the mechanism of coating formation and the variables involved in the anodic oxidation procedure is limited. I n connection rr-ith the development of procedures for measuring coating thickness by stripping methods, R. B.Mason (.i) discovered that a solution of phosphoric and chromic acids quickly dissolves the oxide coaring and has practically no effect 011 tl!ci aluminum. The w i g h t of the coating can thus be readily alid accurately obtained. The value of such a method to detcmiiiic the efficiency oi anodic oxidation and to show the effect of citli~jr minor or major changes in the coating procedure \vas inimediately apparent. Edn-ards ant1 Iceller (6) u.icd the method t o detcrminc the apparent current efficiency of the anodic p r o r e ~ s : they showed that, while the aluminum was oxidized m i l tiissolved a t approximately 1007, cfficiency, only about 70'7' oi the aluminum lost \vas accounted for by the weight of the coating, assuming that it consisted entirely of aluminum oside. The r, t h a t this assumption is only :In authors pointed out, how approsirnation because the coating, when it is formed in suli'uric acid, is known to contain some sulfate and water. GENERAL PROCEDURE
Preliminary experiments indicated that even very i~iiall changes in the operating conditions wcre readily dctectcd b v changes in the weight of coating or in the weight of m e t d removed. A convenient unit for expressing the results w:is obtained by dividing the weight of coating formed undcr tlii, couditions of test by the weight of mctal removed during ihc for-
