1
U. S. Department of Agriculture, Forest Servic e
FOREST PRODUCTS LABORATOR Y
In cooperation with the University of Wisconsi n
MADISON, WISCONSIN
PRETREATMENT OF WOOD WITH HOT DILUTE ACI D
AND ITS EFFECT ON LIGNIN VALUES
By WILBY E . COHEN
Commonwealth Fello w
and
ELWIN E . HARRI S
Chemis t
.
;
[pi
Published in
Analytical Edition o f
INDUSTRIAL & ENGINEERING CHEMISTR Y
May 15, 1937
4
i• .i
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-, 4m
NT O .xI)QOD WITHHeA&ILD,u
:OAT LNIN VALUE'1
By
WILBYB• O
2
an d
ELWIN E, jHOD*til
The infl ,nce of p,Fetreat.ments of wood to rameve, by mean s
of hydrolysis, carboty-dWate substance whit . al Agedly causes error,
in the lignin determination,, has been stadied to determine whether ,
or not lignin is dissolved during such t-reatnveRis .. Rot water, t o
some extent, and boiling 3 percent salfa ra,e avid, to a much greatexteato. dissolved lignin from mgple woe,el- ;.. A . a ara+ nation o f
a reme -04 wa s
metho%yl contents demonstrated that the Wt. ;I
lignin and not some carbohydrate material wrIeh m2ght polyyOeriz e
to at insoluble residue,. All the filtrates gave signifi nIt; color
reactions for lignin with chlorine and soldi snlfiteL . In ead.040d.
the acid filtrates deposited a brown fl-occulext subatame which
contained 20 .3 percent of methoxyl an-d which bel u * 1 Da ligniig
when cklorAnated and subsequently dissolved in sotdii ni scal itt
Spruce wood is reported to be affected it a sim .aX mat`s- It
is re•eca ended that all forms of mild hydwolys4 A,• inc, ut4i g WAt -ments with hot water, be avoided when preparing wae' for e :i
the determination or isolation of ligniunp :
i
For the purpose of removing carboh.y at,e subet
Ls• w
allegedly polymerize when subjected to the conditt-ove employed in th e
estimation of lignin-, various forms of pretreat•menta, using hot dilut e
acid, have been suggested . Norman and Jenkiins (4), foe recommended that the wood be boiled with dilute sulfuric aid loAt tt definit e
period, mat tag this recommendati:e with the r-eservatiAx t1h ,t Xgt° u•g wa s
known of the effect of such treat text on lignin as it ©,c !rs- rn e wood :
_Presented before the Division of Cellulose Chemistry at tale 93rd Meetin . l
of the American Chta3~,1 goei.ety, Ohepel H>sll~ N : C., Air it 12 to 1 5,193 7
-Commonwealth Fund (Mis r York) Service Fellow t'ro x Divis}on of Fores t
Products, Melbourne ., Australia .
-Chemist, U . S . Forest Prig facts Laboratory, NI c ison, Tlis,
3
R113g
1
4
1121 ttlkrO$t such trr :atment *m4
Campbell and amfoVil 1(_
.
.
A
dtli
a
co
e-~
merization of some car ol,y 1t.o substances
, chat
definite increape in the 46004 yield, .
aigre
he
sphere
frmm
dissolved ca +
r-ate s bs ;nces be removed ion-0 _ tk1'
.
Qf
only
a f"IOV
e~c2a
of action 1~r lie gpeteaOmPma of several acid tre.tta,
psibi{L
it
the
minutes l
. In neither of the above instances was
it
wamf4art
en
of
O
of an a ammm , ying lignin extraction considered. AA.
this
p
r
.
was therefore inItle4-404 bud tA19 :4na.lts are p 'sented in-
:
;11vAgitlal Procedur e
II
,
enxkl ft U0 mesh in a Wiley m :tA„i was extraotec+1 4a
succession with cold alcbhel-b-enzene (1 to 2), co-l- m$eNkel, cold water ,
and water at 9S° C . The alcohol-benzene and ale0k$1,1 ;q ctractions were -mad!
in a continuous, percolating extractor (earth for 24 .rs' durati-Om), a;40the water treatments were applied in a glass-lin.ad eaA raaise m -erl the
former for 2L lc
s 'and the latter in two pe.r4o:Gis- orf j hot ra each.. Tl e weo:d<
was finally washed on a suction filter until the hot wash water resna- e d
colorless . CoIrrespord.i•ng q , ,t,itative tre_pVzents, qatirg glass e p ;r ,,u,zs4
indicated that the total loss by extraction ; a1 n d to 4-3 perae utr,
wood was then extracted continuously for 50 . horirs va- th water at 91 g° Cc ice.
d s &r l t
an apparatus which emptied tough a siphon tube ()ace every
quantitative tiReatflent -le &laps showed a further loss of 6.3 pe eent ; m~z___V
a total loss by extracfion and* possibly, by mild hydrolysis of 10 .6 pe ce x«
PrOparatory to stib'sequent treathei t$ the 'wood zas dried * t roo m
temperature (20° to 2.5 6 C .), and then grou?d i=n-an aftsition m -1~7 :-u it a3:~~i
of it passed through a No . SO. sieve .
Separate portions of the prepared wood were then boenled. i l
reflux with a 3 percent solution of sulfuric acid (50 ml . per .m rof wg® j
for periods of 2, 4, g, 16, and 30 hours . .finatrhur 3aeYtion wpm Aimed in a
filter crucible with a boiling 5 percent soltfio n out shl'tri.c acid (10 ml .
Withdrawn b*
Per gram of wood) for a} mAiihites and the acid sel vt ,a r
suction and rOnewed. This prelcedizre was co3 itr Ra.+oc **U. the ?odd had received 10 such tre ttmemts :
Yollovving each qt. the f o rego :ing tr
QKAs.,„ tips i e ed
by suction and the wood w; + hed free of e•c .c by m n°m sot &IA Mat. -e%' . All
samples were seb
ti-ly 1r•'ied 'at ro
t
-r i *
Cor
conditions, f,~.~~ .~. data relative to
t
sed b
'cal
ysis .
Det.e ama6 onac of lignin, using the U . S•.u'Forest ,
Laboratory's modilf e1
ocedure, and of f: rfi l yi -l4 usY r' :1Qkw '~~~•liens .19i
method with phloxlucinol as precipitant, were made on ail samples_.
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+-,?-
`
Additional samples of lignin were prepared and dried at room temperatur e
, preparatory to the determination of methoxyl content .
'1
r
The data, corrected to the basis of unextracted, moisture-fre e
wood, are presented in Table 1 .
_
0 .
r. .. . ~
Discussion
_
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0.
'
_1
While the furfural yield was only slightly decreased by water treatments, acid hydrolysis caused a rapid decrease which continued steadil y
with time of hydrolysis until finally, after 30 hours' treatment, only abou t
I percent of furfural was obtained . Lignin values showed significant de creases when the wood was heated with hot water, but the losses were not o f
the same order as that caused subsequently by boiling 3 percent sulfuri c
acid . The latter, in 2 hours, decreased the lignin value to 17 .7 percen t
which seemed to be the limit, because longer periods of boiling, up to 3 0
hours, caused no significant losses of lignin . The results (Table 1, D t o
H) suggested a solubility relationship, the acid solution (a) having becom e
saturated with the slightly soluble lignin in the first 2 hours, (b) havin g
seached a condition of e q uilibrium between dissolution of lignin and it s
deposition as a less soluble polymerized material, or (c) having dissolve d
all the soluble lignin during that period .
The methoxyl content of the isolated lignin remained unifor m
throughout and, therefore, demonstrated that lignin, rather than carbohydrat e
substances which might polymerize to an insoluble residue, was being re moved . This was verified when the lignin methoxyl contents were calculate d
as percentages of wood .
It was evident that lignin is soluble in hot dilute acid and a n
examination of the various filtrates supported this contention . Thes e
filtrates were chlorinated and, on the addition of sodium sulfite in exces s
of that required to react with the acid present, a wine-red color, characteristic of chlorinated lignin dissolved in sodium sulfite, developed .
Furthermore, after standing at room temperature for a few days, a brown
flocculent substance resembling lignin separated out from the filtrates .
Some of this was collected and chlorinated . It brightened in color t o
orange yellow and dissolved in sodium sulfite to a wine-red solution whic h
changed to a brown color on heating . The methoxyl content of this substanc e
was found to be 20 .3 percent .
Apparently the dissolved lignin polymerized to a less solubl e
form when left in contact with the dilute sulfuric acid for several days .
Campbell and Bamford (1) postulated such a reaction involving carbohydrat e
substances when they recommended the short successive treatments to whic h
reference has already been made (Table 1, I) . It is conceivable that th e
phenomena are identical and that the substance which Campbell and Bamfor d
' found to be redeposited was really lignin . Although their method, whe n
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In the 401y, did not decrease the lignin yield as much as did th e
tt
other acid treatments, the filtrate gave defaAte color reaction fo r
lignin .• Probably continued t atment by th .e t r'6t4iod wo-i 1i et e tuall y
have decreased the lignin yield to about 17 .7 percent .
-
The results obtained with dilute acid natumalrly •dzr-e,ct,ad .attentio n
to the possibility of similar effects resulting from milder forms oaf hydrolysis, such as extractions with hot water which had been used in prep urr-ing th e
wood . The lignin content had been appreciably decreased by the prolonge d
extraction (Table 1, C) and the effluent from the extractor gave a slight color reaction for ligxiin with chlori=ne and sodiudl sulfite . This rea on,
although slight as collpared with that obtained with the acid filer~artes, war s
very significant because about 10 liters were used in the course !e .if th e
extraction and, as the effluent was run to waste r ther than c .0
, %trated;
the lignin concentration was consequently small . I4igesti2ori. with water' at
98° C . for s.?order periods such as 3 hours also result-e'd to some diss glutio .
of lignin ('ble 1, B) because the filtrate from this treatme it caves a ve*'
dafini-ta ;WOW 0i4iistrimi if or lignin .
The action of boiling dilute sulfuric acid on wood
comparabl e
with that of methanol-hydrochloric acid when it is used in the wim0ptlon . of
soluble lignin . In both instances, there is an acidic condition 4-Eh
enhances hydrolysis of souse carbohydrate lxI
. TtP
.Se ie apparentl y
associated chemically with the soluble 11 gal-tel.
cl ei re l sets i t
free to di s=solve M . %alteer on dine hand, or in EftlOgubilk t m lit e: ether . In
fact, the d a, exfei e be w&en the lignin yields!_ (1) from' o4l . 20NNO
st'b
~l only to alaoYiol-benzene, alcohol,
id
. c~l►~ w
and
(2) from wood which, iri addition, has been tre ed with zt
' and r=' t ute
sulfuric acid (Table 1, D), is approximately 6 , ; xaent -- t ilaj
, of tl
sate order at the yield of soluble lignin by tile me-hemol-h0beAlloric *af
t
('~ ,
n
.~y ,r'
process (4),..
-
The influence os mild hydrolysis in releasing soluble Li
to a solvent was demonstrated by one of the authors (2) when desbing
the effects of alkaline extraction on the lignin yield . Sodium k. roxid e
(0 .5 percent) was found to have a much greater solvent action on 1AVN4w 4- f
the wood was first subjected to an extended extraction, amounting -t-orn%d
hydrolysis with hot water .
he polymerization of the dissolved lignin to a less sol-at,D &
aggregate under the influence of cold dilute sulfuric acid may b& •n rde d
as a milder form of the action of the 72 percent sulfuric acid emplo A in,
the lignin determination . It is significant that the filtrate from a ILAq%a
determination does not give any form of color reaction with chlorine .06
so
I
.■
_
i
I
Alation vai
results, indicating that in addition to maple s
dissolved by dilute sulfuric acid, have be&ri rec o
report of the U . S . Forest Products Laboratory by R . L . Mitchell .
81138
-6-
It is concluded that any form of pretreatment involving hydrolysi s
will cause significant errors in lignin determinations and may even resul t
in the complete loss of that portion of lignin which is known as solubl e
lignin . It is believed that all forms of pretreatment using hot dilut e
acids are not only unnecessary for the purpose of removing carbohydrat e
material, but that they should be avoided, and that, if water must be use d
to remove interfering extraneous matter, cold extraction should preferabl y
be employed .
Literature Cite d
(1) Campbell, W . G ., and Bamford, K . F ., Biochem . J ., 30, 1.19 (1936) .
(2) Cohen, W . E ., Division of Forest Products, Techncal Paper 20, Counci l
Sci . Ind . Research, Australia, Pamphlet 62 (1936) .
(3) Harris, E . E ., J . Am . Chem . Soc ., 58, 894 (1936) .
(4) Norman, A . G ., and Jenkins, S . H ., Nature, 131, 729 (1933) .
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