Chemistry Reference
In-Depth Information
9.7.1.2 Determination of Reducing Sugars
Thefollowingreagentsareneededfordeterminationofreducingsugars:
•
FehlingAsolution:69.278gof(CuSO
4
.
5H
2
O)isdissolvedinwateruptoavolumeof1l.
•
FehlingBsolution:346gof(C
4
H
4
KNaO
6
.
4H
2
O)isdissolvedinwateruptoavolumeof1l.
•
Potassiumiodide:30%v/v(30gdissolvedin100mlofH
2
O).
•
Sulfuricacid:25%(1+6).
•
Na
2
S
2
O
3
.
5H
2
Osolution,0.1N:25gofNa
2
S
2
O
3
.
5H
2
Oisdissolvedin1000mlofdistilledwaterand
0.1gofNa
2
CO
3
isaddedasapreservative.
•
Starchsolution(indicator):2gofsolublestarchisaddedto25mlofH
2
O,andtheformedpasteis
addedslowlyandundercontinuousstirringin250mlofboilingwater.Boilingcontinuesfor1-min,
andthesolutioniscooledandkeptinglasslaskswithlids.
•
HCI:0.5N.
•
NaOH:0.25N.
Theprocedureisdescribedasfollows:
•
Ina250-mlconicallask,a25-mlsampleistakenforexamination,and10-mlFehlingA,10-ml
FehlingB,andH
2
Oareaddedtoreachainalvolumeof50ml.
•
ThemixtureisheatedinaBunsenburnerfor5min.
•
Coolingfollowsafter2min,adding10-mlKI30%and10-mlH
2
S0
4
25%,andtitrationwith0.1N
Na
2
S
2
O
3
.
5H
2
Ofollows.
•
Attheendoftitration,starchsolutionisadded,andtitrationcontinuesuntilthesolutionchangescolor.
•
Blankdeterminationisthencarriedout.Thedifferenceofthetwotitrationsgivesthereducedquantity
ofcopper,andthroughthecorrespondingtable,thequantityofsugarasinvertedsugariscalculated.
Resultsareexpressedasfollows:
(Inverte
d sugar from table) 8 100
(Weight of sample in
× ×
grams) 100
Reducing sugars (%), as inverted sugar
=
.
×
A5-gsampleiscalculatedasfollows:
Reducingsugars%,asinvertedsugar=(invertedsugarfromtable)×0.16.
9.7.1.3 Determination of Total Sugars as Inverted Sugar
Forthedeterminationoftotalsugars,saccharosecontainedinthesolutionshouldbeinvertedby
heatingitinanacidenvironment.A25-mlsugarsampleistransferredtoa250-mlconicallask,and
2.5-mlHCI0.5Nisadded.Heatingat100°Cinawaterbathfor30min,followedbycoolingandneu-
tralizationwith5-mlNaOH0.25N,isthencarriedout.Then,theproceduredescribedabovefollows.
Resultsareexpressedasfollows:
(inverted sugar from table)
8 100
(Weight of sample in grams) 100
× 0 95
.
× ×
×
Saccharose (%)
=
.
A5-gsampleiscalculatedasfollows:
Saccharose(%)=(invertedsugarfromtable)×0.152.
InTable9.1,sugarsareestimatedinmilligramsfromtheusedN/10Na
2
S
2
O
3
.
5H
2
Osolutionin
millilitersaccordingtoSchoorlandRegenbogen.EachmilliliterofN/10Nα
2
S
2
O
3
correspondsto
6.36-mgcopper.
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