No. 322 e
Application Bulletin
Of interest to: General analytical laboratories; organic chemistry; plastics
Automated determination of the hydroxyl number
(HN) according to ASTM E 1899-08 and DIN 53240-2
Summary
The hydroxyl group is an important functional group and knowledge of the hydroxyl number, i.e. the
hydroxyl group content, is required for the production of intermediate and finished products such as
polyols, resins, paint resins and lubricants (petroleum industry). In the method presented here, primary
and secondary hydroxyl groups are determined. The hydroxyl number (HN) is defined as the amount
of potassium hydroxide in milligrams that is equivalent to the hydroxyl amount of 1 gram of the sample.
The most frequently described method for determining the hydroxyl number is the reaction with acetic
anhydride in pyridine with subsequent titration of the free acetic acid:
H3C-CO-O-CO-CH3 + R-OH Î R-O-CO-CH3 + CH3COOH
However, this method has the following disadvantages:
•
•
•
•
The sample must be boiled under reflux for 1 hour (long reaction time and expensive, complicated handling)
The method cannot be automated
Small hydroxyl numbers cannot be determined exactly
Toxic pyridine, which is unpleasant to use, is required
Both standards, ASTM E 1899 and DIN 53240-2, describe an alternative method that requires no
sample preparation and can therefore be fully automated.
The method described in ASTM E 1899 is based on the reaction of the primary and secondary hydroxyl groups with excess p-toluenesulfonyl isocyanate (TSI) with the formation of an acidic carbamate. This latter can then be titrated with the strongly basic tetra-n-butylammonium hydroxide
solution (TBAOH) under non-aqueous conditions.
The method proposed in DIN 53240-2 is based on the catalytic acetylation of the hydroxyl group. After
hydrolysis of the intermediate product, the remaining acetic acid is titrated with alcoholic KOH solution
under non-aqueous conditions.
This paper demonstrates and discusses a simple analysis method for determining the hydroxyl
number according to ASTM and DIN using a fully automatic titrimetric system that can be used for
numerous industrial samples.
Instruments and accessories
Both applications use the same instruments and the same number of reagents. The main instruments
are: 814 USB Sample Processor with two towers, 120 mL PP beakers with covers, 809 Titrando, 741
Magnetic Stirrer and 800 Dosinos.
If differences exist between the ASTM and DIN methods then these will be described separately in this
paper.
Both methods can also be carried out manually in sealed titration beakers.
Application Bulletin
No. 322
Potentiometric determination of the hydroxyl number as per ASTM and DIN
Page 2
Instruments and accessories
Both methods require the following equipment:
Pos. Order No.
Description
1
2.814.0130 814 USB Sample Processor (2T / 0P)
2
6.1458.040 Titration head
3
6.9914.158 Titration head with Dis-Cover
4
6.2041.470 Rack 22 x 120 mL PP beakers
5
6.1459.300 120 mL PP Beakers (100 pieces)
6
6.9920.164 Dis-Cover Lids for beakers (6.1459.300)
7
2.800.0010 800 Dosino
8
6.1805.120 FEP tubing /M 6/100 cm
9
2.741.0010 741 Magnetic stirrer for sample changers
10
6.1903.030 stirring bars/25 mm
11
6.0229.100 Solvotrode (LiCl sat. in EtOH)
12
6.2104.030 Electrode Cable 2 m/F
13
2.809.0010 809 Titrando
14
6.2151.000 Controller cable USB
15
6.6056.122 tiamo™ 1.3 full
Pieces
1
1
1
1
1
25
4
8
2
25
1
1
1
1
1
To comply with the requirements of the ASTM standard, the following additional dosing units are
required:
1
6.3032.250 50 mL Dosing Unit for acetonitrile
1
2
6.3032.220 20 mL Dosing Unit for TSI reagent and titrant *
2
3
6.3032.120 2 mL Dosing Unit for deionized water
1
* For the results presented here two 10-mL dosing units were used; it may be necessary to adapt the
sizes of the dosing units.
To comply with the requirements of the DIN standard, the following additional dosing units are required:
1
6.3032.250
50 mL Dosing Unit for catalyst solution and titrant
2
2
6.3032.210
10 mL Dosing Unit for acetylation mixture and deionized water 2
Reagents
This Bulletin contains no information about handling the reagents mentioned; nor does it indicate any
safety measures that have to be observed. Please obtain the necessary information from the appropriate regulations and safety data sheets.
Reagents for ASTM method
Acetonitrile, purity > 99.5%
p-Toluenesulfonyl isocyanate (TSI), purity > 95%
1-Octanol as reference material, hydroxyl number (HN) = 430.08 mg KOH/g
Ethanol (electrode rinsing solution), purity > 99.8%
Water, deionized (electrode rinsing solution)
Titrant: Tetra-n-butylammonium hydroxide solution in 2-propanol/methanol (TBAOH), 0.1 mol/L (ready
to use)
Preparing the TSI reagent
TSI must not become moist (vigorous reaction); working under protective gas (N2) under a fume hood
is strongly recommended.
20 mL TSI is pipetted from a glass volumetric pipet into a dry 500-mL volumetric flask half-full with
acetonitrile. The solution is made up to the mark with acetonitrile and thoroughly shaken. The TSI
reagent has a shelf life of one month.
Application Bulletin
Potentiometric determination of the hydroxyl number as per ASTM and DIN
No. 322
Page 3
Reagents for the DIN method
N-Methylpyrrolidone (1-methyl-2-pyrrolidone,NMP)
Acetic anhydride
4-N-Dimethylaminopyridine
1-Octanol as reference material, hydroxyl number (HN) = 430.08 mg KOH/g
Ethanol (electrode rinsing solution), purity > 99.8%
Water, deionized (electrode rinsing solution)
Titrant: Potassium hydroxide solution, 0.5 mol/L in methanol (ready to use)
Acetylation mixture: Acetic anhydride solution, 10% in NMP (store in amber glass bottle)
Catalyst solution: 4-N-Dimethylaminopyridine, 1% in NMP
Calculation
ASTM
HN
ASTM=
(EP2 - EP1) x CONC x TITER x 56.106 / Sample size
HN:
Hydroxyl number [mg KOH per g sample]
EP2:
Titrant consumption up to second potentiometric endpoint [mL]
EP1:
Titrant consumption up to first potentiometric endpoint [mL]
CONC
Concentration of titrant [mol/L]
56.106
Molecular weight of KOH [g/mol]
Titer
Titer of titrant used [ - ]
Sample size: Weight [g]
DIN
HN DIN = [(Blank - EP1) x CONC x TITER x 56.106 / Sample size] + TAN
HN:
Hydroxyl number [mg KOH per g sample]
EP1:
Titrant consumption up to first potentiometric endpoint [mL]
Blank*
Blank value of solvent and reagents [mL]
CONC
Concentration of titrant [mol/L]
56.106
Molecular weight of KOH [g/mol]
Titer:
Titer of titrant used [ - ]
Sample size: Weight [g]
TAN:
Previously determined acid number (TAN) of sample [mg KOH/g]
*The blank value is extremely important for the calculation of the hydroxyl number according to the
DIN method and is approx. 40 mL! It must be determined every day.
The titer determinations are carried out according to AB-206.
Sample weight
The amount of sample required for the determination according to ASTM/DIN depends on the expected hydroxyl number (HNe) in mg KOH/g. The sample weight E (in gram) is calculated according to
the following equation (example: 1-octanol):
Application Bulletin
Potentiometric determination of the hydroxyl number as per ASTM and DIN
No. 322
Page 4
Calculating the sample weight for 1-octanol
ASTM
DIN
Equation
E = 40/HNe
E = 300/HNe
1-Octanol
ASTM
DIN
Sample
weight
40/430.08 = 0.093 g
300/430.08 = 0.698 g
The following diagram is intended to clarify the automated analytical steps of the ASTM and DIN
methods:
ASTM
Sample & magnetic stirrer
DIN
Sample & stirring bar
Add 10 mL acetonitrile
Stir sealed for 30 s
Add 10 mL TSI
Add 30 mL catalyst
solution and 10 mL
acetylation mixture
Stir sealed for 15 min
Stir sealed for 5 min
Add 3 mL deion. water
Add 0.5 mL deion. water
Stir sealed for 60 s
Add 40 mL acetonitrile
Stir sealed for 12 min
Dynamic endpoint titration
(DET U) with c(KOH)=0.5
mol/L in methanol
Dynamic equivalence point
titration (DET U) with
c(TBAOH) = 0.1 mol/L in
isopropanol/methanol
Analysis time approx. 12 min
Analysis time approx. 40 min
At the end of the titration the electrode accessories are automatically rinsed with ethanol (dip rinsing)
and conditioned in deionized water (see AB No. 80/3 and instruction leaflet for Solvotrode).
Application Bulletin
No. 322
Potentiometric determination of the hydroxyl number as per ASTM and DIN
Page 5
Detailed description of the working steps for the ASTM method
The manually homogenized sample is weighed out into a 120-mL PP beaker, a stirring bar is added
and the beaker is sealed with a Dis-Cover lid. The beaker is placed on the sample changer. After the
method has been started, 10 mL acetonitrile is added automatically. After a stirring time of 30 seconds
and dissolution of the sample, 10 mL TSI is added. Then the beaker is resealed and stirred slowly for
5 more minutes.
The chemical reaction is interrupted by opening the lid and adding 0.5 mL deionized water. The lid is
then closed again and the resealed beaker is stirred for 60 more seconds. The sample beaker is
reopened and 40 mL acetonitrile is added. After resealing the beaker, the sample is stirred at a
moderate speed and the titration with 0.1 mol/L TBAOH in 2-propanol/ methanol is started.
The two equivalence points are determined using the conditioned Solvotrode. The evaluation requires
particular care.
At the end of the titration the Solvotrode is first immersed in a PP beaker containing ethanol and then
remains until the next measurement with only the membrane immersed in deionized water. At the end
of a series of measurements the electrode is automatically immersed in a beaker containing ethanol
until the start of the next analysis. When not in use for longer periods, the Solvotrode is stored in LiCl
sat. in EtOH.
Modification in comparison with the ASTM standard
For automation, the acetonitrile addition at the end of the analysis is increased from 30 mL to 40 mL,
in order to ensure that the Solvotrode is immersed to a sufficient depth.
Detailed description of the working steps for the DIN method
The calculated amount of sample is weighed out directly into a 120-mL PP beaker, a stirring bar and
Dis-Cover lid are added and the beaker is placed on the sample rack. The lid is raised automatically
by using a magnet, 30 mL catalyst solution and 10 mL acetylation mixture are added and the beaker is
resealed. The sealed sample is stirred for at least 15 minutes (the reaction time depends on the type
of sample). The lid is then again raised automatically and the reaction is interrupted by addition of 3
mL deionized water. The beaker is resealed and stirred for a further 12 minutes. The electrode is then
automatically cleaned and conditioned by immersing it in ethanol and then in deionized water. The
titration for determining the hydroxyl number is carried out using 0.5 mol/L KOH in methanol. At the
end of the titration the electrode is automatically cleaned in ethanol and conditioned in deionized
water.
Examples of measurements
The following customer samples were analyzed for the development of the automatic determination of
the hydroxyl number according to ASTM/DIN methods:
Sample name
Oxooil 9N
Oxooil 13
Sample composition
Hydroformylation product of
C8-alkenes
Dodecene hydroformylation product
Lupranol 2090
Polyether polyol, branched
Guideline values
120...130 mg KOH/g
(ideal value = 100 mg KOH/g)
120...130 mg KOH/g
(ideal value = 100 mg KOH/g)
approx. 30 mg KOH/g
Lupranol VP 9346
Polyether polyol, branched
approx. 30 mg KOH/g
Application Bulletin
Potentiometric determination of the hydroxyl number as per ASTM and DIN
No. 322
Page 6
Statistical evaluation of the results for the ASTM method
Sample: Oxooil 13
Sample
Weight [g]
0.3217
0.2961
0.2958
0.3077
Mean
RSD
expected value: 120...130 mg KOH/g and titration curve
Oxooil 13
HN [mg KOH/g]
128
132
134
127
130.0
2.54%
Sample: Lupranol 2090
Sample
Weight [g]
1.2742
1.3323
1.2533
1.3030
Mean
RSD
expected value: 30 mg KOH/g and titration curve
Lupranol 2090
HN [mg KOH/g]
29.8
29.9
29.9
29.7
29.6
1.39%
When the sample weights are taken into account, a constant consumption of 0.7 … 9 mL was obtained for the ASTM method.
Statistical evaluation of the results for the DIN method
Sample: Oxooil 9N
Sample
Weight [g]
1.3057
1.2262
1.1109
Mean
RSD
expected value: ~100 mg KOH/g and titration curve
Oxooil 9N
HN [mg KOH/g]
102
104
103
103
0.97%
Sample: Lupranol VP 9346
Sample
Weight [g]
0.5350
0.5383
0.4304
Mean
RSD
expected value: 400...450 mg KOH/g and titration curve
Lupranol VP 9346
HN [mg KOH/g]
446
448
451
448
0.56%
Application Bulletin
Potentiometric determination of the hydroxyl number as per ASTM and DIN
No. 322
Page 7
General remarks
The use of a fully automatic system for this type of analysis is highly recommended, because in this
way contact with toxic solvents is minimized. Electrode treatment is very important for non-aqueous
titrations. An automated system ensures that electrode handling is always carried out in exactly the
same way.
Summary
The titration system presented here is able to carry out a fully automatic determination of the hydroxyl
number (HN) according to ASTM or DIN. The method permits the analysis of polyols und oxooils
without boiling under reflux and is therefore an enormous step forward for laboratories that have to
analyze a large number of such samples every day. Above all, the ASTM method is characterized by
its simple handling without blank value determination and much, much shorter reaction times.
Expression of thanks
We kindly thank BASF in Ludwigshafen for providing the samples.
Literature
ASTM E 1899-08, Standard Test Method for Hydroxyl Groups Using Reaction with p-Toluenesulfonyl
Isocyanate (TSI) and Potentiometric Titration with Tetrabutylammonium Hydroxide
DIN 53240-2, Bestimmung der Hydroxylzahl – Teil 2: Verfahren mit Katalysator (November 2007)
AB-206, Titer determination in potentiometry (http://products.metrohm.com, search for AB-206)
AB-080, Determination of the acid and base numbers in petroleum products, “Treatment of the
electrode(s), control of the electrode’s slope (http://products.metrohm.com, search for AB-080)
81091586, 6.0279.300 LL Solvotrode Combined pH electrode for non-aqueous acid/base titrations
(http://products.metrohm.com, search for 81091586)
Tips for handling the Solvotrode, Metrohm Information 1/2007, Vol. 36 (www.metrohm.com)
AB-200, Determination of the acid number, hydroxyl number and isocyanates in raw materials for the
fabrication of plastics by automatic potentiometric titration (http://products.metrohm.com, search for
AB-200)
Annex
Method and titration parameters, ASTM/DIN