The hydroxyl number denotes the quantity of potassium hydroxide (in milligrams) needed in order to neutralize acetic acid required for acetylation of 1 gram of an unsaturated polyester resin (UPR) with free hydroxyl groups. This application note describes the determination of hydroxyl number using DIN EN ISO 2554 compliant automated potentiometric titration method utilizing KOH in methanol as titrant.
Sample
Unsaturated polyester resin (UPR)
Sample Preparation
Sample preparation is not required.
Analysis
Blank
The blank is determined the same way as the sample, just without UPR.
Sample
Approximately 5.6 grams of UPR is weighed and placed in the sample beaker. 10 mL of acetylating solution is added to the sample using an automated program. The mixture is allowed to react for 45 minutes in a water bath with 50 °C ± 1 °C temperature.
10 mL of pyridine/water and 2 mL of deionized water are then added using an automated program. 20 mL solvent mixture is added just prior to the titration. Titration is then carried out using KOH with a 0.5 mol/L concentration and continued until after the equivalence point is reached.
Configuration
| . |
. |
| 907 Titrando |
2.907.0010 |
| 800 Dosino, 5x |
2.800.0010 |
| 814 USB Sample Processor (2T/0P) |
2.814.0130 |
| Dosing unit 50 mL, 2x (solvent mixture, titrant) |
6.3032.250 |
| Dosing unit 10 mL, 2x (acetylating solution, pyridine/water mixture) |
6.3032.210 |
| Dosing unit 2 mL, (deionized water) |
6.3032.120 |
| 741 Magnetic stirrer, 2x |
2.741.0010 |
| 843 Pump station (peristaltic) |
2.843.0150 |
| Sample rack, PP, 22 x 120 mL |
6.2041.470 |
| Titration head, 3x NS 14 |
6.1458.040 |
| Sample beaker, PP, 22 x 120 mL |
6.1459.300 |
| Titration head with DIS-Cover |
6.9914.158 |
| DIS-Cover lids for 120 mL PP beakers |
6.9914.164 |
| Solvotrode easyClean, c(TEABr) = 0.4 mol/L in ethylene glycol |
6.0229.020 |
| Tiamo™ 2.5 |
6.6056.252 |
Solutions
| . |
. |
| Titrant |
c(KOH) = 0.5 mol/L in methanol, if possible this solution should be bought from a supplier. |
| Acetylating solution |
w(Toluene-4-sulfonic acid) = 1.14% in ethyl acetate and acetic acid, Φ (ethyl acetate) = 90.2% (v/v) |
| Solvent mixture |
1-Butanol / Toluene, Φ (1-Butanol) = 66.6% (v/v) |
| Pyridine/water mixture |
Pyridine / Water, Φ (pyridine) = 60.0% (v/v) |
Parameters
| . |
. |
| Mode |
DET U |
| Start volume |
5 mL |
| Signal drift |
50 mV/min |
| Stirrer speed |
8 |
| Max. waiting time |
26 seconds |
| Meas. point density |
4 |
| Min. increment |
10 µL |
| Max. increment |
Off |
| EP criterion |
5 |
| EP recognition |
Greatest, evaluation with window |
| Lower limit mL |
2 |
| Upper limit mL |
50 |
| Stop volume |
50 mL |
| Volume after EP |
2 mL |
Results
| . |
. |
| Hydroxyl number / (n = 5) |
s(rel) |
| 42.60 mg KOH / g |
0.5% |
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Remarks
The hydroxyl number is finally calculated by taking into account the total acid number (TAN), determined separately by DIN EN ISO 2114.
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This information has been sourced, reviewed and adapted from materials provided by Metrohm AG.
For more information on this source, please visit Metrohm AG.