Determination of the acid number, hydroxyl number and isocyanates in raw materials for the fabrication of plastics by automatic potentiometric titration
Summary
The determination of the acid number, the hydroxyl number and the isocyanates plays an important part in the analysis of raw materials for plastics. The present bulletin describes the determination of these characteristic values by automatic potentiometric titration.
Instruments and accessories
702 SET/MET Titrino, 716 DMS Titrino, 736 GP Titrino, 751 GPD Titrino or 785 DMP Titrino or 796 Titroprocessor with 700 Dosino or 685 Dosimat
2.728.0040 Magnetic Stirrer
6.3014.XX3 Exchange Units
6.0229.100 LL Solvotrode [reference electrolyte: tetraethylammonium bromide c(TEABr) = 0.4 mol/L in ethylene glycol, Metrohm no. 6.2320.000] with 6.2104.020 electrode cable
1. Determination of the acid number
Reagents
Titrant: potassium hydroxide solution, c(KOH) = 0.05 mol/L in methanol, CO2-free
Solvent A: methanol : dist. water = 3 : 1 (volume ratio)
Solvent B: ethanol : toluene = 1 : 1 (volume ratio)
Benzoic acid, p.a. for the titer determination
Analysis
Weigh 30 … 50 g sample into a glass beaker and add 50 mL solvent, resulting in an homogeneous mixture. Preferably, solvent A is used. If, however, this does not mix well with the sample, solvent B has to be applied (preliminary tests). In this case it is also possible to heat the mixture in order to improve solubility. The sample is then titrated slowly with c(KOH) = 0.05 mol/L using the MET mode. In the same way the blank of the used solvent has to be determined. This blank consumption is stored as common variable C31 in the titrator. Upon completion of the titration, rinse the Solvotrode with ethanol and dist. water. When not in use (e.g. over night), store the electrode in dist. water.
Calculation
The acid number is expressed in mg KOH / g sample.
acid number = (EP1 – C31) * C01 * C02 / C00
EP1 = titrant consumption in mL
C00 = 30 ... 50 (sample weight in g)
C01 = 2.8 [= c(KOH) in mol/L * M(KOH) in g/mol = 0.05 * 56.1]
C02 = titer of the titrant (can be determined with benzoic acid)
C31 = blank consumption in mL
2. Determination of the hydroxyl number
Reagents
Titrant: sodium hydroxide solution, c(NaOH) = 1 mol/L
Reaction solution: c(phthalic acid anhydride) = 1 mol/L in anhydrous pyridine
Solvents: pyridine, p.a., anhydrous and dist. Water
Analysis
Weigh approx. 2 g sample into an Erlenmeyer flask with standard ground-glass joint and dissolve in 20.0 mL reaction solution. Attach a reflux cooler and heat up to 130 C for 45 min. After cooling down, rinse the cooler three times with 10 mL pyridine each, then three times with 10 mL dist. water each into the Erlenmeyer flask. Afterwards titrate the formed phthalic acid with c(NaOH) = 1 mol/L. In order to determine the titrant consumption for the reaction solution and solvents, a blank sample is treated and titrated in exactly the same way as the actual sample. This blank consumption is stored as common variable C31 in the titrator. The titrations are carried out with a start volume that is approx. 20 mL for the sample and approx. 35 mL for the blank.
Calculation
The hydroxyl number is expressed in mg KOH / g sample.
hydroxyl number = (C31 – EP1) * C01 / C00
EP1 = titrant consumption in mL
C00 = approx. 2 (sample weight in g)
C01 = 56.1 [= c(NaOH) in mol/L * M(KOH) in g/mol]
C31 = blank consumption in mL
Remarks
The sample must be completely dissolved before heating up.
For hydroxyl numbers between 100 and 350, the acid number – if it is greater than 0.5 – is added to the hydroxyl number.
An alternative method (without heating of the sample) is described in standard ASTM E 222-94, method C.
3. Determination of the isocyanates (CNO)
Reagents
Titrant: hydrochloric acid, c(HCl) = 1 mol/L in methanol
Reaction solution: c(dibutylamine) = 1 mol/L in toluene
Solvents: toluene, p.a. and methanol, p.a.
Analysis
Weigh approx. 2 g sample into an Erlenmeyer flask with standard ground-glass joint and, heating slightly if necessary, dissolve in 30 mL toluene. Add 20.0 mL reaction solution, stopper the flask and allow to react for 10 min on the magnetic stirrer. Afterwards add 30 mL methanol and titrate back the excess dibutylamine with c(HCl) = 1 mol/L. In order to determine the titrant consumption for the reaction solution and solvents, a blank sample is treated and titrated in exactly the same way as the actual sample. This blank consumption is stored as common variable C31 in the titrator.
Calculation
% CNO = (C31 – EP1) * C01 * C02 / C00
EP1 = titrant consumption in mL
C00 = approx. 2 (sample weight in g)
C01 = 42.0 [= c(HCl) in mol/L * M(CNO) in g/mol]
C02 = 0.1 (conversion factor for %)
C31 = blank consumption in mL
Answer for acid number
736 GP Titrino 04268 736.0011
U(init) 7 mV MET U SZ
smpl size 14.2055 g
EP1 2.280 ml -215 mV
SZ 0.374 mg/g
stop V reached
Answer for hydroxyl number
736 GP Titrino 04268 736.0011
U(init) 39 mV MET U OH-Z
smpl size 1.9713 g
EP1 25.730 ml -191 mV
OH-Z 334.1 mg/g
stop V reached
Answer for % isocyanates
736 GP Titrino 04268 736.0011
U(init) -115 mV MET U CNO
smpl size 2.0754 g
EP1 4.008 ml 271 mV
CNO 30.96 %
stop V reached
============
Literature
Metrohm, Application Bulletin No. 200/2 e
No comments:
Post a Comment