I have two questions in determining OH value of polyol:
1. While OH number of polyol is expressed as mg KOH/g sample, the determination of OH value (such as ASTM D4274-11) doesn't use KOH but NaOH. Has anyone know the scientific reason behind that?
2. How to determine the sample weight of polyol when we don't know / don't have any hint for its OH value? Can we use any weight such as 1 gram, any rule of thumb of max weight for it? Is it correct to say that any gram of sample will give the same OH value? and how about the volume of NaOH during the titration of sample, will it increase when we use more weight of sample or it will happen in the opposite way (less volume of NaOH will be consumed for more sample weight)? Thank you in advance
Usually an ethanolic solution of KOH is used in back titration, NaON will give a very slow and bad colour change of the indicator. When using NaOH the formula to calculate the OH value has to be corrected due to change in molecular weight of the respective hydroxide.
The amount of substance to be measured is not so much dependent on OH value but it is useful to use adjusted amounts, e. g. 1 g in the range of OH No. 100 to 30 mg KOH/g, or 0.2 g in case of 300 to 600 mg KOH/g. You should not exceed 5 g.
In some cases it is favourable to use the phthalic anhydride methode when the OH No. is completely unknown.
0.5–1.0 g of the sample was dissolved in 25 cm3 of a phthalation reagent
and heated at 115 C for 1 h under reflux. The phthalation reagent consisted of 115 g of phthalic anhydride and dissolved in 700 cm3 of pyridine. Followed by the addition of 50 cm3 of pyridine through the condenser. The mixture was back titrated with a 0.5 mol L of sodium hydroxide solution. The indicator was a 1% phenolphthalein solution in pyridine. The hydroxyl number, defined as mg KOH/g of a sample can calculated as follows:
OH=(B-A)*M*56.1/w +acid number
A is the volume of the 0.5 mol/L sodium hydroxide solution required for the titration of sample(cm3), B is the volume of the sodium hydroxide solution
required for the titration of the blank solution (cm3), M is the molarity of the sodium hydroxide solution (mol/L), and w is the amount of the sample (g)
There is one point missing in your description: when do you hydrolyze the residual anhydride? Usually it is done after 1 h and 10 min cooling down of the mixture through the condenser. You did not mention. Without this hydrolysis the results will become obscure. Further, I strongly recommend to use KOH instead of NaOH because of a better transition of the indicator. The colour change of this has to be stable always the same time (we used 15 s). If you do not dtandardize your procedure you'll not receive solid results.
Another question is which standard you use. It should be similar in structure to the substance you investigate. A standardization is absolutely necessary.
ASTM D 1957 and ASTM E222-10 describe several versions of this method of determining hydroxyl value.
The "OH value" of a polyol refers to the hydroxyl value or hydroxyl number of the polyol. It is a measure of the concentration of hydroxyl (OH) groups in the polyol compound. The hydroxyl value is typically expressed in milligrams of potassium hydroxide (KOH) equivalent to the hydroxyl groups in 1 gram of the polyol.
The hydroxyl value is an important parameter in the field of chemistry, especially in the synthesis of polyurethanes and other polymers. It is used to determine the reactivity of the polyol, which is crucial in various chemical reactions, including the formation of polyurethane foams, coatings, adhesives, and more.
The higher the hydroxyl value of a polyol, the more hydroxyl groups it contains, and the more reactive it is in reactions involving isocyanates to form polyurethanes. The hydroxyl value is a key factor in formulating polyurethane materials with desired properties, such as flexibility, hardness, and thermal stability.
Different types of polyols can have varying hydroxyl values, and chemists often choose polyols with specific hydroxyl values to achieve the desired characteristics in their polymer formulations. Therefore, knowing the hydroxyl value of a polyol is important for formulating and optimizing various polyurethane and polymer products.
l With this protocol list, we might find more ways to solve this problem.
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