How to Calculate Free NCO Content Percentage

Determine Isocyanate Content or NCO content in aromatic isocyanates (IPDI, MDI, PMDI, TDI, HDI) by titration methods. 

APPARATUS

a). Magnetic Stirrer

b). Magnetic Bead

c). 500ml conical flask.

d). 100ml measuring cylinder (Class-A).

e). 25ml pipette (Class-A).

f). Burette (Class-A).

REAGENTS & SOLUTIONS

a). Dry Toluene (LR Grade)

b). 1 N HCL reagent.

c). Rectified Spirit (Take 95.6ml Ethanol & add 4.4ml Distilled water obtained by distillation)

d). Bromophenol Blue indicator.

e). Di-Butyl Amine/Toluene mixture 1.0N (Take 67.5ml of Di-Butyl Amine & add to it 250ml of dry Toluene)

The procedure of NCO content by Potentiometric Titration

a). Weigh about 2.0gm of the sample accurately in a 500ml clean dry stopper conical flask.

b). Add 100ml Dry Toluene & 200 ml rectified spirit.

c). After complete dissolution add 25 ml Di-butyl amine/Toulene mixture to the flask containing a sample.

d). Now add 7-8 drops of indicator.

e). Heat the solution while stirring to dissolve.

f). Cool the solution to ambient temperature. Titrate the solution with standard 1N HCl solution.

g). Prepare a blank determination of taking a 250 ml empty conical flask. Add 100 ml Toluene, 200 ml rectified spirit, and 25 ml DBA/Toluene mixture with 7-8 drops of Bromophenol blue indicator and titrate with 1N hydrochloric acid.

R-NCO +R'-NH₂  = R-NHC=ONH-R

Calculation of Free NCO:

% Free NCO = (B-S) x N x 4.2/Wt. of the sample.

Run a blank.

% NCO = 4.2 x [(ml HCl blank) - (ml HCl sample)]/sample weight

Where:

B = Volume of HCL used in the blank titration.

S = Volume of HCL used with sample titration.

N = Normality of HCL

Precaution

Transfer the sample from the pipette to avoid moisture.

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