In this paper, the on-line estimation of reaction heat in continuous stirred tank reactors, from noisy temperature measurements is addressed. An alternative representation of the original system is proposed in order to transform the output system disturbance on a system disturbance; this allows using a high gain-type observer without noise amplification problems. The uncertainty observer developed consists of integral (I)-type action and it is related with differential-algebraic concepts, where the observability condition for the uncertainty estimation from temperature measurements can be corroborated easily and the observer structure is simple enough to leads a feasible implementation. The structure of the proposed observer is robust against noisy measurements and model uncertainties. The performance of the observer is illustrated with numerical experiments.