Dear Sahar,

I believe that the following text taken from a publication contained in the following link may be helpful in solving your experimental problem:

Gravimetric determination

Analysis of the dynamic swelling behaviors of PVA films were performed in a PBS buffer solution. PBS solution was used as the solution medium in the swelling process in order to provide a constant pH,  7.2 . The PBS solution contained NaCl (140 mM), KCl (3 M), Na2HPO4  (8.1 mM), and KH2PO4 (1.5 mM). After humidity regulation in a constant humidity (65% RH) and temperature (25 oC) chamber,

the circular shaped PVA films, 12 mm in diameter, was used in all swelling experiments. 

The swelling experiments were as follows:

Experimental Scheme A (Continuous method):

film was weighed before swelling and the weight was recorded as m0 . The sample was then immersed in a 200 mL PBS solution. At the same time, a stop watch was initiated to record the swelling time (the time for the sample immersed in PBS solution) of the sample. After a definite swelling time t1, the sample was removed from the solution and the surface water was quickly removed with a filter paper.

The sample was weighed on an electronic balance (0.1 mg accuracy) and the weight was denoted as m1 for the first immersion step. The sample was immersed in the solution for the second time, and the procedure was repeated until the equilibrium of swelling was reached. At time t, the weight of the sample was denoted as mt. The swelling ratio at different swelling time can be calculated by Eq. (1). 

Experimental Scheme B (Separated method):

Different from the experimental scheme A, the swelling ratio can be determined with a series of samples. In this method, the pre-weighed films numbered from

1 to n were immersed in the PBS solution, removed out, and weighed at different swelling time t1 , t2 , t3 4n , respectively. For example, at time t1 , sample 1 was removed out and weighed after the absorbing the surface water. The sample 1 was not used again. At time t2 , sample 2 was removed out and weighed.

The same procedure was repeated until the last sample n was weighed. Each sample has the same initial weight, m0 . Although sometimes the same level may be kept, it is better to accurately record the weight of the samples.

Swelling behavior was described by the normalized swelling ratio (Mt) at time t. It can be obtained in grams of water per gram of dry sample using Eq. (1).

Mt = (mt - m0/ m0) x 100%   Eq. (1)

where m0 and mt are weights of samples before swelling and after swelling time t, respectively. Swelling curves of films can be obtained by the plot of Mt vs.

time. As mentioned above, m0 in Eq. (1) may be different in separated method, and only the corresponding initial weight of the sample for determining the Mt can be used.

http://www.ipme.ru/e-journals/RAMS/no_53313/11_533_liu.pdf

Hoping this will be helpful,

Rafik

Maybe NMR or correlation of volume increase with percent reaction completion.