I recommend you a couple solutions:
Dichromate Reflux A Proposed Method for Chemical Oxygen Demand Chloride Correction in Highly Saline Wastes by Frank J. Baumann in Analytical Chemistry Vol. 46, NO. 9 (1974) 1337
(The complexing method, as currently practiced, using a weight ratio of HgS04:Cl equal to 1O:l
will yield reproducible results at chloride concentrations up to 5,000 mg/l.)
and Paper from Malaysian Journal of Chemistry (2003) 5:067-072, intitled: Minimizing the use of Mercuric Salts for Chloride and Bromide Corrections in Chemical Oxygenb Demand Test by Choon Fah et.al. (Chloride <10000mg/L; also in this paper "neutralize" interferences by Chloride
using HgSO4 up to 3 g/L...)
In the last paper, the authors suggest change of HgSO4 by Cr(NO3)2 and Al2(SO4)3
I can send you a copy of these papers to your a private email....please send me a email to firstname.lastname@example.org...