Try to dissolve the other half in Na2S2O3, sodium Thiosulphate solution.
"An Introduction to Chemical Science"
R.P. Williams
Nitrites are oxidised by free chlorine and consequently do not interfere with the estimation of it by the Thiosulphate method; the influence of ferric salts can be overcome by substituting 3 c.
"Chlorination of Water"
Joseph Race