de hy de ▁to ▁the ▁car box y lic ▁acid , ▁introdu cing ▁the ▁th i ol ▁via ▁sod ium ▁sul f ide , ▁and ▁coupling ▁the ▁mon omer ▁by ▁ox id ization ▁with ▁i od ine . ▁Today , ▁this ▁re agent ▁is ▁readily ▁available ▁commer cially . ▁ ▁Ell man ' s ▁test ▁Th i ols ▁react ▁with ▁this ▁comp ound , ▁cle aving ▁the ▁dis ulf ide ▁bond ▁to ▁give ▁ 2 - nit ro - 5 - th io ben zo ate ▁( T NB − ), ▁which ▁ion izes ▁to ▁the ▁T NB 2 − ▁di an ion ▁in ▁water ▁at ▁neutral ▁and ▁al kal ine ▁p H . ▁This ▁T NB 2 − ▁ion ▁has ▁a ▁yellow ▁color . ▁ ▁This ▁reaction ▁is ▁rapid ▁and ▁sto ichi ometric , ▁with ▁the ▁addition ▁of ▁one ▁mole ▁of ▁th i ol ▁re le asing ▁one ▁mole ▁of ▁T NB . ▁The ▁T NB 2 − ▁is ▁quant ified ▁in ▁a ▁spect roph ot ometer ▁by ▁meas uring ▁the ▁abs orb ance ▁of ▁visible ▁light ▁at ▁ 4 1 2   nm , ▁using ▁an ▁ext inction ▁coefficient ▁of ▁ 1 4 , 1 5 0   M − 1   cm − 1 ▁for ▁dil ute ▁buffer ▁solutions , ▁and ▁a ▁coefficient ▁of ▁ 1 3 , 7 0 0   M − 1   cm − 1 ▁for ▁high ▁salt ▁concentr ations , ▁such ▁as ▁ 6   M ▁gu an id in ium ▁hydro ch lor ide