Abstract: In order to rapidly characterize the free-base nicotine (FBN) in e-liquids, two indicators, bromocresol green (BCG) and chlorophenol red (CPR), which could provoke color reactions with FBN, were employed to develop a spectroscopic method for FBN quantification in e-liquids. The developed method was compared with the pH method and the NMR method. The results showed that: 1) The two color indicators were weakly acidic and could bind with FBN but not with protonated nicotine, thus indicating the mass fraction of FBN. 2) The correlations between the spectroscopic responses and the mass fraction of FBN in e-liquids were established based on this principle. The calibration curve was lgA₆₁₆=0.017 2c－0.537 8 (r²=0.998 9, LOD=0.11 mg/g, LOQ=0.37 mg/g) when adopting BCG, while the calibration curve for CPR was lgA₅₈₇=0.015 4c－0.813 0 (r²=0.993 6, LOD=0.19 mg/g, LOQ=0.63 mg/g). 3) Sixteen commercial e-liquid samples were tested using the developed method. The average proportion of FBN (α_fbn) in e-liquids was 19.2% for the e-liquids with higher than 20 mg/g nicotine and 76.1% for those with lower than 20 mg/g nicotine. In addition, the mass fractions of FBN obtained by this method were higher than those by the pH method when α_fbn was lower. 4) There was a good correlation between the FBN mass fractions obtained by the spectroscopic method and by the NMR method. As an indicator, CPR was better than BCG. The spectrometry method could realize the rapid and visual determination of FBN in e-liquids.