Spot test for fast determination of hydrogen peroxide as a milk adulterant by smartphone-based digital image colorimetry

•Greener and cost-effective detection of hydrogen peroxide as a milk adulterant.•Adulterant determination without sample pretreatment.•Direct measurements in microtubes by smartphone-based photometry.•Reliable approach applicable to different stages of milk production. A novel approach for the detection of hydrogen peroxide as a milk adulterant via a colorimetric spot test and smartphone-based photometry is proposed. The procedure is based on analyte oxidation of iron(II) followed by formation of a red iron(III)-thiocyanate complex. The acid medium required for complex formation doubles as a protein precipitation solution, while digital images are taken directly in disposable microtubes under controlled illumination conditions without sample pretreatment. Determination of the adulterant takes about 2 min and generates as little as 350 μL of waste per determination. A linear response was attained from 2.5 to 25.0 mg L−1, with a limit of detection of 1.7 mg L−1 at a 95% confidence level. Repeatability and reproducibility, expressed as coefficients of variation, were 4.8% and 6.8%, respectively. Due to these characteristics, along with the portability and wide availability of smartphones, the proposed method is attractive for determination of hydrogen peroxide adulterant at different stages of production chain.