石蕊试纸

shí ruǐ shì zhǐ

(chemistry) litmus paper

shí

abbr. for Shijiazhuang 石家莊|石家庄[Shi2 jia1 zhuang1], the capital of Hebei

ruǐ

variant of 蕊[rui3]

shì

to test

zhǐ

paper (CL:張|张[zhang1],沓[da2])