In this work, we study the effect of the continuous phase on crystal nucleation of potassium nitrate (KNO3) from aqueous solution by carrying out isothermal crystallization experiments on a set of solution droplets in a microfluidic device. To this end, rates of primary crystal nucleation of KNO3 are derived from induction time measurements. Nucleation is found to follow different heterogeneous nucleation mechanisms, regardless of the continuous phase used. Depending on supersaturation, heterogeneous nucleation centers provoke fast nucleation in a fraction of droplets, whereas slow nucleation is observed in the remaining fraction of droplets. Furthermore, our results suggest that nucleation occurs preferentially at the liquid-liquid interface of the droplets. Based on our results, we derive kinetic parameters and discuss them in the context of classical nucleation theory (CNT).