Milk and other dairy products are known for their calcium content. Calcium content is reported on the finished packaging, making it a necessary parameter to measure for dairy manufacturers. Unlike simpler aqueous solutions, determination of calcium in milk cannot be performed in the same manner. Aside from calcium, milk is full of proteins which can block the membrane of the calcium ion-selective electrode. In this Application Note, the determination of calcium in milk is shown with the copper ion-selective electrode which is less sensitive to contamination from proteins.