Closure device for drinking bottles (2), which has a closure body (4) for closing the drinking
bottle (2), which is designed in such a way that it can be placed in or on an opening of the drinking
bottle (2) in order to close it, wherein the closure body (4) has a sensor device (24, 30, 32) and wherein the sensor device (24, 30, 32) is designed in such a way that a component of a liquid that can be absorbed in the drinking
bottle (2) can be determined, characterized in that the sensor device (24, 30, 32) has an optic (24), a
light source (30) and a light sensor (32) for detecting the light emitted by the
light source (30), wherein the light of the
light source (30) can be radiated via the liquid to the light sensor (32) in order to determine the component, wherein the
optics (24) have a receiving space (44) for the liquid, which is arranged in the light path between the light source (30) and the light sensor (32); wherein the
optics (24) are designed such that they refract the light emitted by the light source (30), wherein the light sensor (32) is configured to determine a
refractive index of the
optics (24) on the basis of the detected light, and wherein the
refractive index is dependent on the component to be detected in the liquid arranged in the receiving space (44) of the optics (24).