An analyzer comprising: an analysis vessel that accommodates a test fluid, a counter electrode, and a reference electrode; a working electrode assembly comprising a working electrode and an access control mechanism, the working electrode assembly being immersible in the test fluid; and a voltage source that applies an electrical potential to the working electrode; a measurement apparatus that measures an electrical current passed between the working electrode and the counter electrode when the electrical potential is applied to the working electrode; wherein: the working electrode and the counter electrode form an electrochemical cell; the access control mechanism of the working electrode assembly comprises a movable barrier that exposes a measuring portion of the working electrode to the test fluid through an access gap and forms a fluid tight seal around a covered portion of the working electrode, the fluid tight seal preventing exposure of the covered portion to the test fluid; and movement of the movable barrier exposes a fresh measuring portion of the working electrode, characterized in that the access control mechanism comprises a first sleeve and a second sleeve disposed over opposite ends of the working electrode and extending lengthwise over the covered portion of the working electrode, each of the first sleeve and the second sleeve having a proximal end, the proximal ends of the first sleeve and the second sleeve defining the access gap between the proximal ends of the first sleeve and the second sleeve, the measurement portion of the working electrode being exposed to the test fluid through the access gap. an analysis vessel that accommodates a test fluid, a counter electrode, and a reference electrode; a working electrode assembly comprising a working electrode and an access control mechanism, the working electrode assembly being immersible in the test fluid; and a voltage source that applies an electrical potential to the working electrode; a measurement apparatus that measures an electrical current passed between the working electrode and the counter electrode when the electrical potential is applied to the working electrode; the working electrode and the counter electrode form an electrochemical cell; the access control mechanism of the working electrode assembly comprises a movable barrier that exposes a measuring portion of the working electrode to the test fluid through an access gap and forms a fluid tight seal around a covered portion of the working electrode, the fluid tight seal preventing exposure of the covered portion to the test fluid; and movement of the movable barrier exposes a fresh measuring portion of the working electrode, characterized in that the access control mechanism comprises a first sleeve and a second sleeve disposed over opposite ends of the working electrode and extending lengthwise over the covered portion of the working electrode, each of the first sleeve and the second sleeve having a proximal end, the proximal ends of the first sleeve and the second sleeve defining the access gap between the proximal ends of the first sleeve and the second sleeve, the measurement portion of the working electrode being exposed to the test fluid through the access gap.