In group II A. Analysis two methods are used to dissolve precipitated Mercury Sulphide – potassium chlorate and concentrated hydrochloric acid or aqua regia. These methods are essentially the solution of mercury sulphide by means of nascent chlorine. A third method has been described whereby mercury sulphide is dissolved in a sulphuric acid solution of potassium iodide. This mixture liberates hydrogen iodide which is the dissolving agent. From the equations [complicated equations here] is seems possible that the dissolving action is due to complex formation. It will be seen from equation (1) that the lower the concentration of the sulphideions the greater will be tendency for mercury sulphide to dissolve. This furnishes an explanation why potassium iodide alone cannot be used with same success as hydrogen iodide. With potassium iodide potassium sulphide instead of hydrogen sulphide will be formed during the solution reaction. As potassium sulphide is the salt of a strong base there will remain in solution a high concentration of sulphideions which must promote the back reaction shown in equation (1) and an equilibrium is all that can be attained. It is not possible to reduce the sulphide ion concentration by boiling as it is with hydrogen sulphide since potassium sulphide is non volatile. If the ordinary methods and this new method are compared it is seen that whereas nascent chlorine converts mercury sulphide into mercury chloride, the action of hydrogen iodide is to form a complex salt which has a very low concentration of mercury ions. It is possible therefore that this new method may not give so sensitive a test for mercury. The object of this portion of the work is to ascertain the possibility of testing for mercury in Group II analysis by using the hydrogen iodide method of dissolving the precipitated mercury sulphide and to compare the sensitivity of the test with that of the methods usually employed.