A REVISION OF THE ATOMIC WEIGHT OF IODINE. 95 



factory medium for washing the silver iodide, for it was found that the precipi- 

 tate could be washed indefinitely with even i per cent nitric acid without passing 

 into the colloidal state. The washings obtained in this way, when treated with 

 an iodide, gave no trace of opalescence even on long standing, showing that 

 silver iodide is essentially insoluble in the dilute acid. When silver iodide which 

 has been washed with nitric acid only is dried at a high temperature, a loss of 

 iodine takes place, owing to action on the silver iodide by the nitric acid. This 

 was evident from the dark color of the iodide. In order to avoid this difficulty, 

 the precipitate was finally transferred to the Gooch crucible with pure water, 

 sometimes after one rinsing with water. The nitric acid was so completely dis- 

 placed by this means that the precipitate did not darken even on fusion. Un- 

 fortunately, even this slight washing with water in most cases caused some of the 

 precipitate to pass through the crucible in a colloidal condition, consequently 

 these last washings were collected separately. 



In the earlier analyses the silver iodide in this colloidal solution was deter- 

 mined by first boiling the solution with a small quantity of ammonium iodide 

 until the silver iodide was coagulated, and then collecting the precipitate upon 

 a small filter paper, together with the asbestos shreds and any silver iodide 

 which were contained in the remainder of the filtrate and washings. As a rule 

 this operation was performed only after the wash-waters had stood for 2 or 3 

 days, in order that insoluble matter might have time to settle. The bottom of 

 each flask was rubbed gently with a rubber-tipped stirring-rod to detach adher- 

 ing particles. Finally the filter paper was burned in a weighed porcelain crucible 

 at as low a temperature as possible, for if a high temperature is employed, a loss 

 of silver iodide by volatilization occurs. The ash was treated with a drop of 

 nitric acid, and, after warming, a drop of ammonium iodide solution was added. 

 The excess of acid and ammonium salt was driven off and the crucible was 

 weighed. 



In these earlier analyses it was found necessary to detach small particles of 

 adhering silver iodide from the neck of the precipitating flask by rubbing with 

 a rubber-tipped rod. Later a better method was devised for collecting this small 

 amount of precipitate together with that contained in the colloidal washings. 

 First the flask was rinsed with a small quantity of a solution of potassium 

 cyanide, and this solution was poured into the colloidal wasliings. Then the 

 solution was evaporated to small bulk and was electrolyzed in a weighed plat- 

 inum crucible, which was heated to 130° in an electric oven and was finally 

 reweighed. The film was dissolved in dilute nitric acid and the solution was 

 precipitated with an excess of ammonium iodide. This precipitate of silver 

 iodide, if over 0.0005 g^i- ^^ weight, was collected on a small weighed Gooch 

 crucible, and the filtrate, together with the original filtrate and wash-waters, 

 was passed through a small filter repeatedly until clear. If the precipitate 

 weighed less than the above quantity, it was collected wholly upon a filter. 

 These filters were, of course, ignited and treated as before. The weight of the 



