Journal of the Society of Motion Picture and Television Engineers (1950-1954)

but more readily soluble in hydrobrornic acid. Roman has described a method of solution in which sulfur dioxide is bubbled through a mixture of vanadium pentoxide and hydrobromic acid. The sulfur dioxide reduces the vanadium from the pentavalent to the readily soluble tetravalent state, as illustrated by the following equation: V206 + 2HBr + SO2 = VOSO4 + VOBr2 + H20. (1) The resulting solution can then be electrolyzed to form the divalent vanadium. This method was at first used for the preparation of the various solutions used in the current experiments. However, traces of sulfur dioxide left in solution eventually formed compounds which greatly increased the fogging propensity of the developer and, since removal of the last traces of sulfur dioxide proved to be quite difficult, alternative methods of dissolving the vanadium pentoxide were sought. One method consisted in heating the pentoxide with a warm mixture of hydrobromic and sulfuric acids, so that the bromide ion reduces the vanadium to the tetravalent state, and bromine gas is liberated, as in the equation: , V2O6 + 6HBr = 2VOBr2 + 3H20 + Br2. (2) This method was used for some time but was always considered objectionable because it was necessary to dispose of a considerable quantity of the highly corrosive and noxious bromine gas byproduct. More recently, it was found that the vanadium pentoxide dissolves quickly in a solution of oxalic acid in concentrated sulfuric acid. The vanadium is reduced to the tetravalent state while carbon dioxide is formed as a byproduct. V2O6 + H2G2O4 + 2H2S04 = 2VOSO4 + 2G02 + 3H2O. (3) This method is much more desirable, especially when preparing large batches of the developer. Once the solution containing the tetravalent vanadium ion has been formed, the extra acid as required by the developer formula is added and the solution is diluted to volume. To prepare the photographically active divalent vanadium ion, the solution is placed in an electrolytic cell and electrolyzed. A cell essentially similar to that described by Roman has been used for all the preparations, consisting of a noncorroding cylindrical tank made from materials such as Plexiglas or Type 316 stainless steel. A sheet-lead liner insulated from the tank constitutes the cathode, and an unglazed porcelain cup centered in the tank contains a carbon rod, such as is used in chlorine cells, which serves as the anode. The vanadium solution serves as the catholyte while the anolyte consists of a solution of sulfuric acid equal in total acidity to that of the vanadium solution. Direct current is supplied to the cell from a selenium rectifier or a d-c generator. Suitable provision is made to stir the solution in the cathode compartment, and the whole unit is surrounded by a cold water cooling-jacket. The laboratory cell described has a total cathode area of 0.12 sq m, upon which a current density of 250 amp/sq m is maintained, or a total of 30 amp, at a potential of 5 to 6 v across the cell. The reduction process is accomplished at a current efficiency of 70%, with the result that this cell can prepare the usual developer solution at a rate of about 1 liter/hr. The vanadium ion is reduced at the cathode in two steps: VO+++ + 2H+ + e = V+++ + H2O, (4) and V+++ + e = V++. (5) Reaction (4) will proceed to comple Rasch and Crab tree: Development by Vanadous Ion