Notes This solution should work well for modern rapid fixers based on Ammonium Thiosulphate, but for fix formulae based on Sodium Thiosulphate you may need to double the amount of Potassium Iodide. Either solution should work fine as a basic indicator of fix exhaustion. Use: Keep the solution in an eyedropper bottle (mark accordingly for safety). Decant a small amount of fixer (approx. 10-20 ml) into a clear graduated cylinder and trickle a couple drops of the Fix Test into the cylinder. If a white precipitate forms then the fix should be replaced. If the solution remains clear then the fix is fine.