Black Oxide Bath

Dropping Bottle Control Procedure

A sample of a freshly prepared Black Oxide production bath should always be taken as a control solution prior to running any parts through the bath.  If a sample was not taken, a laboratory prepared solution at the same concentration may be used as the control solution.  Titration of this “new”” solution will provide the figure for D1.

  1. Transfer a 5 ml sample of the production bath into a 125 ml Erlenmeyer flask.
  2. Dilute with water to the 50 ml mark.
  3. Add  2 ml 6N (1:1) Hydrochloric Acid to the flask.
  4. Add 4 ml of the 15% by weight Potassium Iodide solution.
  5. Add 2 ml of Starch solution.  The solution will become a dark blue to almost black color.
  6. Add the 0.5N Sodium Thiosulfate solution, from the dropping bottle – drop by drop – counting the drops while swirling the flask.
  7. The end point is marked by a sudden change in color from dark black to light brown.

Note: Upon standing, the light brown color will turn dark again, but additional Sodium Thiosulfate solution should not be added.  The first end point is correct.

  1. Calculate the amount of concentrate to be added as follows:


                C2= Black Oxide concentrate in gallons to be added to the bath

                D1= Number of drops of Sodium Thiosulfate used to titrate the new production bath.

                D2=Number of drops of Sodium Thiosulfate used to titrate the used production bath.

                C=Volume of Black Oxide concentrate in gallons used to make up the original new” bath.