HF Neutralization Procedure EXP396

Owned by Chieh Peng (Unlicensed)
Last updated: Feb 08, 2022 by jr_barnes
2 min read

During EXP396, palynologists used the following method to process silica rich sediment

  1. Sediment (~10 grams)

  2. Add small amounts of HCl (10 or 30%) at a time until the reaction with carbonate is over. Fill up with water.

  3. Centrifuge (2500 rpm, 5-10 min) and decant.

  4. On the vortex make sediment loose and add small quantity of HF (38-40%) and let react. Add more HF until reaction is over (in total about 50-70 ml HF) and put on shaker for 2 hours (not heated).

  5. Add water until bottle is filled and centrifuge (this step I have never done, so I guess also 5-10 min at 2500 rpm?) and decant

  6. Make sediment loose again from bottom of the bottle on vortex (if it is under the fume hood otherwise do not).

  7. Add HCl to disaggregate silica gel up to less than half bottle

  8. Fill up the bottle with water

The Question here if we want to centrifuge again or we go upstairs to the paleo lab with this bottle (still containing some acid).

Sample through 250- and 10-micron sieve and residue collected for slides.

 

Due the high concentration of HCL used in the sample process, the HF test paper turned yellow even in the Acid Eater diluted (1:1) waste solution. We could not determine if the solution has been neutralized for -F. Therefore, we adopted the following procedure:

  1. Place 2 (if space allowed) 1L Nalgene bottles in the HF hood. Each bottle filled with Acid Eater up to half full. Scientist(s) pour HF/HCl waste solution directly into these bottles. Acid Eater should change color to indicate the neutralization of -F

  2. When the above bottle is 3/4 full, entire bottle is transferred into a 9L plastic carboy

  3. When the 9L carboy is half filled, we moved them into another hood.

  4. Calcite sand was added slowly into the 9L waste carboy. Caution: this action causes foaming, add calcite little at a time and very slowly to allow the foaming reaction to subside.

  5. Test pH of the waste solution frequently, treat the solution as if F-ions still exist. 

  6. When pH is above 5, take a small aliquot out. Add a few drop of concentrated HCl to reduce the pH back to 1.  Test the aliquot with HF test paper. If paper doesn't change color, the waste solution now can be treated as -F neutralized. (We have found that the Acid Eater does neutralize free F, we just cannot prove it with high concertation of HCl, as HF test paper turned yellow)

  7. Solution can be pour down the chemical sink, and the calcite sand can be disposed off overboard to prevent clogging the drain line.

  8. Carboy should then be rinsed and re-used for the next waste batch.