HowTo – Preserving aliquots for cations analysis [90%]

Priority: low (cations data are low priority)
Updating: rare

This Howto covers how the project should preserve aliquots from field samples for cations analysis. This is performed upon return from a road trip or after receipt of a lake sample.

Change log:

When Who Comment
2023 02 25 Sp17 Began. Derived from sp17’s Catskills work, EPA-inspired.
2023 05 03 Sp17 Minor refinements to 90%. It has been taking 1 mL of acid to bring 45 ml of sample to pH 2. Makes provision for lakes samples.
2023 06 15 Sp17 Convert to Markdown.

Related howtos:

Summary: We pour off and acidify an aliquot of 40 mL to pH 2 before freezing the main field sample. This preserves the aliquot for later cations analysis. (The 40 mL is large, allowing for three tests of the same sample, perhaps for replication.)

This was recommended by Shree Giri, former SWL staff member and current staff member at the USDA Plant, Soil and Nutrition lab at Cornell’s Ithaca campus. He observed the formation of precipitated in our samples that had been frozen for a long time.

1. Objective

  • Preserve samples for total cations determination by preventing precipitation during storage.

2. Quality assurance considerations

  • Field containers are kept on ice upon return to lab, cations aliquots made within 24 hours after return to lab, field samples are frozen shortly after aliquots are made, and aliquots are placed in fridge soon after they are acidified.
  • Keep from mixing up samples or aliquots by proper labeling and logging.
  • Record processing dates and acid amounts added in paper lab logbook.
  • Periodically verify that acidification reaches pH 2.0.

3. Safety considerations

  • This uses hazardous (“Danger”) 1.0M nitric acid. Wear gloves when handling acid and washing acid containers.

4. Supplies and Equipment Needed

4.1 Equipment

  • 250 mL media bottle to hold 1.0M nitric acid. Marked with hazard icons and contents according to safety rules.
  • Pipettor 100-1000 µL to transfer 1.0M nitric acid from beaker to samples.
  • Meter that can read to pH 2 (approximate), and 0-6 pH strips.
  • Glass stir rod to transfer acidified sample drops to pH paper.
  • When making 1.0M nitric acid:
    • Magnetic stirrer plate, cleaned magnet bar, cleaned 500 mL beaker, PPE goggles.
  • Project lab logbook and pen.
  • Refrigerator.
  • Rack to hold 50 mL centrifuge tubes.

4.2 Expendable supplies

  • Nitrile gloves.
  • 1.0M nitric acid. Working with this dilution instead of full strength for safety.
    • Recipe: Working under fume hood wearing gloves and lab goggles, add 16 mL of 70% nitric acid to 234 mL of deionized water, in glass beaker large enough to use stirring rod on magnetic stirring plate. Magnetic stir under fume hood for 10 minutes. Room temperature.
  • New 50 mL centrifuge tubes, one per sample.
  • Rack for 50 mL tubes.
  • Deionized water in spray bottle. Something to catch washwater from sprayer.
  • pH paper that can read down to 1, such as 0-6, if not using meter.
  • One 1 mL blue pipettor tip.

5. Steps

  1. Gather samples returned from field, before they were first frozen. (Thaw one bottle of each Lake samples if they arrived frozen.) This should be done on the day of return from sampling or day after so that main samples can be frozen with minimum delay, or lake samples can be re-frozen.
  2. Begin a fresh, dated lab logbook page to record which samples are being processed, and the volume of nitric acid added to each.
  3. Label 50 mL centrifuge tubes with sample identity (site, sampling point, date, time if more than one sample from point on a day) and words warning about acidification. “For Cations. HNO3 acidified to pH 2.” This can be on bottom row of customary field sample labels.
  4. Pour 45 mL aliquots per sample into the centrifuge tubes. OK to use graduations on tube. Keep volumes consistent over time. Leave room for freezing (though initially the acidified aliquots are stored in refrigerator).
  5. Bring out 1.0M Nitric acid from acid cabinet. We will load pipettor from this container.
  6. If volume of acid needed has not been predetermined:
    1. Begin acidifying one sample in 50 mL cent tube one 0.1 mL volume increment of 1M HNO3 at a time.
    2. For each increment:
      1. Transfer acid from media bottle to aliquot container using pipettor. Cap container and invert three times to mix. Remove cap.
      2. Use glass stir rod to transfer a drop from sample container to pH paper.
      3. Record in logbook the final pH for this dropperful count.
      4. Repeat i-iii until pH reaches around 2.0. Say 1.7-2.3.
    3. Record volume of HNO3 it took for this sample to reach final pH around 2.0, and the final pH. The volume represents dilution of the sample.
    4. This discovered volume can be reused for later samples and batches, with occasional checks of final pH values of samples (as in 5.b.ii). Note: Samples from different environments may need different amounts of acid.
  7. Acidify all samples with predetermined volumes of 1M HNO3. Record volumes added in lab logbook.
    • If you are using the stirring rod to check pH, rinse it with spray deionized water between samples being checked, to avoid cross contamination of samples.
  8. Put rack of acidified aliquots into fridge. Samples go into freezer.

6. Cleanup

  1. Rinse stirring rod several times with deionized water. Return to DEC drawer.
  2. Put 1.0M acid back in inorganic acid cabinet, put other tools in their homes.