HowTo – Chloridometry

Priority: low
Updating: mature from 2020

Objective:

Change log:

When Who Comments
2020 08 14 Sp17 Adapted from “individual tiitration” method in Buchler manual.
2021 03 12 Sp17 Reformatted from Microsoft Word to OneNote for DEC HowTo collection.
2022 04 13 Sp17 Updates and addition of calculation section.
2022 04 17 SP17 Discovered recipe for the gelatine reagent powder.
2023 06 15 Sp17 Convert to Markdown.

This is a low precision, minimal cost method intended primarily so less experienced users can obtain quick results. It is possible for this to be run outside of a lab environment, with due care about the handling of high strength acetic acid solution and waste.

This can be done outdoors if it is not raining. It needs a table or bench to hold samples, working containers, and the chloridometer. There can be small acetic+nitric acid spills so probably there should be a liner on the table (newspaper?) to avoid stains. One AC outlet is needed for the chloridometer.

This uses the “individual titration” method of the chloridometer, at the “low range” setting.

1. Equipment and supplies needed

  • Equipment

    • Buchler Chloridometer. Its printed manual.
    • Optional: Repeater pipettor for dispensing acid reagent and deionized water.
    • Single shot pipettors, adjustable 100-1000 uL and 10-100 uL.
    • Chloridometer glass vials and their styrofoam rack.
    • Three clean 50-500 mL glass or Nalgene-plastic containers for session:
      1. acid sample waste (starting with glass spaghetti sauce jar),
      2. deionized water supply,
      3. acid reagent supply.
    • Racks to hold sample containers in order of processing.
    • Heavy plastic or glass container to neutralize wastes. Starting out with a thick glass spaghetti sauce jar.
    • Deionized water spray bottle for cleaning electrodes.
    • Logbook for recording results, and pen.
    • ~1L heavy glass media bottle with tight cap (for acid reagent).
    • >=1L Pyrex beaker for making gelatin reagent, and glass stirring rod (can use glass pipettor) (not every session)

  • Supplies and reagents

    • All reagent ingredients:
      • Glacial acetic acid
      • Nitric acid
      • Thymol
      • Thymol blue
      • Gelatine powder

  • Gelatin reagent:

    • This involves boiling water using a bunsen burner or some other heat source, and hot glassware. Take appropriate precautions while handling container, such as a heatproof glove.
    • Mix 6g of gelatine power with 0.1g of Thymol powder and 0.1 g Thymol blue powder in a >=15 mL clean plastic or glass container.
    • To prepare in 1L heatproof flask/beaker:
      • Boil 1L deionized water.
      • Dissolve the above powder mixture into boiling water. Stir with glass stirring rod until powder is dissolved.
      • Divide into smaller containers so only a small amount is removed from fridge during later analysis sessions. Date containers.
    • 6 months useful life.
    • Store in fridge.
    • The resulting reagent is not particularly hazardous.

  • Acid reagent mix:

    • The stocks from which this is made, and the solution, are hazardous to skin and eyes. Wear appropriate protective equipment and clothes. See safety data sheets in chloridometer manual.

  • 40% glacial acetic acid, 0.4N nitric acid.

  • Using heavy glass media bottle with cap, graduated. Make 500 mL at a time: start with 250 mL deionized water, add 12 mL conc nitric acid, add 200 mL glacial acetic acid. Bring total volume to 500 mL with more deionized water.

  • Prepare this in lab, don’t take handle concentrated nitric and glacial acetic acid outside of lab, recommend using fume hood while mixing.

  • Indefinite shelf life without refrigeration if stored capped.

  • Store in acid cabinet in lab tightly capped.

  • Optional confidence building chloride concentration standard 1000 mg/L

    • Into 50 mL of deionized water in a centrifuge tube, dissolve 0.0824g of dried NaCl crystals: NaCl: 0.05*(Na + Cl)/Cl = 0.05*(23.0 + 35.5)/35.5 = 0.0824 g

  • Indefinite shelf life if in tightly capped container, without refrigeration.

  • This is not hazardous.

  • Note: Mark all reagent containers with date of preparation, contents including hazard symbols for the acid mix, and contact person’s netID.

  • Pipette tips: 50 mL repeater tip for acid (reusable after washing with DI); 50 mL repeater tip for deionized water. Alternative: use single shot adjustable pipettor.

  • Pipette tips: Single shot 1 mL blue - for dispensing normal strength samples and acid reagent.

  • Pipette tips: Single shot 100 uL yellow - for dispensing high chloride samples.

  • Disposable plastic droppers with micro tips to dispense gelatin droplets.

  • Sodium bicarbonate powder (“baking soda”) for neutralizing acid sample waste.

  • Kimwipes.

  • Silver polish - fresh 2020.

  • Deionized water for cleaning and dilution. Bring in gallon plastic jugs if working outside of lab.

  • Gloves to protect hands from acid and to protect samples from sweaty hands.

  • (not every session) heatproof gloves or other protective items while boiling water to make gel reagent.

2. Setup before tests:

  1. Thaw any frozen samples; bring all samples and gelatin reagent to room temperature before starting.
  2. Expose more silver wire in the chloridometer if necessary (electrode thins over time).
  3. Remove oxidation from electrodes (not always necessary):
    1. Apply silver polish using small sponge or kimwipe. All electrodes.
    2. Wipe off firmly, removing oxides.
    3. Rinse with deionized spray bottle or deionized-moistened kimwipe.
    4. Dry with clean kimwipe.
  4. If there is none left from prior sessions or leftovers have expired: prepare stock containers of gelatin reagent, acetic+nitric acid mix, and/or 1000 mg/L chloride standard.
  5. Prepare session containers of (1) deionized water and (2) acid mixture, for dispensing via pipettors into chloridometer vials. Set up supply of orange gel mixture for dispensing into vials.
  6. Set up racks of chloridometer vials and samples near chloridometer.
  7. Ready logbook for results recording. Make a table in advance to hold sample identities, dilution levels, and readings.

3. Operation:

  1. Wear nitrile gloves. Eyeglasses or protective goggles. Recommend lab coat and long pants.

  2. Power up chloridometer. Set for “auto” reading and LOW range.

  3. Set blank value.

    1. Prepare three or four chloridometer vials each with 1 mL acid, 4 gelatin droplets, and 3 mL deionized water.
    2. Read them consecutively with chloridometer. Average at least the last two, preferably all. It is okay to omit far outliers from the average.
      1. Reading is done by putting the vial into the holder on chloridometer’s front and raising the vial until the electrodes are immersed. The machine will spin the liquid until titration is complete, at which point its readout will stabilize.
    3. Set the chloridometer’s blank value to this average using front panel dials.

  4. Prepare several chloridometer vials in their rack for a given dilution ratio

    1. Dispense 1 mL of acetic+nitric acid mixture into each vial, then dispense four drops of gelatin into each vial. No mixing is needed since the chloridometer swirls these.
    2. Depending on expected concentration, dispense using single shot pipettor (Fresh tip per sample):
      1. 100 uL of sample plus 3 mL deionized water, good up to 1000 mg/L chloride
      2. 0.5 mL sample 2.5 mL deionized water
      3. 3 mL sample 0 mL deionized water, good down to 2 mg/L chloride
      4. Best to dispense deionized water first with one tip, every vial in group, then sample water using a different tip per sample.
    3. Insert vial below electrode, raise into electrodes, let it run, record the stopping measurement, lower vial.
    4. If there is no response after a couple of minutes, the sample is too dliute. Try in next batch using a larger proportion of sample.
    5. If the counting overflows, the sample is too concentrated. Try a deeper dilution in next batch. You can dilute more deeply than the above 100 uL + 3mL.
    6. Record result in logbook. Include the sample size 0.1, 0.5, or 3.0 mL and thee corresponding dilution multiplier.
    7. Probably it is best to finish a batch at its original dilution rather than interrupt a batch to try a different dilution. Probably it is best to do the extra dilutions as singletons.
    8. Consecutive time series samples will often be correlated so you can.

  5. Dump completed vials into waste acid beaker. Rinse vials with deionized water and air dry.

  6. Repeat above steps 3 and 4 for remaining samples.

  7. Optional to build confidence: test artificial samples with known chloride concentrations (made from 1000 mg/L stock solution), diluting them as in 3.b. to have values in range.

4. Cleanup:

  1. Wear nitrile gloves.
  2. Neutralize waste acid in a heavy plastic container or a heavy glass container using powdered baking soda and cold tap water. This will be exothermic thus the container will get warm. Keep adding a little powder at a time until fizzing stops, which indicates close to neutrality. Dump the mixture in sink while flushing with plenty of cold tap water. [Can also be dumped after neutralization outdoors on soil or grass.]
  3. Rinse waste beaker and neutralization container liberally with tap water.

5. Data interpretation

Adapted from the chloridometer manual page 11:

Typical conditions for low concentration of chloride over more than a hundredfold range are as follows:

Anticipated Conditions meq/L OR mg/L Blank 0.05-0.9
1.8-32		 0.9-5.0
33-177		 5.0-30.0
177-1065		 1000 mg Cl/L check standard
mL sample 0 3.0 0.5 0.1 0.1
mL 4N HNO3 + 40% CH3-COOH 1.0 1.0 1.0 1.0 1.0
drops gelatin 4 4 4 4 4
mL distilled H20 3 0 2.5 3.0 3.0
total volume 4.0 4.0 4.0 4.1 4.1
display reading — 15-270 45-250 50-300 282 expected
Multiplier to obtain meq Cl /L from display — 1/300 1/50 1/10 1/10
Multiplier to obtain mg Cl /L from display — 1/300*35.5=0.1183 1/50*35.5=0.71 1/10*35.5=3.55 3.55