DIY Reference Solution for Alkalinity tests that use pH Titration

Discussion in 'Reef Chemistry by Randy Holmes-Farley' started by louisvillereefer, Feb 6, 2018.

Tags:
  1. louisvillereefer

    louisvillereefer Member

    Joined:
    Feb 11, 2017
    Messages:
    43
    Likes Received:
    38
    Location:
    Louisville CO
    Randy and Co,

    Can you please recommend the best way to create a 8 dKH alkalinity reference solution that could be used to verify the accuracy of alkalinity tests such as Salifert and the KH Guardian?

    I have a small jewelers scale and I can get some graduated cylinders if that's what you recommend.

    Ideally, the solution would be accurate within 1%.

    Thanks!
    Brent
     
    Tags:

  2. Randy Holmes-Farley

    Randy Holmes-Farley Reef Chemist Staff Member Team R2R R2R Supporter R2R Excellence Award Article Contributor Expert Contributor

    Joined:
    Sep 5, 2014
    Messages:
    25,393
    Likes Received:
    13,449
    Location:
    Arlington, Massachusetts, United States
    Sure, but 1% is tricky since the pH endpoint of an alk titration is not necessarily the same in seawater and freshwater, and in both situations it changes a little with the actual alk being tested. But it will be reasonably close.

    Do you have sodium bicarbonate (baking soda) or sodium carbonate (washing soda) on hand?
     
  3. louisvillereefer

    louisvillereefer Member

    Joined:
    Feb 11, 2017
    Messages:
    43
    Likes Received:
    38
    Location:
    Louisville CO
    Yes, I have both on hand. Thanks!
     
  4. Randy Holmes-Farley

    Randy Holmes-Farley Reef Chemist Staff Member Team R2R R2R Supporter R2R Excellence Award Article Contributor Expert Contributor

    Joined:
    Sep 5, 2014
    Messages:
    25,393
    Likes Received:
    13,449
    Location:
    Arlington, Massachusetts, United States
    Sodium carbonate has a molecular weight of 106 grams per mole.

    An 8 dKH standard will contain 2.88 meq/L of sodium carbonate, and since each carbonate carries two units of alkalinity, we want half that, or 1.44 mM sodium carbonate, or 1.44 millimoles per liter.

    1.44 millimoles is 1.44 millimoles x 106 mg/mmole = 152.6 milligrams per liter.

    So dissolving 152.5 milligrams in a liter (1.00 kg) of water will give an 8 dKH standard.

    If that is not able to be weighed out accurately (or the 1 L measured accurately), you can make a higher concentration and dilute it down, but that too potentially introduces errors, in the volume measurements.

    I might also suggest baking the sodium carbonate first to be sure it has no moisture in it and it is all present as carbonate and very little as bicarbonate. 1 h at 400 deg C should be sufficient.
     
    louisvillereefer likes this.
  5. louisvillereefer

    louisvillereefer Member

    Joined:
    Feb 11, 2017
    Messages:
    43
    Likes Received:
    38
    Location:
    Louisville CO
    Thanks so much Randy! That is really helpful.

    What type of measuring device would you recommend to obtain an accurate volume measurement? Graduated Cylinder?

    Also, I'm not sure that my oven reaches 400 degrees C but I can turn it up all of the way.

    After making the solution, assuming it is capped in an airtight bottle how long will it be good for?
     
  6. Randy Holmes-Farley

    Randy Holmes-Farley Reef Chemist Staff Member Team R2R R2R Supporter R2R Excellence Award Article Contributor Expert Contributor

    Joined:
    Sep 5, 2014
    Messages:
    25,393
    Likes Received:
    13,449
    Location:
    Arlington, Massachusetts, United States
    Top bet is a volumetric flask. Second to that is either mass on a scale, or a graduated cylinder.

    What is the range of your scale?
     
    louisvillereefer likes this.
  7. Randy Holmes-Farley

    Randy Holmes-Farley Reef Chemist Staff Member Team R2R R2R Supporter R2R Excellence Award Article Contributor Expert Contributor

    Joined:
    Sep 5, 2014
    Messages:
    25,393
    Likes Received:
    13,449
    Location:
    Arlington, Massachusetts, United States
    Oops, my error, I meant 400 F. :)
     
    louisvillereefer likes this.
  8. Randy Holmes-Farley

    Randy Holmes-Farley Reef Chemist Staff Member Team R2R R2R Supporter R2R Excellence Award Article Contributor Expert Contributor

    Joined:
    Sep 5, 2014
    Messages:
    25,393
    Likes Received:
    13,449
    Location:
    Arlington, Massachusetts, United States
    It is good for as long as evaporation does not change the liquid volume significantly. The entry or exit of CO2 does not alter the actual alkalinity, just how much of it is carbonate and how much is bicarbonate.
     
    louisvillereefer likes this.
  9. louisvillereefer

    louisvillereefer Member

    Joined:
    Feb 11, 2017
    Messages:
    43
    Likes Received:
    38
    Location:
    Louisville CO
    Great. Thanks again. I'm surprised that no one seems to sell a standard reference solution.

    ;) Thanks for clarifying.

    My scale ranges up to 500g in 0.01g increments and is calibrated with a 500g weight

    I will probably get another scale that 0 - 20g in 0.001g increments with a 10g calibration weight since my other scale will not be accurate for mgs.
     
  10. Randy Holmes-Farley

    Randy Holmes-Farley Reef Chemist Staff Member Team R2R R2R Supporter R2R Excellence Award Article Contributor Expert Contributor

    Joined:
    Sep 5, 2014
    Messages:
    25,393
    Likes Received:
    13,449
    Location:
    Arlington, Massachusetts, United States
    2 batches of 500 g is a good way to get 1 L of water pretty accurately. :)
     
  11. louisvillereefer

    louisvillereefer Member

    Joined:
    Feb 11, 2017
    Messages:
    43
    Likes Received:
    38
    Location:
    Louisville CO
    Thanks so much. I really do appreciate the help!
     
  12. Randy Holmes-Farley

    Randy Holmes-Farley Reef Chemist Staff Member Team R2R R2R Supporter R2R Excellence Award Article Contributor Expert Contributor

    Joined:
    Sep 5, 2014
    Messages:
    25,393
    Likes Received:
    13,449
    Location:
    Arlington, Massachusetts, United States
    Alternatively, here's a better method using the scale you have.

    EDITED DUE TO A MISTAKE

    Dissolve 1.52 g in 498.5 g of water.
    Take out 100 g and add to 500 g of water. Conc ~ 0.304 grams in 600 grams total,

    Take out 89.5 g and add to 210.5 g of water. Conc ~ 0.0453 grams in 300 grams total, ~0.1512 grams/L, ~ 1.426 mM (8.01 dKH)
     
    Last edited: Feb 13, 2018
    louisvillereefer likes this.
  13. louisvillereefer

    louisvillereefer Member

    Joined:
    Feb 11, 2017
    Messages:
    43
    Likes Received:
    38
    Location:
    Louisville CO
    Thanks Randy. That's a great option.
     
  14. Randy Holmes-Farley

    Randy Holmes-Farley Reef Chemist Staff Member Team R2R R2R Supporter R2R Excellence Award Article Contributor Expert Contributor

    Joined:
    Sep 5, 2014
    Messages:
    25,393
    Likes Received:
    13,449
    Location:
    Arlington, Massachusetts, United States
    Oops, made a mistake in the last protocol. I am editing it.
     
  15. Randy Holmes-Farley

    Randy Holmes-Farley Reef Chemist Staff Member Team R2R R2R Supporter R2R Excellence Award Article Contributor Expert Contributor

    Joined:
    Sep 5, 2014
    Messages:
    25,393
    Likes Received:
    13,449
    Location:
    Arlington, Massachusetts, United States
    Fixed now. Forgot the factor of 2 in mM to dKH conversion due to 2 units of alk for each carbonate.
     
  16. louisvillereefer

    louisvillereefer Member

    Joined:
    Feb 11, 2017
    Messages:
    43
    Likes Received:
    38
    Location:
    Louisville CO
    Ok thanks.
     
  17. louisvillereefer

    louisvillereefer Member

    Joined:
    Feb 11, 2017
    Messages:
    43
    Likes Received:
    38
    Location:
    Louisville CO
    Hi Randy,

    The carbonate solution worked perfectly on the Salifert test. However, when I tried it out on the KH Guardian, the KH Guardian would not test the solution because the pH was too high. I'm guessing that a baking soda solution would have a lower pH.

    Can you please tell me how much baking soda is need for the 8 dHK solution? Also, do you have an idea what pH the baking soda solution would be (approximately)? Is there anything special I need to do to ensure that the baking soda is almost pure bicarbonate?

    Thanks again!
    Brent
     
  18. Randy Holmes-Farley

    Randy Holmes-Farley Reef Chemist Staff Member Team R2R R2R Supporter R2R Excellence Award Article Contributor Expert Contributor

    Joined:
    Sep 5, 2014
    Messages:
    25,393
    Likes Received:
    13,449
    Location:
    Arlington, Massachusetts, United States
    We can try a modification of the last recipe using sodium bicarbonate instead of sodium carbonate, but it is hard to ensure sodium bicarbonate is pure sodium bicarbonate without added water or loss of CO2:

    Dissolve 2.41 g sodium bicarbonate in 497.6 g of water.

    Take out 100 g and add to 500 g of water. Conc ~ 0.482 grams sodium bicarbonate in 600 grams total

    Take out 89.5 g and add to 210.5 g of water. Conc ~ 0.0719 grams in 300 grams total, ~0.2397 grams/L, ~ 2.853 mM ( 7.99 dKH)
     
  19. louisvillereefer

    louisvillereefer Member

    Joined:
    Feb 11, 2017
    Messages:
    43
    Likes Received:
    38
    Location:
    Louisville CO
    Thanks Randy. I'll give that a try. I have that 20g scale now that read in milligrams. So, I'll just add 240mg of sodium bicarbonate to 1 L of water.

    I also found a good source of 0.1N HCl for your DIY alkalinity test, which I can use to test the solution. For other people's reference, Hanna Instruments sells a 1L bottle of 0.1N HCl for $20 plus shipping.
     
  20. Randy Holmes-Farley

    Randy Holmes-Farley Reef Chemist Staff Member Team R2R R2R Supporter R2R Excellence Award Article Contributor Expert Contributor

    Joined:
    Sep 5, 2014
    Messages:
    25,393
    Likes Received:
    13,449
    Location:
    Arlington, Massachusetts, United States
    Sounds good! :)
     
    louisvillereefer likes this.
Draft saved Draft deleted

Share This Page

Loading...