Alkalinity measurement

Updated 10-11-2025 by: Rachel Bober

This protocol is based on Robotic Titrosampler 855 titration system connected to 800 Dosino (Metrohm, Switzerland) including a glass electrode (Metrohm, Switzerland).

General Comments

Due to the sensitivity of the titrator and the alkalinity measurement and in order to prevent unnecessary malfunctions, the analyses will be performed only during working hours (08:00-16:00) and by prior arrangement, so that another person is present in the room. *Seawater samples can be stored for several weeks in the refrigerator until measurement. *Ensure that all samples and buffers are allowed to equilibrate to room temperature prior to use.

  • Since you are working with acid. You must wear a lab coat, nitrile gloves always, before starting the experiment. -Safety Goggles is optional. -Do not bring any food or drink into the laboratory!!!
  • Do not touch or use tools, devices and materials not related to alkalinity measurement!!!
  • Please maintain order and cleanliness!

Materials and equipment

Prepare:

  1. Robotic Titrosampler 855 titration system connected to 800 Dosino (Metrohm, Switzerland( including a glass electrode) Metrohm, Switzerland).
  2. Laptop with Tiamo software
  3. For saving files – Disk on key
  4. Analytical balance with an accuracy of 0.001 grams.
  5. MANUAL pH measurements (see details below) or External pH bench meter for compresence - Adwa User Manual for pH/mV/ISE/Temperature Bench Meters AD1030 https://www.adwainstruments.com
  6. Reservoir of 1 Liter of HCL 0.05N for seawater samples HCl Titrisol: One ampoule to keep and the other to dilute in 2 liters of DDW and divide into two bottles of 1 liter each. HYDROCHLORIC ACID Hydrochloric acid for 1000 ml, c(HCl) = 0.1 mol/l (0.1 N) Titrisol
  7. Acid calibration Dickson standard: CRM (Certified Reference Materials) for Oceanic CO2 Measurements Batch
  8. Spritzer bottle with DDW water
  9. Metrohm 120 ml plastic sample beakers
  10. Metrohm Buffer solutions pH 4, 7 and 9 (25 °C) in single use sachets, colourless, box of 3 x 10 x 30 mL 62307230
  11. Large Beaker to be used for excess
  12. Mertrohm 62323000 storage solution for pH glass electrodes with KCl 3 mol/L as electrolyte Make sure that the filler opening is close while in storage!!!
  13. Kimwipes
  14. 50 mL no needle Luer-Lock™ sterile syringe for single use
  15. Disposable sterilized individually syringe filters 0.45 micron-meters
  16. Two thin tubes for connecting to the syringe and filter
  17. Permanent marker
  18. 110 ml dark glass sampling bottles with plastic screw caps (this volume is enough for 3 replicates)

Sampling Process

  1. Rinse the sampling bottle 3 times with the sample water.
  2. After rinsing, sampling will be done by inserting the tube into the bottom of the bottle and releasing the water through the tube in such a way that the water level in the bottle slowly rises. At the end of filling, the sample water should be allowed to flow out of the bottle, remove the tube while the water flows out and close the bottle with a stopper.
  3. The samples should be kept in a cool, shaded place until they reach the laboratory, then the samples should be stored in a refrigerator at 4 degrees Celsius.

##Before starting the experimental procedure

  1. Remove the samples from the refrigerator and wait until they reach room temperature.
  2. Make sure that the waste tank is empty and that at least one third of the DDW water tank is full (the large caps should be left slightly open).
  3. Connect the Disk on key to the keyboard. Important: Connecting and disconnecting the Disk on key must only be done when the device is turned off, otherwise the software will freeze.
  4. Turn on the device, the pump and the analytical balance.
  5. Shake the bottle containing the titrator to mix the HCl.
    1. Do not forget to fill the beaker in the Special position with DDW double distilled water and make sure that the water level is high enough.
  7. When the device is turned on, a message will appear regarding the preparation of the burette for work.
  8. Make sure that the Lift is down and the titrant tube (tip) is inside the beaker, so that the HCl is released into the beaker.
  9. Press OK to confirm and continue according to the following details:
  10. MENU–>OK
  11. MANUAL CONTROL–>OK
  12. POSITION LIFT–>OK (the LIFT goes down Below)
  13. Rinsing the piping and burette to remove air bubbles according to the following menu:
  14. MENU–>OK
  15. MANUAL CONTROL–>OK
  16. DOSING Make sure that YES is marked and press OK At this stage, the burette and piping are rinsed. If there is a concern for additional air bubbles, you can repeat the process of emptying and filling the piping with HCl
  17. At the end of the process, manually pump out the HCl liquid that has accumulated in the beaker and rinse using the pump- about a quarter of a beaker
    • At any stage, you can go back in the menu by pressing BACK
    • Acid calibration/normalization- wear gloves because the standard contains mercuric chloride.
    • For calibration, at least 5 good values for the standard must be obtained and to determine the acid concentration, a triple value must be calculated.

Electrode calibration

  1. Calibration must be performed before starting work Important! Do not expose the electrode to air for more than a few minutes. If you take a long break (over 10 minutes) from work, immerse the electrode in undiluted seawater and make sure the level in the beaker is high enough. Prepare the electrode: Open the cap that closes the hole on the top of the electrode (The filler opening)
  2. Check the level of the electrolyte solution inside the electrode. The electrolyte level should reach 0.5 cm below the hole. If the level is lower, add 3M KCl storage solution.
  3. External rinsing: Rinse the electrode with DDW water and gently absorb the excess with kimwipes without rubbing and move it to its designated place in the titration position.
  4. It is recommended to calibrate the device every 24 hours.
  5. Check the room temperature and change the buffers accordingly.
  6. Fill 3 beakers with buffer solutions: 4.00, 7.00, 9.00 pH and place them on the carousel in positions 1, 2, 3 (from lowest to highest).
  7. Go back to MENU by pressing BACK. In the menu, select:
  8. METHOD–>OK
  9. Select the pH cal method. Make sure that LOAD is marked and press OK to confirm. Ignore the message regarding the changes to make sure YES is marked and press OK again
  10. At this stage, press START and you will receive the following menu:
  11. No samples–>OK –>1 –>accept –>OK
  12. Next sample position –>OK–>1 –>accept –>OK
  13. Sample table –>OK –>off –>OK –>Start Building a table
  14. At this stage, the electrode is calibrated. When the calibration is complete, the zero pH and slope values will appear on the device screen:

pH[0] 7.443 Slope 98.3%
Variance 1.729 Calibration temperature (manual) 22.0 ◦C

T [s] T[◦C] U [mV] pH 140 22.0 198.2 3.994 Buffer 1 87 22.0 25.9 7.012 Buffer 2 37 22.0 91.7- 9.024 Buffer 3

  1. If the slope values are between 96-103% and the gap between pH 3 to pH 7 is 170 mV then the device is ready for samples’ measurements.
  2. Remove the cups containing the buffers and close them with the lids.
  3. Prepare the first sample in triplicates.
  4. The buffers will be replaced at least once a month.

Titration of the samples

  1. Mixing the sample.
  2. Weigh the sample as follows: Place the cup on the scales and zero it.
  3. Aspirate a small amount of the sample using the syringe and tube and rinse the inside of the syringe.
  4. Aspirate approximately 35 ml of the sample again, screw a 45 micron spinner filter onto the syringe and release approximately 33.5 ml (minimum 32.5 ml) into the beaker. Make sure that there are no drops on the side of the beaker and if they have to be collected, these drops will be included in the sample weight but will not participate in the titration.
  5. Record the bottle number and sample weight.
  6. Weigh 11 samples, as described in sections 1-5 and place them on the carousel in positions 1-11 (according to the recording). Return to the MENU by pressing BACK and using the arrows select: METHOD –>OK Using the arrows, select the desired method, for example: AT1-START-V2 and press OK Return to the MENU by pressing BACK and using the arrows select: MENU –>OK Sample table –>OK At the bottom of the screen, highlight: Edit –>OK METHOD –>OK Using the arrows, select the desired method, for example: AT1-START-V2 and press OK ID1–>OK In ID, type the sample number and press accept Sample size –>OK Type the sample weight and press accept The data for the first sample appears on Line 1 and now the data for the following samples must be added by adding lines using the right arrow and so on. When all the data has been entered, press Start and you will reach the Sample series and continue according to the following menu: No samples –> OK –> table –> OK –> accept –> OK Next sample position –> OK –> 1 –> accept –> OK Sample table –> OK –> ON –> OK –> Start At this point, the samples are automatically weighed and the next set can be weighed and the steps repeated.

At the end of the work:

  1. The electrode’s diaphragm must be immersed in the storage solution. The filler opening for the reference or bridge electrolyte must be closed!!
  2. Use the pump to wash the titration station - electrode, tip, and stirrer and finally pump out the water, wipe the beaker and return the electrode to the test tube containing the preservation liquid.
  3. The sample beaker must be emptied and wiped, and the entire lift assembly (stirrer, tip, etc.) must also be wiped.
  4. Turn off the titrator (long press), the pump and the scales (before turning off the titrator, make sure that the titration station - Lift is down).
  5. To Rinse the syringe and tubes with DDW and allow to dry.
  6. Wipe off any water or salt residue that has accumulated on the scales and the carousel to prevent corrosion.
  7. Empty the waste container.
  8. Keep the work station clean and tidy.

Troubleshooting and recommendations

B. Manual pH measurements:

  1. On the computer screen in the lower left panel select MANUAL –> MEASURING INPUT 1
  2. Make sure that the option MEASURED QUANTITY: pH is selected
  3. Fill sample beaker with about 2 envelopes of the desired pH buffer solution (volume should be enough for electrode to be immerse)
  4. Place the glass electrode in the sample tube
  5. Press START
  6. On screen it will show the pH value that was measure
  7. Write down the pH buffer measurement value.
  8. Wash twice the electrode in DDW and proceed on to measure the second buffer solution and the third.
  9. Wash twice the electrode in DDW.

C. Electrode treatment (to improve repeatability):

  1. Soak in the supplier’s cleaning fluid for about half an hour while stirring and finally rinse with DDW
  2. Soak in a 3M KCl solution: 22.36 g KCl in 100 ml DDW for half an hour while stirring and finally rinse with DDW.
  3. Drain the electrode from the electrolyte and refill.
  4. In extreme cases, the ceramic membrane can be gently scraped with a needle and if all the etch is gone, the electrode should be immersed in a dilute HF solution for a minute and then rinsed thoroughly with DDW
  5. A hydrochloric acid with a higher ionic strength can be prepared and is suitable for titrating seawater samples and improves repeatability.

49.78 g NaCl in 0.5 Liter DDW

Add 0.5 Liter HCl 0.1N

Formula for calculating alkalinity:

(100,000,000 X acid concentration X acid volume in ml in inflection point)/ Sample weigh in grams

Purchasing information

  1. Robotic Titrosampler 855 titration system connected to 800 Dosino (Metrohm, Switzerland).
  2. LL-Ecotrode Plus WOC 60256100 Glass electrode SAP SKU: 7003027 shipping services SAP SKU: 6000231 (Order number 20048171 from 6.5.2024 Supplier 44002737: ISI, Israel Scientific Instruments Ltd. www.isil.co.il 03-9232202).
  3. Adwa User Manual for pH/mV/ISE/Temperature Bench Meters AD1030 www.adwainstruments.com

Or

select MANUAL on the lower left corner –> MEASURING INPUT 1

Make sure that the option MEASURED QUANTITY: pH is selected

Put the glass electrode in the desired buffer solution and press START On screen it will show the pH value that was measured Wash twice in DDW and proceed on with the second buffer solution.

  1. Reservoir of 1 Liter of HCL 0.05N for seawater samples Purchase 2 ampoules of HCl Titrisol: One ampoule to keep and the other to dilute in liters of DDW and divide into two bottles of 1 liter each.

HYDROCHLORIC ACID Hydrochloric acid for 1000 ml, c(HCl) = 0.1 mol/l (0.1 N) Titrisol

SAP SKU: 7002926 Chemical Order number 20050598 from 10.7.2024 Supplier: Mercury Scientific and Industrial Products Ltd. 44045294

  1. Acid calibration Dickson standard: CRM (Certified Reference Materials) for Oceanic CO2 Measurements Batch number Do not place an order yourself but order through Jack Silverman from the Institute for Seas and Lakes Research. Once a year Jack orders a large quantity from the US and we offset it against him.

The glass bottles should be saved for reuse, because they are sent back to be filled in a special, temperature-controlled Styrofoam box to the US to Prof. Andrew G. Dickson.

If you are ordering yourself then look for additional details in purchasing order 294129 dated 9.5.2017. Make sure that you fill out an up-to-date CRM order form and send it by FedEx in a special, temperature-controlled Styrofoam box to the USA, to:

Prof. Andrew G. Dickson

Certified Reference Materials Laboratory

Scripps Institution of Oceanography

University of California, San Diego

Andrew Dickson

Deep Sea Drilling West Room 15

2215 Downwind Way

La Jolla, CA 92037

co2crms@ucsd.edu

858 534 8388

  1. Spritzer bottle with DDW water
  2. Metrohm plastic sample beakers
  3. Metrohm sample beakers, plastic (PP), 120 mL, 20 pieces 6.01400.200 Sample beakers, plastic (polypropylene, PP), 120 mL, suitable for 602041030 OMNIS sample rack, 20 pieces Metrohm Buffer solutions pH 4, 7 and 9 (25 °C) in single use sachets, colourless, box of 3 x 10 x 30 mL 62307230 OR Buffer Solution pH4 (30x30mL) 62307200 Buffer Solution pH7 (30x30mL) 62307210 Buffer Solution pH9 (30x30mL) 62307220 SAP SKU: 7002932 Shipping Services SAP SKU: 6000231 (Order Number 20048171 from 6.5.2024 Supplier: ISI Ltd. 44024787).
  4. Large Beaker to be used for excess
  5. Mertrohm 62323000 storage solution for pH glass electrodes with KCl 3 mol/L as electrolyte Make sure that the filler opening is close while in storage!!!
  6. Kimwipes
  7. 50 mL no needle Luer-Lock™ sterile syringe for single use
  8. Disposable sterilized individually syringe filters 0.45 micron-meters
  9. Two thin tubes for connecting to the syringe and filter
  10. Permanent marker
  11. 110 ml dark glass sampling bottles with plastic screw caps (this volume is enough for 3 repetitions)
Written on November 10, 2025