User Guide: Autotitrator pH/Alkalinity
Manual information
| Author(s): | L. Brandt |
| Reviewer(s): | D. Houpt, K. Fujine |
| Approval: | D.J. Houpt, Supervisor of Analytical Systems |
| Audience: | AD, MLS, RS, Scientists |
| Origination date: | 2/14/08 |
| Approved: | Version 2.0 (June 2013) |
| Revision: | IODP-II V3.0 (Suhonen, X382, May 2019) |
| Domain: | Chemistry |
| System: | Titration and Refractometry |
| Keywords: | Titration, pH, Alkalinity |
User Guide contents
| Table of Contents |
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Introduction
Method
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overview
Alkalinity is the measure of how much acid it takes to lower the pH of a water sample enough to convert all bicarbonate (HCO3–) and carbonate (CO32–) to carbonic acid (H2CO3). Although total alkalinity is equal to the stoichiometric sum of all bases in solution, not just carbonates, ~97% of alkalinity in seawater is due to carbonates.
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Apparatus, supplies and reagents
Laboratory
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apparatus
Figure 1. Metrohm Autotitrator (Note: dispenses 0.1 M HCl).
Hardware and
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software
- | Metrohm Titrino 794 autotitrator |
- | LabVIEW Alkalinity program v6.5 |
- | Laboratory oven |
- | Desiccator |
- | Water bath |
- | Analytical balance system |
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- | Pipettes, Class A: 0.5, 2, 3, 5, 8, 10, and 20 mL |
- | Adjustable pipettor, 100–5000 µl |
- | Volumetric flasks, Class A: 100 ml, 1 l |
- | Combination electrode (Metrohm, combined pH glass electrode, model 6.0234.100) |
- | pH paper, to bracket pH 7 (pH 1–12 paper) |
- | 5 ml snap-cap vials |
- | Magnetic stir bars |
Laboratory
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reagents
- | Hydrochloric acid (HCl), ultrapure |
- | Potassium chloride (KCl), certified ACS |
- | Borax (Na2B4O7·10H2O) |
- | Sodium bicarbonate (NaHCO3), certified ACS |
- | Sodium carbonate (Na2CO3), certified ACS |
Reagent
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solutions
- | 0.1 N HCl solution (premade from Fisher, AMS# CH5009) |
- | 3 M KCl solution (224 g KCl in 1 L l reagent water) |
Stock
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standard solutions (1
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l)
- | 0.1 M borax solution (38.1 g borax in 1 L l reagent water) |
- | 0.5 M NaHCO3 (42 g sodium bicarbonate in 1 L l reagent water) |
- | 0.1 M Na2CO3 (10.6 g sodium carbonate in 1 L l reagent water) |
- | 0.5 M Na2CO3 (53 g sodium carbonate in 1 L l reagent water) |
Standard
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solutions (100
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ml)
- | 5 mM Na2CO3 alkalinity (pipet 2.5 mL ml 0.1 M Na2CO3 into 97.5 mL ml 0.7 M KCl) |
- | 20 mM Na2CO3 alkalinity (pipet 10 mL ml 0.1 M Na2CO3 into 90 mL ml 0.7 M KCl) |
- | 40 mM Na2CO3 alkalinity (pipet 20 mL ml 0.1 M Na2CO3 into 80 mL ml 0.7 M KCl) |
- | 50 mM NaHCO3 alkalinity (pipet 10 mL ml 0.5 M NaHCO3 into 90 mL ml 0.7 M KCl) |
- | 100 mM Na2CO3 alkalinity (pipet 10 mL ml 0.5 M Na2CO3 into 90 mL ml 0.7 M KCl) |
- | IAPSO standard seawater (alkalinity ~2.325 mM) |
Reagent
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water
18.2 MΩ millipore deionized water
Buffer
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solutions
NBS buffers: commercially obtained low ionic strength solutions at pH 4.00, 700, and 10.00 (stable indefinitely; store in the chem lab refrigerator when not in use).
Tris buffer: tris(hydroxymethyl)amino methane or 2-amino-2-(hydroxymethyl)-1,3-propanediol
Bis buffer: bis(hydroxymethyl)methylamino methane or 2-amino-2-methyl-1,3-propanediol (not commercially available because of its highly hygroscopic nature; prepare as follows):
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Before an electrode can be used, it must be calibrated against pH buffers in the range expected in samples. Generally, calibration at pH 4, 7 and 10 covers the necessary range.
Figure 3. Electrode Calibrationcalibration.
- Make sure the water bath temperature is set to 25°C. Ensure no air bubbles are present in the acid dispensing line. Press DOS on the body of the titrator to push acid through the line to remove potential air bubbles. Select Calibrate Electrodes from the Main instrument panel.
- Enter your range of buffers (4, 7, 10).
- Select your Drift Span. The default drift span is 30.
- Place 3 mL of the first buffer solution in the vessel. Add stir bar. Remove the electrode from the storage solution, rinse with DI water, and blot dry with a Kimwipe. Do not rub the electrode, as this can cause a static charge. Insert the electrode tip into the titration vessel (not touching the bottom of the cup or the stir bar). Confirm that the frit is in the solution.
- Select Cal 1 and then Start. Measure until the drift gets close to 0.0. Usually approximately 500 seconds will be adequate. Select Stop when satisfied with measurement.
- When finished, clean vessel and the electrode.
- Repeat steps 4–6 with each calibration buffer, selecting Cal2 and Cal3 when appropriate.
- When all three buffers have been run, the slope value of the regression curve should be close to –59 pH/mV. Select OK-Save to save the calibration.
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A drift span of 30 (default) indicates that a minimum of 30 measurements will be taken after each addition of titrant (acid). The difference between the first and last measurements is compared to the stability criteria specified in the dispensing rate program. stability Stability criteria acts as follows:
If the difference between the first and the last measurement is smaller than the drift tolerancestability criteria, the next increment of acid will be dispensed.
If the difference between the first and the last measurement is larger than the drift tolerancestability criteria, measurement will continue at that increment until the drift tolerance the stability criteria is satisfied.
Figure 10. Drift Span.
Sample
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preparation and analysis
Sample Storage and Preservation
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Use 3 ml of interstitial water sample (usually pipetted from the IWS vial). Store residual sample in a 5 ml snap cap tube for additional shipboard analysis.
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Entering Sample Information
Click on SAMPLE in the Main Instrument Panel. Select the sample (IWS) from LIMS tree or type in Text_ID. Click Continue. If you type in a Filter Code IWS, the software will only bring up the IWS sample, which can be handy if your IW has a lot of children.
The system should now be calibrated and dispensing rate as well as the standard ratio correction selected. Generally, start with the slowest dispensing rate, assuming the alkalinity will be around the value of IAPSO. Same with the standard ratio correction, start with the IAPSO standard ratio correction and adjust according to what is measured in the samples.
Select SAMPLE from the Main instrument panel.
Figure 11. Entering sample information.
Click START. The software will guide you through.
Figure 12. Reminders before starting the pH measument.
After completing all the steps in the "We are watching you!" box, click GO. The pH measurement will commence.
Measuring pH
The titrator measures and records the pH value for each sample before titration for alkalinity begins.
Figure 13. Measuring pH.
Once the pH measurement is complete, you will see a guide box again.
Figure 14. Acid dispenser reminder prior to starting the alkalinity measurement.
Once the pH measurement is complete, you will see a guide box again.
Measuring Alkalinity
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1.
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On the Alkalinity/Standard tab, ensure dispensing rate, drift span, and standard ratio are set correctly (Figure 7).
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2.
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Ensure the water bath temperature is set to 25°C and pipette sample in titration vessel.
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3.
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Select the sample (IWS) from LIMS tree. Alternatively, type or scan in a Text_ID. If you use a Filter Code IWS, the software will only bring up the IWS sample, which can be handy if your IW has a lot of children.
Place 3 mL of the sample in the vessel. Add stir bar. Remove the electrode from the storage solution, rinse with DI water and blot dry with a Kimwipe. Do not rub the electrode, as this can cause a static charge. Insert the electrode tip into the titration vessel (not touching the bottom of the cup or stir bar)
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. Confirm that the frit is in the solution.
Select Continue.
Figure 12. Alkalinity data acquisition window.
Click START. The software will guide you through.
Figure 13. Reminders before starting the pH measument.
After completing all the steps in the "We are watching you!" box, click GO. The pH measurement will commence.
Measuring pH
The titrator measures and records the pH value for each sample before titration for alkalinity begins.
Figure 14. Measuring pH.
Once the pH measurement is complete, you will see a guide box again.
Figure 15. Acid dispenser reminder prior to starting the alkalinity measurement.
Insert acid dispenser probe when prompted. Click GO.
Measuring Alkalinity
The alkalinity titration is automatic once you have inserted the acid dispenser probe and clicked GO
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Click SAMPLE.
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5.
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Select measurement type from the list, enter operator name (this must be the same as LIMS user ID), text_ID, and sample volume (Figure 8). Click OK.
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6.
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After taking the initial mV and pH readings, the software prompts the user to insert the acid dispensing tip and click GO (Figure 9). If you make a mistake, click ABORT and no acid will be added to the sample. After clicking GO, the sample will be titrated to completion.
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7.
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Click Done/Save to upload data (see LIMS Integration).
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. The plot on the left side
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displays the signal coming directly from the electrode in real time. The y-axis is the mV reading, and the x-axis is time in seconds. The readings will continue until the stability criteria is satisfied, which provides a final mV reading. The green trend on the plot right side of the figure is the trend of mV readings vs. acid additions in ml. The mV readings come from the final value reached upon satisfying the stability criteria.
Figure 16. Analysis complete.
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Measuring Alkalinity.
The alkalinity titration will continue through the three stages of the dispensing rate program. The software will let you know once the analysis is complete.
Figure 17. Analysis complete.
Analysis is complete. Click OK. This will take you to the Gran-method window.
Figure 18. Gran-method window.
The Gran-Method window appears with the results of the titration. Write the pH and the Alkalinity Cor value in the blue book. This is the alkalinity
When finished, clean vessel and the electrode.
Click Ok/Save to save the results to the LIMS database. The value saved to LIMS and the value you need to note in the blue book is the “Alkalinity Cor” value. This is your result with the standard ratio correction applied to it. Click Ok/Save. This will upload the result to LIMS assuming the MUT uploader is active.
When finished, transfer the residue into a 5 ml cryo vial. Clean the vessel, the electrode and the acid dispensing probe.
Cleaning up after
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measurement
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1.
- Remove electrode from cup
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- Rinse electrode with DI water in squeeze bottle.
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3.
- Blot the electrode dry with a Kimwipe. Do not rub the electrode, which could cause static charge buildup.
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4.
- Place the electrode in a storage container containing 3 M KCl
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- Rinse and dry the acid dispensing tip and stir bar.
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6.
- Pipette or pour the titrated alkalinity IW sample into a container to ship to repository or scientist.
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7.
- Write on label how much HCl was added during titration
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- Rinse the titration vessel cup with DI water and dry.
Data Handling
Editing Gran Factor Points
Outlier Gran factor points (Figure 17) can be deleted from the linear portion of the curve to improve the accuracy of sample results as follows.
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