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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 | 6/13/13 | |
Revision: | IODP-II V2.1 | 1/3/2014 371T | X382 April 2019 |
Domain: | Chemistry | ||
System: | Titration and Refractometry | ||
Keywords: | Titration, pH, Alkalinity |
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Figure 1. Metrohm Autotitrator with Workstation (Note: dispenses 0.1 M HCl).
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- | Pipettes, Class A: 0.5, 2, 3, 5, 8, 10, and 20 mL |
- | Adjustable pipettor, 100–500 100–5000 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 |
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1. | Heat reagent water in a glass beaker while adding Bis and stirring. |
2. | When precipitation is observed (saturation), add HCl until pH = 7 as tested with pH paper. |
3. | Cool solution (a gel forms in the beaker), then decant the water and place in a warm oven until dry. |
4. | Scrape dry remains from the bottom of the beaker and place in a vacuum desiccator to cool. |
5. | Take first weight quickly upon removal from desiccator when making up the buffer. See ODP Tech Note 15 for complete instructions on making up the powdered buffer. |
6. | Redry in oven and cool in desiccator each time prior to weighing. |
7. | Make new Bis buffer at the beginning of each expedition. |
Instrument Calibration
Overview
Main instrument panel
Instrument calibration
Instrument calibration
Before the titrator can be used to measure samples, the electrode must be calibrated against pH buffers in the same range expected in samples. To ensure accuracy of sample measurements, the titrator settings must also be optimized. Calibration of the titrator system includes calibrating the following steps: Calibrate the electrode, Select or create selecting a dispensing rate program , and Calculate the calculating a standard ratio. Each of these steps are discussed in the following sections.
Calibrating the
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electrode
Before the titrator can be used to measure samples, the The pH electrode must be standardized for linearity across the pH range of the calibrated against pH buffers in the range expected in samples. Generally, calibration at pH 4, 7 , and 10 covers the necessary range. The electrode is calibrated at the beginning of each expedition as follows (calibration dataflow is shown in pH Dataflow).
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1.
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Select the Calibration tab from the main menu of the Alkalinity program (Figure 2).
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2.
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In the Buffers fields, enter pH of at least 2 buffers to bracket the expected sample pH values (generally 4, 7, and/or 10).
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3.
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Place enough buffer solution in the titration vessel to cover the electrode tip and add a stir bar.
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4.
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Immerse the electrode tip in the buffer solution (not touching the bottom of the vessel).
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5.
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Set water bath temperature to 25°C, and activate stirrer.
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6.
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Set the Drift Span field (below plot) to a value >0 (30 readings is recommended), and click Measure 1.
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7.
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Measure with stirring until drift (mV) vs. time (sec) on the plot is near 0.0 (this may take an hour or longer).
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8.
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When the drift is nearly 0.0, click Done.
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9.
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Repeat Steps 2–8 with each calibration buffer, selecting Measure 2 and Measure 3, as applicable.
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10.
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Click Calibrate to view the regression curve. The slope value should be close to –59 pH/mV.
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11.
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Click Save Calibration to accept values.
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Setting the Dispensing Rate
The rate at which the titrator dispenses the acid into the sample can be adjusted according to expected alkalinity value. Based on experience, for samples with alkalinity <20 mM, a 3 mV rate is usually appropriate. A 3 mV rate dispenses as follows:
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Volume of 0.1 N HCl (µL)
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30
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150
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4
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220
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1
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220–240: through the linear Gran portion of the titration curve
Higher alkalinities may require faster dispensing rates. The dispensing rate can be selected from a list of predefined rate programs or a new dispensing rate program can be created.
Selecting a Predefined Dispensing Rate Program
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1.
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Open the Rates & Standard Corrections tab in the Alkalinity main menu (Figure 3).
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2.
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Under Rates, select a predefined rate from the list. These rates are multistep rate programs such as the 3 mV rate example in the previous section.
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3.
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Click Load This Rate to use the predefined rate program.
Figure 3. Selecting a Predefined Dispensing Rate Program.
Creating a New Dispensing Rate Program
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1.
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Open the Alkalinity/Standard tab in the Alkalinity main menu (Figure 4).
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2.
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Edit the Rates fields for each step in the rate program as follows:
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–Level (mV) up to which this rate is applicable (150 in the example shown below).
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–Acid (mL) to add in each increment (0.030 mL = 30 µL as below).
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–Drift tolerance (mv/s) for acid increment (0.02 as below). Measurement continues until drift tolerance is satisfied.
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–Seconds until rate program times out (timeout(sec)) if drift tolerance is not satisfied (180 as below).
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3.
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Click Save This Rate.
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4.
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Enter a rate program name in the dialog box and click OK.
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Figure 4. Creating a New Dispensing Rate Program.
Drift Span
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 drift tolerance specified in the rate program (see Creating a New Dispensing Rate Program). Generally, drift tolerance starts at 0.10 mV/s. Drift tolerance acts as follows:
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–
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If the difference between the first and last measurements is smaller than the drift tolerance the next increment of acid will be dispensed.
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–
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If the difference between the first and last measurements is larger than the drift tolerance measurement will continue at that increment until the drift tolerance is satisfied
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- Select Calibrate Electrodes from the main Alkalinity interface.
- Enter your range of buffers (4, 7, 10).
- Select your Drift Span. 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 measurement is compared to the drift tolerance specified in the rate program.
- Place 3 mL of the first buffer solution in the vessel. Add stir bar and immerse electrode in vessel.
- Select Cal 1 and then Start.
- 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, select OK-Save to save the calibration.
Calibrating the Electrode
The pH electrode must be standardized for linearity across the pH range of the samples. Generally, calibration at pH 4, 7, and 10 covers the necessary range. The electrode is calibrated at the beginning of each expedition as follows (calibration dataflow is shown in pH Dataflow).
1. | Select the Calibration tab from the main menu of the Alkalinity program (Figure 2). |
2. | In the Buffers fields, enter pH of at least 2 buffers to bracket the expected sample pH values (generally 4, 7, and/or 10). |
3. | Place enough buffer solution in the titration vessel to cover the electrode tip and add a stir bar. |
4. | Immerse the electrode tip in the buffer solution (not touching the bottom of the vessel). |
5. | Set water bath temperature to 25°C, and activate stirrer. |
6. | Set the Drift Span field (below plot) to a value >0 (30 readings is recommended), and click Measure 1. |
7. | Measure with stirring until drift (mV) vs. time (sec) on the plot is near 0.0 (this may take an hour or longer). |
8. | When the drift is nearly 0.0, click Done. |
9. | Repeat Steps 2–8 with each calibration buffer, selecting Measure 2 and Measure 3, as applicable. |
10. | Click Calibrate to view the regression curve. The slope value should be close to –59 pH/mV. |
11. | Click Save Calibration to accept values. |
Figure 2. Electrode Calibration.
Setting the Dispensing Rate
The rate at which the titrator dispenses the acid into the sample can be adjusted according to expected alkalinity value. Based on experience, for samples with alkalinity <20 mM, a 3 mV rate is usually appropriate. A 3 mV rate dispenses as follows:
Volume of 0.1 N HCl (µL) |
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30 | 150 |
4 | 220 |
1 | 220–240: through the linear Gran portion of the titration curve |
Higher alkalinities may require faster dispensing rates. The dispensing rate can be selected from a list of predefined rate programs or a new dispensing rate program can be created.
Selecting a Predefined Dispensing Rate Program
1. | Open the Rates & Standard Corrections tab in the Alkalinity main menu (Figure 3). |
2. | Under Rates, select a predefined rate from the list. These rates are multistep rate programs such as the 3 mV rate example in the previous section. |
3. | Click Load This Rate to use the predefined rate program. |
Figure 3. Selecting a Predefined Dispensing Rate Program.
Creating a New Dispensing Rate Program
1. | Open the Alkalinity/Standard tab in the Alkalinity main menu (Figure 4). |
2. | Edit the Rates fields for each step in the rate program as follows: |
| –Level (mV) up to which this rate is applicable (150 in the example shown below). |
| –Acid (mL) to add in each increment (0.030 mL = 30 µL as below). |
| –Drift tolerance (mv/s) for acid increment (0.02 as below). Measurement continues until drift tolerance is satisfied. |
| –Seconds until rate program times out (timeout(sec)) if drift tolerance is not satisfied (180 as below). |
3. | Click Save This Rate. |
4. | Enter a rate program name in the dialog box and click OK. |
Figure 4. Creating a New Dispensing Rate Program.
Drift Span
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 drift tolerance specified in the rate program (see Creating a New Dispensing Rate Program). Generally, drift tolerance starts at 0.10 mV/s. Drift tolerance acts as follows:
– | If the difference between the first and last measurements is smaller than the drift tolerance the next increment of acid will be dispensed. |
– | If the difference between the first and last measurements is larger than the drift tolerance measurement will continue at that increment until the drift tolerance is satisfied |
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Figure 5. Drift Span.
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The titrator measures and records the pH value for each sample before titration for alkalinity begins.
Figure 7. Measuring pH.
Once the pH measurement is complete, you will see a guide box again.
Figure 7. Acid dispenser reminder prior to starting the alkalinity measurement.
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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After rinsing the electrode with DI water and gently blotting dry with a Kimwipe, insert the electrode tip into sample (not touching the bottom of the cup).
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4.
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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).
alkalinity begins.
Figure 7. Measuring pH.
Once the pH measurement is complete, you will see a guide box again.
Figure 7. Acid dispenser reminder prior to starting the alkalinity measurement.
Measuring Alkalinity
1. | On the Alkalinity/Standard tab, ensure dispensing rate, drift span, and standard ratio are set correctly (Figure 7). |
2. | Ensure the water bath temperature is set to 25°C and pipette sample in titration vessel. |
3. | After rinsing the electrode with DI water and gently blotting dry with a Kimwipe, insert the electrode tip into sample (not touching the bottom of the cup). |
4. | Click SAMPLE. |
5. | 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. |
6. | 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. |
7. | Click Done/Save to upload data (see LIMS Integration). |
Figure 8. Measuring Alkalinity. The plot on the left side is 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 drift tolerance 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 in the red arc after satisfying the drift tolerance
Figure 8. Measuring Alkalinity.
Cleaning up after Measurement
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Quality Assurance/Quality Control
Overview
A quality assurance/quality control (QA/QC) program ensures that a measurement system is performing within control limits and therefore provides high-quality data. The QA/QC program for this system includes instrument calibration, calibration verification, and accuracy and precision monitoring.
Instrument Calibration
The instrument is calibrated by the onboard laboratory specialist at the beginning of the expedition. Calibration is verified routinely during operation. (See Instrument Calibration.)
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