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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 | July 2017April 2019 |
Domain: | Chemistry | ||
System: | Titration and Refractometry | ||
Keywords: | Titration, pH, Alkalinity |
User Guide Contents
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| PAGE | |
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| Introduction | 1 |
| Apparatus, Reagents, & Materials | |
| Instrument Calibration | |
| Sample Preparation & Analysis | |
| Quality Assurance/Quality Control | |
| Software Dataflow | |
| LIMS Integration | |
| Health, Safety |
| & Environment | |
| Maintenance/Troubleshooting | |
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To measure alkalinity, a pore water sample is titrated with an acid to an endpoint at which carbonate is converted to bicarbonate and bicarbonate is converted to carbonic acid. In seawater, this endpoint occurs at about pH = 4.2.
H+ + CO32– ¿ xHCO3–
H+ + HCO3– ¿ x ¿ H2CO3
The alkalinity determination in this method (Gran titration) relies on a mathematical evaluation of the second equivalence point of carbonate titration in seawater using the most stable part of the titration curve (i.e., the part beyond the equivalence point on the low pH side). In essence, the Gran method linearizes the titration curve by means of a simple function:
F = (v + V0) × 10E/A,
where:
F = Gran factor,
v = volume of acid added to the solution in the titration vessel,
V0 = original volume of the sample,
E = EMF (millivolts) at v, and
A = slope of electrode determined on the basis of the electrode calibration.
Generally, the slope is ~59 mV at 25°C. Slope is determined during calibration (see Calibrating the Electrode).
The function F, when plotted as a function of the volume of acid added (v), is linear when sufficiently removed from the equivalence point. We measure mV instead of pH to determine the endpoint because this method offers better precision. The optimum range of millivolts for linearity is 220–240 mV. The value of v at F = 0 is the equivalence point from which the alkalinity is evaluated.
The slope of the F vs. v plot changes with variations in the sulfate content of the samples. This is because at lower pH values the following reaction
H+ + SO42– = HSO4–
plays an important role in establishing the pH of the solution through a buffering effect. This change in slope, however, has no effect on the Gran extrapolation intercept with the y-axis and is not accurate enough to estimate sulfate concentrations.
Alkalinity System
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Figure 1. Metrohm Autotitrator with Workstation (Note: dispenses 0.1 M HCl).
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The laboratory technician receives a whole-round sample at the catwalk to squeeze for interstitial water, which is passed through a 0.45 µm filter before analyzing pH and alkalinity. Note that the RHIZON samplers will produce incorrect pH and alkalinity values, and should not be analyzed!
The general procedure for analyzing pH and alkalinity on interstitial water samples is as follows:
1. | Calibrate the electrode (see Instrument Calibration). | 2. | Pipette a 3 mL sample into the titration vessel and add stir bar. |
32. | Take an initial pH reading (see Measuring pH). | ||
43. | Titrate to determine total equivalent alkalinity value (see Measuring Alkalinity). | ||
54. | Upload data to LIMS (see LIMS Integration). | ||
65. | Store titrated sample in a sealed polytube with a new label indicating volume and concentration of acid added. |
Measuring pH
The titrator measures and records the pH value for each sample before titration for alkalinity begins.
Entering Sample Information
Select the sample (IWS) from LIMS tree or type in Text_ID. Click Continue.
Figure 7. Entering sample information.
Click START. The software will guide you through.
Figure 7. 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 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 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 place 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 SAMPLESelect the Alkalinity tab at the bottom of the screen and click Measure. |
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 78. Measuring Alkalinity.
Figure 8. Select Measurement Type.
Figure 9. Insert Acid Tip Prompt.
Cleaning up after Measurement
1. | Remove electrode from cup. |
2. | Rinse electrode with DI water in squeeze bottle. |
3. | Blot the electrode dry with a Kimwipe. Do not rub the electrode, which could cause static charge buildup. |
4. | Place the electrode in a storage container containing 3 M KCl. |
5. | Rinse and dry the acid dispensing tip and stir bar. |
6. | Pipette or pour the titrated alkalinity IW sample into a container to ship to repository or scientist. |
7. | Write on label how much HCl was added during titration. |
8. | Rinse the titration vessel cup with DI water and dry. |
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Data Reports
Data reports are not available at the instrument, but data can be viewed on the
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View Datalog tab (Figure 11). Also, the software appends the alkalinity results to the DAT file C:\ALKALINITY\LOG\ALKALINITYSAMPLES.DAT. The best way to view the alkalinity values to be used in graphs and reports is via LIMS Reports. It is also highly recommended to record the alkalinity and pH values in the blue laboratory notebook to protect against inadvertent data loss
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Figure 11. Data Log Tab. Datalog.
Figure 12. Viewing alkalinity data in LIMS.
Quality Assurance/Quality Control
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$mVthreshold: slope threshold; maximum calculated value for the reading to be considered stable; also “drift tolerance.”
Figure 12. Alkalinity Electrode Calibration Data Flow.
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User-defined variables (Figure 13), with values from the example given in the Setting the Dispensing Rate section:
$MV1: rate 1 mV threshold (150 mV)
$MV2: rate 2 mV threshold (220 mV)
$MV3: rate 3 mV threshold (240 mV)
$Rate1: rate for first mV threshold (30 µL)
$Rate2: rate for second mV threshold (4 µL)
$Rate3: rate for third mV threshold (1 µL)
$SlopeSpan: number of samples used to calculate the slope (default = 30)
$StabilityThreshold: maximum slope value to ensure a stable reading (also referred to as drift tolerance)
Figure 13. Alkalinity Measurement Data Flow.
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When the alkalinity titration finishes, the Alkalinity Calculation GRAN-METHOD screen shows the slope information, correction factors, and final alkalinity value (Figure 14).
Edit outlier Gran factor points, if necessary (see Editing Gran Factor Points). If satisfied with the results, click Done Ok/Save to load the values into LIMS.
Figure 14. Load Data to LIMS.
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Electrode: Metrohm 6.0234.100 or equivalent. |
0.1 M HCLHCl, Fisher CH5009 |
Microvalve buret tip, Metrohm 020683244, CM5129 |
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