...

Stock Standard Solutions (1 L)

...

1. 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 Alkalinity interface.
2. Enter your range of buffers (4, 7, 10).
3. Select your Drift Span. The default drift span is 30.
4. 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.
5. 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.
6. When finished, clean vessel and the electrode.
7. Repeat steps 4–6 with each calibration buffer, selecting Cal2 and Cal3 when appropriate.
8. 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.

...

1. Place 3 mL of standard in vessel. Add stir bar and immerse electrode in vessel. Confirm that the frit is in the solution. Check that your Drift Span is 30.
2. Select Continue.  
3. Click START.
4. Insert the acid dispensing probe when prompted.
5. When finished, clean vessel and electrode. Repeat steps 1–4 until you have at least three consistent measurements per standard.
   
   Now go to the STND Manager, selected from the main alkalinity interface.
   
   

...

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.

Image Added Image Removed
Figure 510. Drift Span.

Calculating Standard Ratio

Calculating the standard ratio (estimated vs. actual alkalinity) for the anticipated range of alkalinity values accounts for measurement error in acid strength. Standard ratio can be calculated using borax solution, sodium bicarbonate solution, or IAPSO standard seawater, as necessary, to most closely match alkalinity values (within 5 mM, to preserve the first-order transfer function) of the unknown samples. Generally, IAPSO standard seawater is used to establish this ratio, and additional calibration standards are used if samples deviate >5 mM from the alkalinity of IAPSO (~2.325 mM). The measurement is repeated until at least 3 consistent measurements are obtained within 5% of actual value for each standard:

Obtain measurements for the standard ratio as follows:

...

1.

...

Open the Alkalinity/Standard tab and select Standard Ratio at the bottom of the window (Figure 6).

...

2.

...

Check to make sure the reservoir is full with 0.1 N M HCl. Set the water bath to 25°C, pipet standard into titration vessel, and add stir bar.

...

3.

...

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 and remove air bubbles.

...

4.

...

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), make sure Drift Span = 30, and click Measure.

...

5.

...

Enter the given Alkalinity and Sample volume (e.g., 3 mL) and click GO.

...

6.

...

Once pH measurement is done and value is OK (>7.0), follow the onscreen instructions to complete the titration and measurement. If pH value is wrong, abort the measurement and go back to Step 4.

...

7.

...

Repeat Steps 4–6 until you have at least 3 consistent measurements per standard.

...

8.

...

Highlight the measurements to include in the average ratio and click Save Standard.

...

9.

...

Name the ratio so it can be retrieved later from the Rates & Standard Corrections tab.

Sample Preparation & Analysis

Sample Storage and Preservation

...

Sample Preparation & Analysis

Sample Storage and Preservation

Use 3 ml of interstitial water sample. Store residual sample in a 5 ml snap cap tube for additional shipboard analysis.

...

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 samples 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.

1. Pipette a 3 mL sample into the titration vessel and add stir bar.

...

2.

1. Take an initial pH reading

...

1. .

...

3.

1. Titrate to determine total equivalent alkalinity value

...

1. .
2. Upload data to LIMS

...

1. .
2. Store titrated sample in a sealed polytube with a new label indicating volume and concentration of acid added.

Entering Sample Information

Select the sample (IWS) from LIMS tree or type in Text_ID. Click Continue.

Figure 711. Entering sample information.

...

Click START. The software will guide you through.

Figure 712. 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 713. Measuring pH.

Once the pH measurement is complete, you will see a guide box again.

Figure 714. Acid dispenser reminder prior to starting the alkalinity measurement.

...

Data Reports

Data reports are not available at the instrument, but data can be viewed on the 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.

Figure 11. Datalog.

Figure 12. Viewing alkalinity data in LIMS.

...