Page History

...

Before the titrator can be used to measure samples, the electrode must be calibrated against pH buffers in the range expected in samples. Generally, calibration at pH 4, 7 and 10 covers the necessary range.

1. Make sure the water bath temperature is set to 25°C. 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 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, select the slope value of the regression curve should be close to –59 pH/mV. Select OK-Save to save the calibration.

Dispensing rate

The rate at which the titrator dispenses the acid into the sample can be adjusted according to the expected alkalinity value. Higher alkalinities may require faster dispensing rates. The dispensing rate can be selected from a list of predetermined programs, or a new dispensing rate program can be created.

Select Edit Rates from the main alkalinity interface.

To create a new rate program:

1. Set your Stability Criteria for each step of the program: Measurement continues until Stability Criteria (mV/s) is satisfied.
2. Select your Increment for each mV level (initial to 150, 150 to 220 and 220 to 240): How much acid is added in each increment.
3. Set the Time Out for each step of the program: Seconds until rate program times out if Stability Criteria is not satisfied.
4. Save To File.

Image Added

Image Modified

Standard ratio

Calculating the standard ratio (estimated vs. actual alkalinity, “fudge factor”) 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:

-         IAPSO = 2.21–2.44 mM

-         20 mM standard = 19–21 mM

-         40 mM standard = 38–42 mM

To start creating a standard ratio, select STANDARDS from the main alkalinity interface.

1. Place 3 mL of standard in vessel. Add stir bar and immerse electrode in vessel. Confirm that the frit is in the solution.
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.
   
   

...