IODP (International Ocean Discovery Program) at MARUM
Alkalinity / pH
pH and alkalinity both are an ephemeral property and will be analyzed offshore in a timely manner after interstitial water acquisition
Analytical approach
The pH value and the alkalinity of the IW sample are analysed from the same designated IW sample aliquot (IW split), with a Radiometer TIM 840 Autotitrator, in a combined 2-step analytical approach.
The IW sample aliquot for pH / alkalinity is first analyzed for its pH value, which is then followed by a single-point titration to a certain pH value.
pH value
Pore water pH is measured using an ion-specific electrode (Mettler Toledo) mounted on the Autotitrator unit. The pH meter integrated in the Autotitrator unit is calibrated once per day, as the device requests a fresh calibration latest every 24 hours.
The general 2-point calibration is performed using color coded National Bureau of Standards (NBS) scale buffer solutions of pH = 4.01 and 7.00 (and if necessary sample wise of pH 10.00 as well).
Temperatures are monitored with a pt1000 T-sensor next to the analytical station.
The Autotirator unit shows the pH value with a resolution of 0.001 pH units and the measurement accuracy is better than +/- 0.02 pH units.
The solutions have to be stirred with the internal magnetic stirrer during all measurements.
To achieve a constant reading and in order to enable measurements / titrations with very small sample quantities (0.5 mL IW ph/alkalinity aliquot), the 2 mL-sample vial is turned around the 3 mm tip of the electrode with a magnetically driven sample cup holder rather than stirring the sample with a mini stir bar.
pH-controlled alkalinity titration
Alkalinity is determined in a second analytical step from the same IW pH/alkalinity aliquot, after the initial pH value of the sample is analyzed, performing a single point titration to pH 3.95 with an 0.01 M HCl (0.05 M HCL for samples with higher alkalinity values) according to standard procedures (Grasshoff et. al., 1983).
The Radiometer TIM840 Autotitrator device is equipped with a 5 mL burette for this step, to deliver the HCl stepwise through a small PTFE tube (0.3 mm ID), placed carefully in the rotating sample vial after the initial pH value analysis.
Note the initial pH of the sample and perform a pH controlled titration to a pH of approximately 3.95 (Set the autotitrator to an endpoint of pH 3.95). Stir well.
Note the concentration and the exact amount of acid and the exact final pH value.
The autotitrator will overtitrate but returns as a result the amount of HCl needed to reach the setpoint of 3.95.
The algorithm used to calculate alkalinity accounts for the activity of seawater and dilution by the titration solution so that the results are comparabe for different end-point pH values. The measurement accuracy is better than 0.2mM.
The algorithm is a corrected version of the algoritm of Grasshoff (1983).
The alkalinity is calculated from:
ALK [M] = (10-pHinitial /fH+initial) + [(cHCL*vHCL)/v0] - (10-pHfinal/fH+final) * [(v0 + vHCL)/v0]
where
pHinitial = original pore water pH
pHfinal = pH at end-point of titration (usally pH 3.95)
fH = activity coefficient of H+ ( for standard seawater 0.755)
v0 = initial sample volume (here: 0.5 mL / 500µL / 0.0005 L)
cHCL = molar concentration of titration solution (here: HCL 0.01 M or 0.05M)
For the autotitrator the final pH is the setpoint and the volume of HCl added is the result shown after the titration is completed.
The above equation accounts for the dilution by the amount of HCl added and is fairly stable to different volume / pH data couples or to measurements with different HCl concentrations.
Back-up: Accustep digital burette
As a possible analytical back-up (only in case needed) for the above approach, a combination of a standard handheld pH-meter with the same electrode type, used together with a digital burette (e.g. Accustep) and a magnetic stirrer can be facilitated for the single-point titration as well.
The Accustep has three parts. The control unit, the handset, and the syringe.
There are two syringe types: Type B (25 mL) and type (C 5 mL). The syringes have a luer slip tip so that you can connect all kinds of disposable luer ware
Assembly:
Do not turn on the control unit before a syringe is attached to the handset. If you do so and play with the buttons the plunger drive might move too far up or down and you will have to disassemble the handset to manually put it in a proper position.
Grip the syringe plunger, pull it out a bit. Slip the upper end of the plunger sideways into the low end of the handset. Screw the body of the syringe onto the headset.
Connect the handset to the control unit. The control unit is connected to mains and switched on by a soft button (low right).
Preparation:
There will be blinking lights reminding you to reset the unit and set the syringe type:
With the syringe attached, the handset is re-zeroed by moving the plunger drive up a bit (<key on the handset) and all the way down (>key).
Set the syringe type to either B or C with the syringe key on the control unit. Like with a normal syringe, you can now fill the syringe and attach the above tubing. Make sure to get rid of air bubbles by holding the handset upright and moving the plunger using the < and > keys on the handset.
To avoid vacuum, fill the syringe first without the small diameter tubing. With microtubing use the C syringe and set the slowest speed possible (speed key). When the system is air free you can fill it. The display will show the amount of liquid in the syringe (25 mL or 5 mL ). For best precision move the plunger down 1 cm again (> key) and use preferably the upper half of the syringes volume.
Modes:
You can use the Accustep in three modes: Dispense, Burette, and Dilute.
Dispense:
a red light will flash until you set an amount in µL (type in a number i.e. 25 and press the set key). Now the unit will dispense 25 µL every time the button on the handset is pressed. The total amount will be recorded on the display.
Burette:
The unit will dispense liquid as long as you press the button on the handset. The total amount is recorded on the display.
You can refill the syringe at any time by using the < key on the handset. The unit will
memorize the dispensed amount.
Caution:
If you switch between the modes the unit will lose the recorded total amount! To change the syringe move the plunger down all the way first. Remember to re zero the new syringe and set then new syringe type (see above).
Analytical approach:
The IW pH/alkalinity sample will be handled in the same manner as above.
The sample is constantly stirred, facilitating the magnetically driven cup holder on a magnetic stirring device.
First the initial pH value is measured with the calibrated pH-meter and the value documented.
Then the sample titration to an end-point of 3.95 is facilitated by adding stepwise the HCl with the digital burette to the sample.