pH AND ALKALINITY MEASUREMENT

 

 

EQUIPMENT

            Orion 720A pH Meter

            Radiometer Model GK2401C Combination Electrode

 

SAMPLE COLLECTION

      Rinse bottle 3 times with small volumes of the water to be sampled.  Collect pH and alkalinity samples in separate 20-ml scintillation vials with displacement caps.  Collect a second pH sample in a 125-ml wide mouth bottle.  Fill the 20-ml vials with a minimum of splashing and air entrainment.  Fill to overflowing and cap, excluding all air bubbles from the sample.  It is important to ensure that no atmospheric gas exchange occurs between the time of sample collection and analysis.  The 125-ml bottles are for air equilibrated pH determination, and may have air in them.  Keep samples cold and dark to minimize biological activity.

 

pH MEASUREMENT

      pH samples should be analyzed as soon as possible after collection, and must be analyzed the same day they are collected.

 

      Warm samples to room temperature in a dark container.  The container may be filled with room temperature water to speed the warming.

      Fill the electrode syringe barrel with an extra sample of the lake water to be analyzed.  Leave the electrode to soak in this sample at least 15 minutes.  The longer the electrode is "conditioned" before pH measurement, the faster it will respond during measurement.

      Bubble air through the samples in 125-ml bottles for at least 15 minutes before analysis using a vacuum pump and the gang valve assembly.  The intake hose should be pulling air from outside the building.  After bubbling, cap samples tightly until analysis.

 

        Uncover the electrode filling solution hole.  Remove the conditioning solution from the electrode syringe barrel using the three way valve and aspiration system.

         With a clean 5-ml syringe take an aliquot of the sample to be measured and inject about 2 ml into the electrode chamber.  Drain chamber.  Inject another 2 ml and flush in and out of the chamber several times.  Drain.

         Refill syringe with another aliquot of the sample and inject 2 ml into the electrode chamber.  There should be enough sample in the chamber to cover the electrode junction.

 

         Press the MEASURE button on the pH meter to take the meter off standby.  The meter should be in millivolt mode; if not, press MODE until the first line of the display indicates MV.  The second line of the display should read:  CH-1   25.0 C   MEAS.  If it reads HLD rather than MEAS, press the MEASURE button again to begin measurement.

         When the electrode response is stable the meter will beep and indicate HLD on the display.  Record the millivolt value on the data sheet.

         Continue to run 2-ml aliquots of the same sample until two consecutive millivolt readings are within 1 millivolt.  Record the last millivolt value in the FINAL  MV column of the data sheet.

              

        When all samples have been analyzed, obtain mV readings for the 3 buffer solutions:  pH 10.00, 7.00, and 4.00.  Only one 2-ml aliquot of each buffer need be run.

 

        Rinse the electrode chamber with Trout Lake tap water and store the electrode in Trout Lake tap water.  Cover the electrode filling solution hole securely with tape.  Rinse all sample bottles and syringes three times with milliQ water and leave out to dry.  Leave the pH meter on standby.

 

(reviewed 2/05 pkm)

 

ALKALINITY MEASUREMENT

Note: This Alkalinity protocol which is used for analyses done at both the Trout Lake Station and the Center for Limnology. The deviations from the standard protocol for the Center for Limnology are shown in parenthesis below):

 

Warm samples to room temperature in a dark container.

        Place the electrode in a sample of water from the lake that the alkalinity samples are from.  Allow to soak for at least 15 minutes.

        Turn the stirrer on 15 minutes before beginning titrations to allow it to come to constant stirring speed.  Do not change the stirring speed during titrations and buffers.  A change in stirring rate introduces a millivoltage error called the 'streaming potential'.

        Fill the Gilmont buret with standard 0.01N HCl as follows (0.05N HC for CFL):  Place the delivery tip of the buret in a waste beaker and deliver acid until there are no air bubbles in the tubing.  Rinse with milli-Q water, wipe dry, and deliver a few more drops of acid.  Place the tip in a beaker of standard HCl and draw acid into the buret.  Rinse and wipe tip again, and deliver acid into the waste beaker until the buret is zeroed.  If there is air in the buret, remove it from the clamp, invert, and deliver the air out the tip.

 

        Run a 'dummy' alkalinity titration on a sample of conditioning water from the lake before running any samples.  This is done because the first alkalinity titration often gives a suspicious value.

 

       Rinse a small stir bar with milli-Q water, dry with a kimwipe, and place in a clean, dry 2-dram vial (25 ml vial for CFL).  Do not touch the stir bar with your fingers; handle with a magnet and forceps only. 

 

Place 3 or 4 ml of sample (16 ml of sample for CFL), depending on the expected alkalinity of the sample, into the vial using an Eppendorf pipet.  Draw up and deliver sample from the pipet slowly so that no air bubbles are drawn into the tip, or sample droplets splashed onto the side of the vial.  Because of the small volume titrated, any loss of sample by spattering or any sample forming a droplet on the side of the vial will not be titrated, thus giving low values.  If you make an error, start over with a clean, dry vial.

 

       Remove the electrode from the conditioning lake water vial.  Rinse the electrode with milli-Q water and carefully blot it dry with a kimwipe.  Place the electrode in the vial containing the sample, slide the magnetic stirrer below the vial, and center it on the 'x' on top of the stirrer.

       Zero the acid buret, wipe the delivery tip, and insert it into the vial.

 

       Note the initial millivolt reading and add small increments of acid until the meter reads >115 mV.  When the electrode response is stable, the meter will beep and the display will indicate HLD.  Record the volume of acid added and the mV reading as your first increment.

      Titrate the sample from +115 to approximately 165 mV using 10 equal increments of acid.  Generally 0.010 ml increments are required.  Record the volume of acid added and the equilibrium millivolt value after each increment.

 

      At the end of the titration, place the meter on standby.  Remove the buret delivery tip from the vial, rinse with milli-Q water, and dry.  Deliver some excess acid from the tip to flush the tip.  Remove the stir bar from the sample with the magnet, rinse with milli-Q water, and dry.

 

      After completing sample titrations, standardize the electrode using fresh buffer solutions (pH 4.00 and 7.00).  Place a few mls of buffer and a stir bar in a vial, and take a mV reading.  Do not place the acid buret tip in the buffer vials.

 

      Turn off magnetic stirrer, place the pH meter on standby, cover electrode filling solution hole.  Rinse electrode and store in Trout Lake tap water.  Rinse sample bottles and titration vials three times with milliQ water and leave out to dry.

 

 

 

(reviewed 2/05 pkm)