US Long-Term Ecological Research Network

 

LTER-Geochemistry Lab
 
The Biocomplexity Cross Lakes Comparison water samples are analyzed once each summer to determine alkalinity values. The "higher" alkalinity samples (>100µeq/L) are analyzed with a Brinkmann-Metrohm autotitrator using a Radiometer GK2401C electrode. Trout, Allequash, Sparkling and Big Muskellunge Lakes have alkalinity values higher than 100 µeq/L. Crystal Lake, Crystal Bog(27-2), and Trout Bog(12-15) have lower alkalinity values and are analyzed by manual titration. A Gilmont microburet and a Radiometer GK2401C electrode are employed for low range analysis.
 
High Range Alkalinity Samples:
 
Equipment
 
Brinkmann 636 Titroprocessor
Metrohm Dosimat E635 w/ magnetic stirrer
Radiometer Model GK2401C combination electrode
 
Sample Collection
 
See the procedure listed under Trout Lake’s LTER PH/ALKALINITY MEASUREMENT method.
 
Analysis Procedure
 
1) Check affluent level of .05 N HCl acid in the Dosimat. Remove the tape from the electrode’s KCl filling hole.
 
2) Power up the Titroprocessor by pressing the button on the back left-hand side of unit. The following message will be displayed.
 
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Enter the date by pressing the appropriate numbers consecutively without punctuation,then ENTER. (eg. 50291 for 5/2/91)
 
3) EN GO will now be displayed. This is a prompt for the user to enter the desired analysis mode--sample titration or buffer mV reading. It’s a good idea to run an initial check of your buffers’ responses so that data in the appropriate mV range can be saved. To select the mV reading mode, press 1,4,Go. The following message will be displayed.
 
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4) Remove the electrode from its soaking solution, rinse with Milli-Q water and blot dry with a clean Kimwipe. Place the clean, dry magnetic stir bar in vial containing 16 mls. of buffer. Place the vial on the magnetic stirrer and gently lower the electrode into the solution until the junction is covered. The stirring rate should be adjusted to allow for good mixing and left at that rate for the entire analysis.
 
5) To take a reading, press Go. Record the buffer’s pH, starting time and finishing time in the Alkalinity lab book.
 
6) Repeat steps 4 & 5 for the remaining buffer solutions. The geochem lab uses 3.557 and 6.88 buffers in the gran titration program used to reduce the data. The range of mV readings which will be used are determined by the mV reading of the 3.557 buffer. A 50 mV span of values is used which ends with the mV reading given by the 3.557 buffer. If the 3.557 buffer gives a reading of 155, then the useful range of data is 105 mV to 155 mV.
 
7) To run a titration of sample or standard, select the titration mode of operation by pressing 1,1,Go. The following message will be displayed.
 
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Before running any samples or standards, the acid injection lines must be thoroughly flushed of air bubbles. Beside the green light on the Dosimat, there is a switch which is labeled by an up and down arrow. Hold that switch up to start flushing the lines, When more acid is needed, and arrow indicator at the base of the Metrohm column will slide to the left so that a downward arrow shows. Release the up/down arrow switch until the column has filled and the indicator slides back to the right. Repeat this procedure until no air is being pumped out the titration tube. (Usually takes 5-7 repetitions).
 
8) To prepare a sample for analysis, place an empty vial on the Mettler balance and tare it. Carefully aliquot approximately 16 mls. of sample into the vial taking care not to splash any on the inside wall of the vial. Record the weight of the sample in the lab book, sample #, lake name, depth, starting and finishing time of the titration, and any comments/observations.
 
9) Place the clean, dried stir bar in the vial and place the vial on the magnetic stirrer. Gently immerse the titrator tube and the electrode into the vial until the junction is covered (both have, of course, been rinsed with Milli-Q water and blotted dry prior to immersion in the sample).
 
10) To save time, you can program the titrator to inject an initial volume of acid into the sample before the actual recorded titration takes place. To enter or modify a predose volume of acid, press 4,MOD,(volume in mls), ENTER, (eg. 4,MOD, .3, ENTER for a .3 ml. predose). Look in the lab book at previous analyses to find appropriate predose volumes for each lake and depth.
 
11) Set the final mV reading to be 170 mV for the Radiometer electrode (210 mV for a Corning) by pressing 5,MOD,1,7,0,ENTER. This parameter needs only to be entered at the beginning of the analysis.
 
12) Enter the acid normality by pressing 1,4,MOD,(acid normality), ENTER. This parameter needs only to be entered at the beginning of the analysis.
 
13) Enter each sample’s weight by pressing 1,5,MOD,(sample wt.), ENTER.
 
14) Set the end limit window by pressing 3,1,MOD,0,ENTER. (The end limit for a Corning electrode would be 100). This parameter needs only to be entered at the beginning of the analysis.
 
15) Once you have the parameters set and the sample is ready for analysis, start the titration by pressing GO. The titroprocessor waits until a stable mV reading has been established for the sample and then injections of acid will begin. When the titration is complete, a listing of acid volumes and corresponding potential readings will be printed. This information will be entered in the alkalinity calculation program for each sample to determine the alkalinity in µeq/L.
 
16) Repeat steps 8,9,10,13, &15 for each alkalinity sample.
 
17) At the end of the day take mV readings of the buffers once again. Pressing 1,4,GO will disable the titration operation and allow direct potential measurements. Repeat steps 4 & 5 for each buffer and record in the lab book along with the room temperature.
 
18) When finished for the day, replace the tape over the electrode’s filling solution hole and immerse the electrode into a vial filled with pH 4.01 buffer. Turn off the power on the Brinkmann 636 Titroprocesser.
 
Low Range Alkalinity Samples:
 
The low range samples are usually analyzed at Trout Lake Station by the chemistry technician visiting from the Madison lab. See the Trout Lake pH/Alkalinity document for description of the method used. If the samples are not analyzed at Trout Lake Station, then they are analyzed in Madison by the same method. Note: The mV range of data may differ at the lab in Madison and must be established by measuring the response of the buffers.
 
(Revised 2001 James Thoyre)
(used for Biocomplexity Cross Lake Comparison Samples from 2003-2004)

 

Protocol Format
Process
Protocol ID
biocom_alkalinity1
Protocol Type
laboratory