User Guide: Autotitrator pH/Alkalinity

Manual

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information

User Guide

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contents

Introduction

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plays an important role in establishing the pH of the solution through a buffering effect. This change in slope, however, has no effect on the Gran extrapolation intercept with the y-axis and is not accurate enough to estimate sulfate concentrations.

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Apparatus, supplies and reagents

Laboratory Apparatus

Figure 1. Metrohm Autotitrator (Note: dispenses 0.1 M HCl).

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Laboratory Apparatus

Hardware and Software

Supplies

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-	Pipettes, Class A: 0.5, 2, 3, 5, 8, 10, and 20 mL
-	Adjustable pipettor, 100–5000 µLµl
-	Volumetric flasks, Class A: 100 mLml, 1 Ll
-	Combination electrode (Metrohm, combined pH glass electrode, model 6.0234.100)
-	pH paper, to bracket pH 7 (pH 1–12 paper)
-	5 mL ml snap-cap vials
-	Magnetic stir bars

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1.	Heat reagent water in a glass beaker while adding Bis and stirring.
2.	When precipitation is observed (saturation), add HCl until pH = 7 as tested with pH paper.
3.	Cool solution (a gel forms in the beaker), then decant the water and place in a warm oven until dry.
4.	Scrape dry remains from the bottom of the beaker and place in a vacuum desiccator to cool.
5.	Take first weight quickly upon removal from desiccator when making up the buffer. See ODP Tech Note 15 for complete instructions on making up the powdered buffer.
6.	Redry in oven and cool in desiccator each time prior to weighing.
7.	Make new Bis buffer at the beginning of each expedition.

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Main Main

Main instrument panel

Figure 2. Main instrument panel.

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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 Main instrument panel.
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.

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Disp Disp

Dispensing rate

The rate at which the titrator dispenses 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 Main instrument panel.

To create a new rate program:

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Make sure to select a correct dispensing rate program for the standard in question. You can access the rates by selecting Edit Rates from the main alkalinity interface Main instrument panel.

To start creating a standard ratio correction, select STANDARDS from the main alkalinity interface Main instrument panel.

Figure 5. Entering standard information.

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Figure 6. Measuring a standard for standard ratio correction.

1. Place 3 mL 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 Main instrument panel.
   
   

Figure 7. Averaging standard measurements.

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This window also shows the electrode calibration values, the path to the datalog file, the default setting for the Drift Span , and where to select the standard ratio correction.

To select a saved standard ratio correction , double-click it.

Drift

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span

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 stability criteria specified in the dispensing rate program (see Creating a New Dispensing Rate Program). Generally, drift tolerance starts at 0.10 mV/s. Drift tolerance acts as follows:. stability criteria acts as follows:

If the difference between the first and the last measurements measurement is smaller than the drift tolerance, the next increment of acid will be dispensed.

If the difference between the first and the last measurements measurement is larger than the drift tolerance, measurement will continue at that increment until the drift tolerance is satisfied.

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Figure 16. Analysis complete.

A Gran-Method window appears with the results of the titration. Write the Alkalinity Cor value in the blue book. This is the result with the standard ratio applied to it. Click Ok/Save. This will upload the result to LIMS assuming the MUT uploader is active.

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When finished, clean vessel and the electrode.

Click Ok/Save to save the results to the LIMS database. The value saved to LIMS and the value you need to note in the blue book is the “Alkalinity Cor” value. This is your result with the standard ratio correction applied to it.

Cleaning up after Measurement

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