Chloride by Autotitrator: User Guide
V1.1 (C. Bennight 10/2/2011; Approved by DJH 10/23/2013)
V371T (D. Houpt, July 2017)
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Introduction
This manual describes the operational methods, standard maintenance, and systems integration of the Metrohm 785 DMP Chloride Autotitration System (Figure 1). Methods describes in this manual are based on the Metrohm manual, Metrohm Application Bulletin 130/2 e (Chloride titrations with potentiometric indication), and email correspondence with Gretchen A. Robertson at the Scripps Institution of Oceanography.
This manual covers the USIO standard method for the determination of chloride via potentiometric titration, Variation A – with Nitric Acid.
Figure 1. Chloride Autotitration System.
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Theory of Operation
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The Cl autotitrator uses a potentiometric endpoint to determine the amount of free (unbound) chloride in a solution. A silver nitrate reagent is aliquotted into an unknown solution with some amount of dissolved chloride in solution. As the silver nitrate is added the following reaction occurs.
Ag
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+
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(aq)
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+
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Cl–(aq)
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® AgCl(s)
~ The product, AgCl, is insoluble in water and precipitates. A silver-specific electrode in the solution measures a value proportional to the amount of free silver in solution.
As the silver chloride reagent is added, a measurement (in mV) is taken from the silver-specific electrode. If we consider the function of volume added versus the electrode reading and find the volume where the second derivative of the function is 0 (or the inflection point), we have found our equivalence point (<span style="color: #1f497d"><strong><em>Figure 2</em></strong></span>). !worddav995eef5467d599571cd1bcccb619a2ce.png|height=253,width=336! \\ *Figure 2. Potentiometric Equivalence Point.* \\ For most purposes, this first equivalence point corresponds to the volume where all free chloride has been converted to AgCl. Knowing the volume of reagent added, the concentration of reagent, and the stoichiometric ratio between reagent and chloride (Figure 2).
Figure 2. Potentiometric Equivalence Point.
For most purposes, this first equivalence point corresponds to the volume where all free chloride has been converted to AgCl. Knowing the volume of reagent added, the concentration of reagent, and the stoichiometric ratio between reagent and chloride (1:1), we can determine the amount of chloride contained in the solution:
[Cl–] = ([AgCl] × Volume(AgNO3))/Volume(Sample)
(Note: A value in brackets ([]) indicates the concentration of the item enclosed in brackets.)
Volume(AgNO3) is simply the volume of reagent added when the first equivalence point is \\ \[Cl{^}-{^}\] = (\[AgCl\] × Volume{~}(AgNO3){~})/Volume{~}(Sample)~ \\ (Note: A value in brackets (\[\]) indicates the concentration of the item enclosed in brackets.) Volume{~}(AgNO3)~ is simply the volume of reagent added when the first equivalence point is reached.
Assumptions
It is assumed that the first equivalence point corresponds to that of the AgCl reaction. Silver forms sparingly soluble precipitates with other anions as well as chloride. Solutions with high concentrations of bromide or iodide may cause an erroneous first equivalence point (as they titrate before chloride). In these cases, the method on the instrument will need to be adjusted to run past the first endpoint and to determine the Volume(AgNO3) value. The reagent volumes for the non-chloride endpoints must be subtracted from the reagent volume for the chloride endpoint. It should be fairly obvious which the chloride endpoint is, and chloride will typically be many times more concentrated in situ than other anions.
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Apparatus, Reagents, and Materials
Hardware
785 DMP Titrino
The 785 DMP is the primary piece of the instrument. All other components connect to it and it provides the logic and outputs to run the titration (Figure 3).
Figure 3. Titrino Autotitrator.
Reagent Exchange Unit
This interchangeable unit contains the following:
–500 mL reagent bottle
–Exchange burette (5 mL)
–Reagent tubing
–Reagent dispensing tip
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To make 500 mL of acid-based dilution solution, add the following to a 500 mL volumetric flask:
SolutionSS | Volume (mL) |
*18 m¿ DI water (reagent water)* | 430 |
2.2 M trace metal grade nitric acid in reagent water | 20 |
0.2% polyvinyl acid (PVA) in reagent water | 50 |
Note: Make this solution fresh for each run (typically keep no longer than 1 day). Adjust proportions of solution to make the desired final volume.
Note: The solution can be prepared without PVA if needed; simply add 20 mL of 2.2 M HNO3 to 480 mL reagent water.
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General Equipment Setup and Operation
Setting Up The Instrument
To install the instrument for the first time:
1. | Remove the 785 DMP unit from the container, locate the following cords, and plug them into the indicated slot (Figure 4): |
2. | Connect the other end of the serial cable to the back of the computer host that will control this device. |
3. | Power on the instrument by pressing the power button on the front panel of the 785 DMP unit. |
4. | Flip the power switch up on the front of the electronic stirrer (stirring will not start until the run actually begins – controlled by the 785DMP unit). |
Figure 4. 785DMP Connector Slots.
Operation
Purging the lines
The lines connecting the reagent bottle to the reagent dispensing tip must be purged before analysis can begin. Wear gloves and safety glasses for this operation.
1. | Place the acid dispensing tip in a Ziploc bag—but make sure it's not completely air tight. |
2. | Tap, clamp, or have an individual hold the bag and tip up so it is the highest portion of the system. |
3. | Turn the dispensing speed knob in the bottom right of the device fully clockwise to its maximum setting (if it isn't there already). |
4. | Press and hold the DOS button on the front of the 785 Titrino. While the unit is dispensing flick the reagent lines with your fingers to dislodge any stuck air bubbles. |
5. | When the burette has exhausted its capacity (5 mL), press the FIL button on the front of the 785 Titrino. This will cause the burette to aspirate reagent from the bottle. While this is occurring, flick the exchange lines between the burette and the reagent bottle to dislodge air bubbles. |
6. | Repeat steps 4 and 5 until no air bubbles are visible in any of the exchange lines. |
Shutting down the instrument
1. | Rinse the dispensing tip in DI water and store in DI water (add a small amount to the housing in the exchange unit where the tip sits). |
2. | Rinse the electrode in a 3 M sodium nitrate solution or 2 M nitric acid solution followed by DI water. |
3. | Store the electrode in 3 M KCl solution (add to the housing in the exchange unit where the electrode sits). |
Analyzing Samples
1. | Place stir bar in bottom of reaction vessel. |
2. | Pipette 100 µL of sample into the bottom of the reaction vessel. |
3. | Pipette 4 mL of dilution solution into the bottom of the reaction vessel. |
4. | Insert the electrode and reagent dispensing tip into the reaction vessel. |
5. | Start the software – ClAutotitrator. |
6. | Log in using LIMS username/password. |
7. | Select the sample to be measured by filling in Expedition, Site-Hole, Sample Name, and Sample (Figure 5). |
8. | Click the measure button (Figure 6). |
9. | While the instrument is running, the plot displays the current data set as well as instantaneous values for volume of reagent added, current value, run time, and current titrator status. No interaction is required. |
10. | When the run is complete a sound will play and a window will pop up telling you that the run is finished (Figure 7). Click Save to upload the data to LIMS. |
Figure 5. Enter Sample Information.
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Figure 6. Measure Sample.
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Figure 7. Run Complete.
QAQC
IAPSO is used a standard for this analysis. The certified value, typically ~559 mM, is printed on the outside of each bottle. For quality control purposes, IAPSO shall be run as a sample as follows:
–Before every run
–As the last sample of any run
–Every 5 samples or 2 hours, whichever comes first.
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LIMS Integration
–Data for this analysis is stored under analysis TITRA_MAN.
–Data are logged against the instrument TITRINO_785.
–LIMS component definitions: Anchor
Component | Alias | Unit | Description |
chloride | Chloride | mM | Concentration of chloride |
| Titrant amount |
| Amount of titrant added to solution to achieve endpoint |
Software
There is one piece of software associated with this instrument, the in-house: Cl Autotitrator piece. It is a WPF application written in C# against the 4.0 .NET runtime. The following table describes its characteristics.
Characteristic | Description |
Supported Operating Systems | Windows 7 – 64 bit |
Source Code | |
Deployment |
The project is deployed as a Click Once application installable without administrative permissions. The install page is at: http://web.ship.iodp.tamu.edu/tasapps/cltitrator/ | |
Web Services | This project makes calls to the Web Services (WAPPS). Specifically, it:
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Health, Safety, and Environment
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