Chloride by Autotitrator: User Guide (from CI_Autotitrator_UG_371T_draft_
V1.1 (C. Bennight 10/2/2011; Approved by DJH 10/23/2013)
V371T (D. Houpt, July 2017)

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.

Theory of Operation

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⁺_(aq)  + Cl^–_(aq) ® 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 (Figure 2).

Open

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 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.

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).

Reagent Exchange Unit

This interchangeable unit contains the following:
–500 ml reagent bottle
–Exchange burette (5 ml)
–Reagent tubing
–Reagent dispensing tip

Titrino Keypad

This auxiliary data entry device can be used to control the Titrino manually. It is not needed for any of the methods described in this manual.

Silver Electrode

Specifically, a Metrohm 6.0430.100, their "Ag Titrode," this electrode combines a standard silver ring electrode with a pH membrane to serve as the reference. It is the only electrode required for this method. It measures the concentration of silver ions in solution. The metal ring on the tip of the silver titrode may be periodically cleaned by scrubbing gently with toothpaste. Cleaning in this manner will reduce the titrode efficacy over time. The titrode should be stored in DI water long term. Trichloroethylene may be used to degrease the probe. See Storage of Metal Electrodes for more info.

Automatic Stirrer

This electronic stirrer is controlled by the 785 DMP Titrino. Note that the stir bar will not activate until the titration has started.

Reaction Vessel

This is a non-water-cooled narrow bottom reaction vessel. The electrode and reagent dispensing tip are inserted down into the narrow portion of the vessel to reduce the volume required for the overall reaction.

Reagents

Dilution Solution

• Add about 500 mL of 18 MΩ deionized (DI) water to a 1L volumetric flask. Pipette in 6.08 mL of concentrated trace-metal grade nitric acid. Make up to 1 L with 18 MΩ deionized (DI) water (reagent water). Note: Always add acid to water.

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.

0.015 M silver nitrate solution

Dilute 150 ml of 0.1 M silver nitrate stock solution with 1 l reagent water, or dissolve 2.548 g silver nitrate in 1 l reagent water.

General Equipment Setup and Operation

Setting Up The Instrument

To install the instrument for the first time:

Operation

The settings for the titrator can be accessed by connecting the titrino's detachable keypad to the instrument. The titrator should be set with the following settings using the keypad.

• Mode

  • Set mode to DET

• Config

  • COM1 and COM2

    • set Handshake to none

  • Peripheral Controls

    • set Stirrer Control to on

• Parameters

  • Titration Parameters

    • set Meas. Pt. Density to 0

    • set Signal Drift to 10

  • Stop Condition

    • set Stop EP to 1

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.

Shutting down the instrument

Calibration

It is recommended to ‘re-calibrate’ the system whenever the silver nitrate solution is replaced, if the electrode needs replacing, if you replace the dilution solution and your checks are not acceptable or during normal operation when once again your checks are not within acceptable limits.

NOTE: when changing the AgNO₃ solution, make sure that you flush all lines, with the new solution, before attempting to re-calibrate.

Analyzing Samples

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.

Data Available in LORE

Interstitial Waters Standard Report 

• Exp: expedition number

• Site: site number

• Hole: hole number

• Core: core number

• Type: type indicates the coring tool used to recover the core (typical types are F, H, R, X).

• Sect: section number

• A/W: archive (A) or working (W) section half.

• Top offset on section (cm): position of the upper edge of the sample, measured relative to the top of the section.

• Bottom offset on section (cm): position of the lower edge of the sample, measured relative to the top of the section.

• Top depth CSF-A (m): position of observation expressed relative to the top of the hole.

• Top depth [other] (m): position of observation expressed relative to the top of the hole. The location is presented in a scale selected by the science party or the report user.

• Sampling tool: tool used to collect sample

• Data columns: header lists parameter measured and concentration units, followed by wavelength (for ICP-AES) and then analysis method.

Expanded TITRA_MAN Report

• Exp: expedition number

• Site: site number

• Hole: hole number

• Core: core number

• Type: type indicates the coring tool used to recover the core (typical types are F, H, R, X).

• Sect: section number

• A/W: archive (A) or working (W) section half.

• text_id: full text ID of sample

• sample_number: sample number of sample. text ID with sample type prefix removed.

• label_id: id combining exp, site, hole, core, type, sect, A/W, parent sample name (if any), sample name

• sample_name: name of sample

• x_sample_state:

• x_project: expedition project the sample is uploaded under. typically the same as Exp.

• x_capt_loc: 

• location: location sample was taken

• x_sampling_tool: tool used to collect sample

• changed_by: name of person who uploaded sample

• changed_on: date and time sample was uploaded

• sample_type: type of sample. typically LIQ, for liquid.

• x_offset: top offset of parent sample where sample was taken in m

• x_offset_cm: top offset of parent sample where sample was taken in cm

• x_bottom_offset_cm: bottom offset of parent sample where sample was taken in cm

• x_diameter: 

• x_idmp: 

• x_orig_len: 

• x_length: length of sample in m

• x_lengeth_cm:  length of sample in cm

• status:

• old_status:

• original_sample: 

• parent_sample:

• standard:

• login_by: name of person logged into LIMS application used for this test

• sampled_date: 

• legacy:

• test changed_on: date of last edit of analysis

• test date_started: date analysis was started

• test group_name: 

• test status:

• test old_status:

• test test_number: number assigned to performed analysis on sample

• test date_received: date analysis was uploaded to LIMS

• test instrument: instrument used to perform analysis

• test analysis: analysis type

• test x_project: project test was assigned to

• test version: 

• test order_number:

• test replicate_test:

• test replicate_count:

• rest sample_number: sample number for sample the analysis was performed on

• Top depth CSF-A (m): position of observation expressed relative to the top of the hole.

• Bottom depth CSF-A (m): position of observation expressed relative to the top of the hole.

• Top depth CSF-B (m):

• Bottom depth CSF-B (m):

• ammonium (mM): concentration of ammonium 

• calcium (mM): concentration of calcium

• chloride (mM): concentration of chloride in mM

• dilution_factor: the amount the sample was diluted by

• magnesium (mM): concentration of magnesium

• phosphate (uM): concentration of phosphate

• ssup_asman_id:

• ssup_filename:

• sulfide (uM): concentration of sulfide

• titrant_amount (mL): 

• sample description: description/comment of sample

• test test_comment: comment added for performed analysis

• result comments: comment added for analysis results

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.

Health, Safety, and Environment

The primary safety concerns for this instrument and method relate to chemical handling and disposal.

Nitric Acid

2.2 M nitric acid is required by this method; if it is made in house then, handling of concentrated nitric acid is required. Nitric acid is an oxidizer and should not be brought into contact with any organic materials.

Nitric Acid, Concentrated

–Open and handle concentrated nitric acid in a working fume hood.
–When diluting, always add acid to water.
–Wear gloves and safety glasses at all times when handling concentrated nitric acid.
–Body contact: rinse immediately with copious amounts of water.
–Clothing contact: remove the clothing and rinse with water and/or acid neutralization solution.
–If a spill occurs apply an acid neutralization solution to the spill area.

Nitric Acid, Dilute (2.2 M)

–Store this solution in the non-organic acid section of a ventilated fume hood storage cabinet.
–This acid can be aliquotted outside of a fume hood.
–Always wear gloves worn when using this solution.
–Neutralize or flush with copious amounts of water for disposal of this solution or waste products.
–Follow the recommendations for the concentrated solution above if contact with skin or clothing.

Silver Nitrate Solution (0.10 M)

The primary hazard from silver nitrate solution is contact with body parts. It forms a silver compound and turns black, staining the skin and other membranes until they are naturally replaced by the body (typically 1–3 weeks). Certain individuals may also exhibit redness, swelling, and rash-like symptoms in the afflicted area. Of particular concern is getting the solution in the eye. Care should be taken to avoid this.
–Use gloves and eye protection when transferring the stock solution to the reagent bottle.
–Use gloves when handling the waste solution—it contains a small amount of unreacted silver.

LIMS Component Table

Although the titrator could theoretically be used to titrate a number of constituents (e.g., Ca²⁺, S^2-), it is in practice only used for chlorinity. Nonetheless, the LIMS component table includes those other analytes should they ever be done by titration.

Archive Versions

Cl Autotitrator User Guide: 29th September 2022

LMUG-pH_Alkilinity Autotitrator_UG-230220-1718-112.pdf