Chloride meters and titrators

Eleni Loizides <eleni.loizidou<-at->cytanet<.>com<.>cy> writes

>We are looking into purchasing a chloride meter to be used for
>monitoring the desalination process of underwater finds (organics,
>metals and ceramics).  We have come across Sherwood Scientific
>Chloride Analyser 926 but have also been advised that Titrators
>could also be used.

There is seemingly a large variety of methods for measuring
chlorides with varying degrees of accuracy.  Working with maritime
archaeological materials, I have used a range of options including
(in order of quantitative effectiveness):

    Basic Chemistry Titration (Mohr method): While this is
    considered to be a low tech way of measuring chlorides, it is
    not a widely used process due to the subjectivity in
    observations between people, the fast "turning point", and the
    need for the disposal of the silver solution.  Specific chloride
    ions can be targeted.

    Total Dissolved Solids (TDS) meter: These are great for working
    in the field and giving a very broad measurement of the
    salts/dissolved solids in your solution compared to a "pure"
    sample of that solution.  I use these in the field and for
    conservation education exercises when I don't have the resources
    for consumables.

    Hach Quantabs: A big leap up from TDS meters, the Quantabs are a
    somewhat economical and field friendly way to measure chloride
    ions specifically.  They can still be subjective in viewing the
    results as you read the outer limits of a color change and there
    is a range of accuracy.  I haven't had to neutralize solutions
    before measurements, but it is chloride specific.

    Bench Conductivity Meters: This is the class of analyzers that
    are similar to the Sherwood Scientific Chloride Analyzer 926.  I
    have experience with a similar model, the Jenway PCLM3.  While
    these models do have advantages, the sampling, regular
    maintenance and calibration of the unit should be considered
    based on your lab environment (time and financial resources).
    Plusses are that it only requires a small sample.  Consider the
    disposal needs of the waste solutions that contain silver.

    Ion Selective Electrodes (ISE): This is a form of ion analysis
    that comes as an electrode wand that is connected to a computer
    unit (benchtop or field).  The Fisher Accumet model was straight
    forward to maintain, but I did find that the calibration of the
    electrodes can be challenging to achieve the perfect slope.  It
    is important to keep the electrode clean and wet.  Additionally,
    your solution will need to be neutralized.

    Ion Chromatography (IC): An ion chromatographer is one of the
    most accurate instruments available to conservators.  While they
    are user friendly, the cost alone can be prohibitive to smaller
    laboratories, but using this for chloride analysis will ensure
    accuracy in your measurements.

Deciding which method is best for your lab depends on a few
questions: Do you have funds or resources to maintain the equipment
and purchase associated consumables?  Are you wanting to use it in
the field?  How accurate do you need it to be?  On this last note,
unless you are utilizing a digestion process to determine the exact
amount of chlorides in the object, we are really only ever getting
an estimate of chlorides extracted from the object.  So your
solution measurements will be fairly subjective to begin with, but
that is what we have to work with when desalinating objects!

Hopefully this will be of some assistance in your choice.

Susanne Grieve


                                  ***
                  Conservation DistList Instance 29:47
                    Distributed: Sunday, May 1, 2016
                       Message Id: cdl-29-47-005
                                  ***