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Subject: Crystal identification

Crystal identification

From: Doug Nishimura <dwnpph>
Date: Sunday, November 7, 1993
Mary Ann Tyrrell:


Saturated salts are often used in humidity control.  From your
description, I suspect that the salt that you have is *sodium
dichromate.*  This salt actually controls at about 52% RH at 20 C (54.2%
at 30 C).  This is about the most common salt used for nominally 50% RH
control.  Other salts that are commonly used are sodium bisulfate (a
white crystal - very acidic!), calcium nitrate (another white or
"colorless" crystal), sodium iodide (a third white "colorless" crystal)
and magnesium nitrate( colorless, white crystal).  They may have also
used chromium trioxide used to maintain about 45% RH at room
temperature. However, my bet is sodium dichromate.  This and chromium
trioxide (dry chromic acid) both appear as orange/red crystals. This
chemical can cause ulceration of the skin and is now listed as possibly
being fatal if absorbed through the skin (chromium poisoning.)  It is a
very strong oxidizing agent (storage instructions caution against
getting this in contact with combustibles).

When I was working for the National Archives of Canada doing research,
all of the incubations of samples were done using desiccators with
humidity control salts and dry ovens.  They really don't work well
unless they are carefully made up.  The "saturated" solution should be
heated well above the use temperature and an excess of crystals should
be sitting in the bottom of the container.  As the chemical cools in
solution, it will re-crystallize.  This is the only way to ensure that
you really have a saturated solution.  There also must be a cone of dry
salt extending out of the solution.  Without this dry mound, many of
these salt solutions just end up saturating the air (causing many ruined
experiments.) Remmington-Rand made these cabinets with good intentions,
but more often than not, I'll bet that people were creating humidities
near 100%.

I don't want to to alarm anyone needlessly.  But care should be taken
when handling this chemical.  In the old days (and even now) this
chemical was and is used as a photographic bleach in various processes
including chromium intensifier.  It has only been recently that it has
been determined to be fairly dangerous.

I don't have an MSDS for SODIUM dichromate, but it will be very similar
to the potassium dichromate one (since it is the dichromate part that is
dangerous.)  Chromium trioxide will look similar except that it is also
a strong acid.  (Chemists make chromium trioxide in solution (chromic
acid) for glass cleaning by adding potassium dichromate to sulfuric

    "US Precautionary Labeling

    Poison Danger

    May cause burns or external ulcers.  Caution:  Contains chromium
    (VI), cancer hazard.  Strong oxidizer.  Contact with other material
    may cause fire.  May be fatal if swallowed or absorbed through skin.
    Harmful if inhaled.  Keep from contact with clothing and other
    combustible materials. Do not breathe dust.  Keep in tightly closed
    container.  Use with adequate ventilation.  wash thoroughly after
    handling.  In case of fire, soak with water.   In case of spill,
    sweep up and remove.   Flush spill area with water.

    Section V Health Hazard Data

        Threshold limit value (TLV/TWA):  0.05 mg/M3
            TLV is for chromium (VI) compounds, water soluble, as Cr.
        Short-term exposure limit (STEL): Not established
        Permissible exposure limit (PEL): 0.1 mg/M3
            PEL (Ceiling) is for chromic acid and chromates, as CrO3
        Toxicity of components
            Intraperitoneal mouse LD50 for potassium dichromate   37
        Carcinogenicity:   NTP: YES  IARC: YES  Z List: NO  OSHA REG: NO
            This substance is listed as a NTP human carcinogen and an
            IARC human carcinogen (Group 1).
        Reproductive effects:  None identified

    Effects of overexposure:

        Inhalation:  Severe irritation of respiratory system, prolonged
            contact may cause perforated septum
        Skin Contact:  Irritation, prolonged contact may cause
            dermatitis, and ulceration
        Eye contact:  Severe irritation or burns
        Skin Absorption:  Is harmful and may be fatal
        Ingestion:  Severe burns, ulceration - mouth, throat, stomach,
            and may be fatal.
        Chronic effects:  damage to liver, kidneys, blood, lungs.
        Target organs:  blood, respiratory system, lungs, liver,
            kidneys, eyes, skin, GI tract
        Medical conditions generally aggravated by exposure:  Damaged
        Primary routes of entry:  Inhalation, ingestion, skin contact,
            eye contact, absorption."

Oh yes, the Baker SAF-T-DATA system uses the following labeling (these
are out of 4).

    Health:       4 extreme (cancer causing)
    Flammability: 0 none
    Reactivity:   3 severe (oxidizer)
    Contact:      3 severe (life)

Good Luck.

-Douglas Nishimura
Image Permanence Institute

                  Conservation DistList Instance 7:38
                 Distributed: Monday, November 8, 1993
                        Message Id: cdl-7-38-001
Received on Sunday, 7 November, 1993

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