Zinc -
Clinical PharmacologyZinc
is an essential
nutritional requirement
and serves as a cofactor
for more than 70
different enzymes
including carbonic
anhydrase, alkaline
phosphates, lactic
dehydrogenase, and both
RNA and DNA polymerase.
Zinc facilitates wound
healing, helps maintain
normal growth rates,
normal skin hydration,
and the senses of taste
and smell.
Zinc
resides in muscle, bone,
skin, kidney, liver,
pancreas, retina,
prostate and
particularly in the red
and white blood cells.
Zinc binds to plasma
albumin,α2-macroglobulin,
and some plasma amino
acids including
histidine, cysteine,
threonine, lysine, and
asparagine. Ingested
Zinc is excreted mainly
in the stool
(approximately 90%), and
to a lesser extent in
the urine and in
perspiration.
Providing Zinc helps
prevent development of
deficiency symptoms such
as: Parakeratosis,
hypogeusia, anorexia,
dysosmia, geophagia,
hypogonadism, growth
retardation and
hepatosplenomegaly.
The
initial manifestations
of hypoZincemia in TPN
are diarrhea, apathy and
depression. At plasma
levels below 20 mcg
Zinc/100 mL dermatitis
followed by alopecia has
been reported for TPN
patients. Normal Zinc
plasma levels are 100 ±
12 mcg/100 mL.
Zinc
1 mg/mL (Zinc Chloride
Injection, USP) is a
sterile, nonpyrogenic
solution intended for
use as an additive to
intravenous solutions
for total parenteral
nutrition (TPN). Each mL
of solution contains
2.09 mg Zinc chloride
and 9 mg sodium
chloride. The solution
contains no
bacteriostat,
antimicrobial agent or
added buffer. The pH is
2.0 (1.5 to 2.5);
product may contain
hydrochloric acid and
sodium hydroxide for pH
adjustment. The
osmolarity is 0.354
m0smoL/mL (calc.).
Zinc
Chloride, USP is chemically
designated ZnCl2,
a white crystalline compound
freely soluble in water.
Reactions
Molten anhydrous ZnCl2 at 500 - 700 °C
dissolves zinc metal and on rapid cooling of the melt a yellow
diamagnetic glass is formed which Raman studies indicate contain the
Zn22+ ion.
A number of salts containing the
tetrachlorozincate anion, ZnCl42−, are known.
"Caulton's reagent," V2Cl3(thf)6Zn2Cl6
is an example of a salt containing Zn2Cl62−.
The compound Cs3ZnCl5 contains tetrahedral
ZnCl42− and Cl− anions. No
compounds containing the ZnCl64− ion have been
characterized.
Whilst zinc chloride is very soluble in water,
solutions cannot be considered to contain simply solvated Zn2+
ions and Cl− ions, ZnClxH2O(4−x)
species are also present.
Aqueous solutions of Zinc
chloride are acidic: a 6 aqueous
solution has a pH of 1. The acidity of aqueous ZnCl2
solutions relative to solutions of other Zn2+ salts is
due to the formation of the tetrahedral chloro aqua complexes where
the reduction in coordination number from 6 to 4 further reduces the
strength of the O-H bonds in the solvated water molecules.
In alkali solution in the presence of OH−
ion various zinc hydroxychloride anions are present in solution,
e.g.ZnOH3Cl2−, ZnOH2Cl22−,
ZnOHCl32−, and Zn5OH2Cl3.H2O
(simonkolleite) precipitates.
When ammonia is bubbled through a solution of
zinc chloride the hydroxide does not precipitate, instead compounds
containing complexed ammonia (ammines) are produced, Zn(NH3)4Cl2.H2O
and on concentration ZnCl2(NH3)2.
The former contains the Zn(NH3)62+
ion and the latter is molecular with a distorted tetrahedral
geometry. The species in aqueous solution have been investigated and
show that Zn(NH3)42+ is the main
species present with Zn(NH3)3Cl+
also present at lower NH3:Zn ratio.
Aqueous zinc chloride reacts with zinc oxide to
form an amorphous cement which was first investigated in the 1855 by
Sorel who later he went on to investigate the related magnesium
oxychloride cement which bears his name.
When hydrated zinc chloride is heated, one
obtains a residue of Zn(OH)Cl e.g.
ZnCl2.2H2O
→ ZnCl(OH) +2HCl +H2O
The compound ZnCl2.½HCl.H2O
may be prepared by careful precipitation from a solution of ZnCl2
acidified with HCl and it contains a polymeric anion (Zn2Cl5
−)n with balancing monohydrated hydronium
ions, H5O2+ ions.
The formation of highly reactive anhydrous HCl
gas formed when zinc chloride hydrates are heated is the basis of
qualitative inorganic spot tests.
The use of zinc chloride as a flux, sometimes in
a mixture with ammonium chloride, involves the production of HCl and
its subsequent reaction with surface oxides. Zinc chloride forms two
salts with ammonium chloride, (NH4)ZnCl4 and
(NH4)3ClZnCl4, which decompose on
heating liberating HCl just as zinc chloride hydrate does.
Cellulose dissolves in aqueous solutions of ZnCl2
and zinc-cellulose complexes have been detected. Cellulose also
dissolves in molten ZnCl2 hydrate and carboxylation and
acetylation performed on the cellulose polymer.
Thus, although many zinc salts have different formulas and different
crystal structures, these salts behave very similarly in aqueous
solution. For example, solutions prepared from any of the polymorphs
of ZnCl2 as well as other halides (bromide, iodide) and
the sulfate can often be used interchangeably for the preparation of
other zinc compounds. Illustrative is the preparation of zinc
carbonate:
ZnCl2(aq)
+ Na2CO3(aq) → ZnCO3(s) + 2
NaCl(aq)
USES
Dry Cell or Batteries:
Zinc chloride is commonly used in dry cell batteries
as an electrolyte where it also acts as a moisture absorbent and
corrosion inhibitor.
It
is an excellent water
soluble Zinc source for uses compatible with chlorides. Chloride
compounds can conduct electricity when fused or dissolved in water.
Chloride materials can be decomposed by electrolysis to chlorine gas
and the metal. They are formed through various chlorination
processes whereby at least one chlorine anion (Cl-) is covalently
bonded to the relevant metal or cation. It is generally
immediately available in most volumes and high purity.
A Zinc chloride battery is a heavy duty variation of a zinc
carbon battery. It is used in applications that require moderate to
heavy current drains. These batteries have better voltage
discharge per time characteristics and better low temperature
performance than carbon zinc batteries. These batteries are
used in radios, flashlights, lanterns, fluorescent lanterns, motor
driven devices, portable audio equipments, communications
equipments, electronic games, calculators, and remote control
transmitters.
Electroplating : Today, there are three primary types of acid zinc plating
baths: straight ammonium chloride, straight potassium chloride and
mixed ammonium chloride/potassium chloride. Acid zinc plating
systems have several advantages over alkaline cyanide and alkaline
non-cyanide zinc plating systems except that in acid zinc plating,
the electrolyte is extremely corrosive.
Ammonium chloride zinc plating. The ammonium chloride bath
is the most forgiving of the three major types of acid zinc plating
because of its wide operating parameters. The primary drawback of
this system is the high level of ammonia, which can cause problems
in wastewater treatment. Ammonia acts as a chelator, and if the
rinse waters are not segregated from other waste streams, removal of
metals to acceptable levels using standard water treatment practices
can be difficult and expensive. Ammonia is also regulated in many
communities.
Potassium chloride zinc plating. Potassium chloride zinc
plating solutions are attractive because they contain no ammonia.
The disadvantages of this system are a greater tendency to burn on
extreme edges and higher operating costs. The potassium bath also
requires the use of relatively expensive boric acid to buffer the
solution and prevent burning in the high-current-density areas,
functions performed by the ammonium chloride in the other systems.
Mixed ammonium chloride/potassium chloride zinc plating.
This bath combines the best of the ammonia and ammonia-free baths.
Because potassium chloride is less expensive than ammonium chloride,
the maintenance costs of the mixed bath are lower than the ammonia
bath, and it does not require boric acid. The ammonia levels in the
rinse waters are low enough that it does not significantly interfere
with wastewater treatment, even if plating nickel and copper in the
same plant with mixed waste streams. If local regulations restrict
the level of ammonia discharged, special waste treatment equipment
will be required, and the non-ammonia bath is most likely the best
choice.
Galvanizing, Soldering and Tinning Fluxes:
Zinc chloride is used in fluxes for galvanizing, soldering and
tinning. Its ability to remove oxides and salts from metal surfaces
insures good metal to metal bonding. It has the ability to attack metal oxides (MO) to give
derivatives of the formula MZnOCl2.
This reaction is relevant to the utility of
Zinc chloride as a flux for soldering
- it dissolves oxide coatings exposing the clean metal surface.
Typically this flux was prepared by dissolving zinc foil in dilute
hydrochloric acid until the liquid ceased to
evolve hydrogen; for this reason, such flux was once known as killed spirits or "Marela".
Agriculture: It is very rarely used in
agriculture. It may be reacted with chelating agents to
form solutions of zinc that are biologically available to plants and
animals. It's the Chelate manufacturing that consumes Zinc chloride.
Petroleum: It is an excellent
emulsion breaker and is used to separate oil from water. It is also an effective packer
fluid in oil and gas wells due to its high specific gravity. However
its a little more costly than the other low specific gravity fluids
used in the process.
Water Treatment: It is used in
specialty corrosion inhibitors in cooling towers, potable water, and
in gas and oil wells.
Resins: It is used in Ion
- Exchange resins production.
Paints: It is used in for
the production of lithopone and as pigment for zinc chromate.
Rubber: It is used as
accelerator in the vulcanizing process of rubber.
Glue, wood working:
Zinc chloride is used in for
the preservation of glue, and for the impregnation of timber.
Printing: It is used in off-set
in the chemical products.
Odor Control: It reacts with sulfide to
minimize release of H2S gas in waste treatment facilities.
Oil-Gas Wells: High-density solutions of Zinc chloride and Calcium chloride give good performance
in well completion and work-over operations; the solutions also used
as packer fluids under certain well conditions. ZnCl2 has
been used in specialty corrosion inhibitors and invert emulsion
breakers.
Vulcanized Fiber & Reclaimed Rubber:
Water-leaf paper is
gelatinized with zinc chloride solution is lesstacky, drier and less
moisture-absorbent than caustic reclaimed rubber. The ZnCl2 not only dissolves the cellulosic fibers in
the scrap, but also catalyzes depolymerization of the
elastomer.Similar method is used for Rubber reclaimed from natural,
styrene-butadiene rubber (SBR), and mixed scrap
Animal drug:
It is used for
the production of zinc bacitracin.
Herbicide: Zinc chloride is used as an herbicide. It is
used to control lichen and moss growing on the roofs of houses and
other domestic dwellings, along walks, driveways, fences, and
wherever moss grows
Chemical:
Zinc Chloride is used in the
production of ethylacetate. It is used as
condensing agent for the production of organic dye-stuff.
It is used as a stabilizing agent
for diazonium compounds. It is used for
the production of active carbon. Zinc
Chloride is used for Friedel Craft
Reaction, Azotropic or
Azeotropic Distillation, Desiccation &
Karl Fischer. Zinc laurate, linoleate Stearate or resinate can be
formed from zinc chloride solutions and solutions of the
corresponding sodium salt. It is a Lewis acid and
therefore electrophilic in character. Its catalytic activity is
milder than that of aluminum chloride in, for example,
Friedel-Crafts type reactions. It is particularly
effective in catalyzing reactions that eliminate molecules of water,
ammonia or mercaptans. Zinc chloride solutions gelatinize cellulosic
materials and induce crosslinking in such polymer formers as the
methylol ureas. It absorbs readily on charcoal or silica
for catalyzing acylations and alkylations by Friedel-Crafts
synthesis. In esterifications and condensation reactions, it facilitates the elimination
of water or ammonia molecules from the reactants. One example is the
Fischer idole synthesis.
Miscellaneous:
Zinc chloride has been used as a catalyst in
production of methylene chloride from methyl alcohol.
In the textile industry it
has found use in resin systems to impart durable press to cotton and
synthetic fabrics.
It has been used in reclaiming
rubber where it dissolves rayon cord.
In conjunction with sodium dichromate it has made an excellent wood
preservative.
It has found use in the manufacture of
glue, diazo dyes, paper, cosmetics, rayon, synthetic fibers,
disinfectants and fire fighting foam.
In ore refining it has been used as a flotation
agent.
It is an excellent source of zinc as a
starting material in the production of other zinc chemicals and is
an effective catalyst for removing molecules of water, ammonia or mercaptants.
Zinc chloride is used for
Friedel Craft Reaction,
Azotropic or Azeotropic Distillation, Desiccation & Karl Fischer.
In the laboratory, Zinc chloride finds wide use, principally as a
moderate-strength Lewis acid. It can catalyze the Fischer indole
synthesis and also Friedel-Crafts acylation reactions involving
activated aromatic rings.
Related to the latter is the classical preparation of the dye
fluorescein from phthalic anhydride and resorcinol, which involves a
Friedel-Crafts acylation.
Hydrochloric acid alone reacts poorly with primary alcohols and
secondary alcohols, but a combination of HCl with ZnCl2
(known together as the "Lucas reagent") at 130°C is effective for
the preparation of alkyl chlorides. This probably reacts via an SN2
mechanism with primary alcohols but via SN1 with
secondary alcohols.
Zinc chloride is also able to activate benzylic and allylic
halides towards substitution by weak nucleophiles such as alkenes
In similar fashion, Zinc Chloride ZnCl2 promotes selective NaBH3CN
reduction of tertiary, allylic or benzylic halides to the
corresponding hydrocarbons.
Zinc chloride is also a useful starting point for the synthesis
of many organozinc reagents, such as those used in the palladium
catalyzed Negishi coupling with aryl halides or vinyl halides. In
such cases the organozinc compound is usually prepared by
transmetallation from an organolithium or a Grignard reagent.
Zinc enolates, prepared from alkali metal enolates and ZnCl2,
provide control of stereochemistry in aldol condensation reactions
due to chelation on to the zinc. This is because the chelate is more
stable when the bulky phenyl group is pseudo-equatorial rather than
pseudo-axial, i.e., threo rather than erythro.
HANDLING
Zinc Chloride Solutions are acidic, astringent and
may cause skin irritation or even burns. Contact with skin or eyes
should be avoided through the use of face shields, rubber aprons and
rubber gloves. Protective clothing should be employed in cases where
contact with the solution is probable. In case of contact, flush
eyes and skin with water for 15 minutes. Remove contaminated
clothing and consult a physician. A paste of baking soda can be
applied to help neutralize acid remaining on the skin. Obtain prompt
medical attention for eyes. Contaminated clothing should be
thoroughly laundered prior to reuse. Do not take internally. If Zinc
Chloride Solution is ingested, administer large quantities of water
or milk. Do not induce vomiting. Avoid inhaling mist or fumes. The
Threshold Limit Value (TLV) of Zinc Chloride is 1 milligram per
cubic meter of air for a 8 hour work-day. Exposure to fumes can be
minimized by adequate ventilation or exhaust fans. In case of
spills, flush area with plenty of water to chemical sewers. Zinc
Chloride Solution is non-flammable.
