greetings to you lets continue our discussion
on the water parameters especially drinking
water parameters using spectrophotometry already
covered about atrium parameters which include
anions and cations and today i am talking
about the other parameters that is total hardness
you may wonder what is this hardness total
hardness to do with spectrophotometry usually
total hardness of water is caused by the presence
of calcium and magnesium salts and the solubility
of calcium and magnesium salts are ah quite
good and the presence of these salts leads
to the 
the presence of these salts causes the water
to taste very hard
you dont feel like drinking it and then when
you want to wash the clothes it will keep
on taking the soap but it wont produce any
lather as you know lather production there
is a an essential characteristic whenever
you want to wash your clothes basically lather
is a type of a foam with very large surface
area and whenever you want to wash the clothes
with the soap the lather ah will catch the
particles dust particles and other things
and then the clothes will get brighter or
get cleaner so hardness has got about another
property that is it cannot be used in several
industrial processes such as cooling and heating
because as the water evaporates the [solu/soluble]
soluble salts will precipitate and coat on
to the inside of the cooling tubes
which are made of copper or iron and presence
of a little bit of silica will produce calcium
silicate and magnesium silicates the both
these salts are essentially like cement because
cement is also a silicate and as we know cement
building etcetera last for hundreds of years
and so the silicate salts also will last for
several years unless you remove them mechanically
the presence of silicates on the inside of
the heat exchange tubes causes low temperature
movement that is low temperature transform
and that causes the requirement or to increase
the heat requirement will increase
so the from the industrial point of view also
presence of hardness cause in chemicals like
calcium magnesium are more desirable hardness
of water the is measured as p p m parts per
million of calcium carbonate become magnesium
carbonate and several other carbonates and
sulphates also cause hardness but for easy
standard in the bureau of indian standard
b i s prescribes a limit of about three hundred
p p m as calcium carbonate and no suitable
alternate source of water is available the
limit may be relaxed up to six hundred p p
m but this works only for drinking water so
usually hardness is determining by titration
with ethylenediaminetetraacetic ethylenediaminetetraacetic
acid as a titration method
but we are not recommending that what we want
to recommend here is the calmagite method
so in this method both calcium and magnesium
react with this reagent at about p h twelve
point five to form a colour complex the changes
the colour of the solution from blue to red
blue the change in the absorbance at five
hundred twenty nanometers is related to the
hardness of waters so this is the structure
of calmagite you can see here it is contains
three benzene rings connected with azo group
and o h group there is one s o three group
this s o three group helps in solubilizing
the salts so we doesnt need to add any sulphate
and something like that and this azo group
and this o h group will help form a complex
of ring closure using calcium and magnesium
here so it is is basically a question of ring
closure ah reaction and the chronopher will
increase the increased chronopher will shift
the lambda max to the bathochromic site that
is only higher valiance site
so calmagite it is also coloured but the complex
will be more coloured with lambda max slightly
towards the right that is at longer wave length
so the reagents include very simple things
now will not going to very much detail about
how to prepare the reagents because all the
numbers are given here so we have to take
concentrated h c l and dilute it to hundred
m l and you can prepare the calcium solution
by ah you have to dry the calcium carbonate
because usually calcium carbonate is associated
with water and you have to remove this water
and that happens around one hundred five degree
[cen/centigrade] centigrade you just try to
weigh out calcium carbonate into hundred m
l beaker add two normal h c l hydrochloric
acid slowly it [terl/swirling] swirling till
the white powder just dissolve shake well
remove and transfer the quantity into hundred
m l volumetric flask and dilute it to hundred
m l
so that is the standard calcium solution hm
you can also prepare magnesium solution also
ah containing point two four six five gram
of magnesium sulphate heptahydrate that is
seven h two o in deionised water and make
to hundred m l for our working reagent with
this both calcium and magnesium ah ah prepare
to prepare hundred p p m equivalent of calcium
because sometime magnesium may be there sometimes
calcium may be there so when we mix equal
volumes of calcium and magnesium solutions
we get about we get hundred p p m of both
but only as the same concentration now since
the reaction occurs around p h twelve we need
several potassium hydroxide solution and this
stock solution is prepared by dissolving potassium
hydroxide in dieonised water and make up to
ah hundred m l
this potassium hydroxide or any other hydroxide
as i have been repeatedly telling you we need
to dissolve in water but store in polyethylene
bottle with a screw cap normally if you store
it in glass bottles there is a tendency that
water will evaporate and it will form ah it
will be very difficult to remove the bottle
to open the bottle this is a practical difficulty
so ah potassium hydroxide working solution
also you can prepare by adjusting the p h
to twelve point five and we have to prepare
point zero one normal potassium hydroxide
for this we have to dilute one m l of potassium
hydroxide stock solution to hundred m l so
all these things you can calculate yourself
but i have giving you the details because
i want to give you a readymade ah method because
sometimes it takes time for you whenever there
is an urgent need for preparation of the reagents
you need the ready reference method to ah
work so i am straight away taking you with
this introduction i am straight away taking
you to the determination to the experimental
level itself
so calmagite reagent also you can prepare
by dissolving point zero seven two five gram
of calmagite in hundred m l deionised water
but this reagent is available in the ah in
the market so there is no difficulty in obtaining
this calmagite solution now the hm hardness
of a given solution always we measure in about
about fifty hundreds etcetera that is if you
take [riv/river] river water now a days in
india you will come across a waters having
about hundred to two hundred ah p p m of hardness
even in river water in bore well it may go
up to ah it depends upon the depth at which
bore well water is extracted and there also
the range may vary from ah few hundreds to
fifteen hundred p p m so basically the range
should vary from fifty to ah five hundred
range
if we can make a calibration curve using the
this king of standard i think we should be
in the business so what we should do is we
normally do not worry about plus or minus
ten percent error in measuring the hardness
of water so we prepare fifty hundred hundred
fifty two hundred and two fifty micro litre
of hundred p p m standard solution that is
both containing both calcium and magnesium
and then you dissolve them and put them in
volumetric flask add one m l of the reagent
dilute by potassium hydroxide solution and
measure you just have to measure ah the ah
absorbance after adding one m l of point zero
one five molar potassium hydroxide solution
we have just try to dilute it and measure
the absorbance of at a wave length of five
hundred twenty nanometer against the blank
dish tells us that the reagent could be somewhat
bluish in colour reddish red violet says the
right colour and as usual
we have to prepare a calibration curve of
the absorbance verses hardness of the water
and determine the hardness of the sample by
referring the absorbance of the with a calibration
curve so recommend a sample volume is about
one ml but this can this refers to only the
standard solution that is if you are take
fifty m l ah in fifty micrograms it gives
you ah that is about five p p m fifty microgram
in ten m l that is five p p m it gives you
an absorbance of about point five two four
plus or minus point zero one that is quite
good absorbance and if you get this absorbance
with you standard reagents you are in business
that means all your preparation standard solutions
and other things are all ah well under good
control so i am showing you the absorbance
of this ah calcium magnesium that is complex
that is cal mag ite complex calcium magnesium
complex and then the lambda max
as you can see here is somewhere around hm
four eight five this is around around five
hundred ten to five hundred twenty the curve
is somewhere flat here in this region not
much absorbance range and so you can be comfortable
when you are plus or minus five nanometers
this side that side that no serious error
will be introduced during the measurement
so this is the calibration curve you can see
that the calibration curve is linear ah we
have done it up to one hundred twenty micrograms
here but we can see that we are already very
near the end of the ah absorbance scale that
is almost one point two to one point five
ah as dictated by the relative standard error
we should be within point two to point eight
and that range and here even around one twenty
p p m we have absorbance of about one point
three so any standard it follows a very nice
linear curve
the least square feet value of r square is
point nine nine eight that means the method
is quite reliable at this this thing and of
course you will always be having some amount
of hm in normal circumstances you will be
having the hardness not within fifty or hundred
but you will ah your range it may go between
two hundred three hundred five six hund[red]
eight hundred to fifteen hundred so what you
should do you do a pilot trial ah with a with
one m l of the given sample where calibration
curve is done anyway but the actual sample
analysis if you want to determine the hardness
of the sample you try it with one m l
if the absorbance is too high then you will
have to dilute it accordingly and then try
to bring it within point two and point eight
and multiply it by the dilution factor and
you will be able to determine the hardness
of water a very simple experiment which can
be conducted and completed within half an
hour if you have all the reagents and the
reagents are all available across the shelf
we will not have any worry regarding this
hm analysis of hardness so now i would like
to take you to the determination of zinc zinc
is an essential element of the plants and
animals but elevated levels it is toxic to
aquatic life as well as human life but it
that is a very standard statement any chemical
that is in excess is toxic that is a fundamental
reveal so but zinc is an a essential micronutrient
if you look at some of the ah soil as well
as water con[tent] water samples all over
the all over the world you will see that most
of the soils and water they do contain certain
amount of zinc and that is how ah it is ah
it is distributed it is widely distributed
element the bureau of indian standards prescribes
a limit of five p p m that is relaxable if
no suitable alternative source is available
then it is up to fifteen p p m now zinc is
a industrial element also it is used for the
determination it is used for anodizing so
you can see that many of the material around
you that is zinc sheets as you might have
heard they are all aluminum and iron which
are galvanized so in galvanization zinc is
coated ah electro electrometrically ah so
the zinc is ah acts as a sacrificial cathode
protecting the metal inside the ah zinc coat
and it is also an alloying element brass is
known ah well known example of copper and
zinc alloy so brass has got a very bright
alloy structure colour and its also used and
the zinc is also used in give a imparting
strength to fibers especially this cellulose
fibers if you want to make cloth or something
like that using viscose or rayon or something
like that then the first the ah wood pulp
is treated with sodium hydroxide and then
dissolved in sodium hydroxide
and then treated with carbon disulfide to
remove the lignin and once we lignin is removed
the fiber becomes white and this fiber you
fiber is dissolved in carbon disulfide or
and then pass through a spinneret containing
number of fibers and the spinneret is ah treated
with zinc to give strength to the yarn so
this kind of yarn is is suitable to this strength
to the yarn so most of the tires ah of cars
cars airplanes cycles etcetera and clothes
what we wear ah viscose clothes that is very
soft materials they are all made of viscose
rayon which is strengthened with zinc and
there are of course several other kinds of
industrial applications for zinc and so it
is a widely distributed element as well as
widely used element so there is no escape
from zinc in your day to day life from ah
this because of the origin because of the
use of zinc in several industrial applications
that we normally come across so the b i s
standard prescribes limit of five p p m and
this is zinc is determined by using a reagent
call as zincon this zincon is a very special
organic compound and its a trigonal of course
and i am not going to give you the actual
chemical name because it will run it about
two pages two lines not two pages and but
i can tell you that essentially it is contains
s o three group here ah in ah this side that
helps solubilization there is n n double bond
and o h group and n h double n h grouping
and then o h group is there there is a triton
group and two three benzene rings are there
so because of this there will be ah complexation
of zinc something like inner complex and let
us see how the reagent works basically what
happens is with this kind of structures we
can expect ah
we can expect several ah types of metals to
complex with zincon so the trick is is there
anywhere that we release zinc from the complex
leaving others in this solution so that trick
has been employed here that is why i have
chosen zinc also to you in this ah presentation
so many other metals including zinc form this
but then we add cycle of zincon to this to
remove the zinc and then we monitor the absorbance
so the reagents the reaction is carried out
around p h nine using sodium hydroxide and
boric acid ah and then zincon reagent is again
dissolved in methanol because obviously many
such compounds having high monocular weight
they do not dissolve in water so you have
to have methanol or ethanol which are miscible
with water because you want to develop aqueous
methods
so the only we know very few ah organic chemicals
which are miscible with water so zincon dissolved
in methanol and methanol is miscible with
water methanol and ethanol among the two methanol
is cheaper compared to ethanol so ah similarly
you can use acetone also but the evap[opation]
rate of evaporation of ethanol acetone is
not much higher than methanol so the concentration
keeps on changing even if you keep it at room
temperature so we normally recommend dissolution
in methanol so that is how we prepare that
and then standard zinc solution we know how
to prepare this are the numbers point four
three nine eight and sodium ascorbate we have
to prepare there using ah ascorbic acid and
adding five hundred five gram of sodium bicarbonate
mixing well this becomes a slightly darker
solution and complexing agent is potassium
cyanide
so there potassium cyanide is a dangerous
chemical but still there are certain situations
in the life of any chemist where you cannot
avoid the contact with some of these chemicals
which are deadly of course there are other
ways of doing zinc analysis but ah potassium
cyanide is not very unusual lot of laboratories
do have it but you may also able to get it
for laboratory purpose the method of the in
the laboratory method you should be able to
determine the zinc and zincon is as i told
you it is easy to prepare working solution
we need ten p p m that is ten microgram so
ah see thousand p p m we need we are preparing
and then working solution is ten p p m so
that is a very simple calculation and the
procedure involves transferring zero to fifty
microgram of zinc ten p p m zinc solution
that is up to five m l ten p p m into five
would be [and/another] that fifty micrograms
and ten divided by ten m l is five p p m so
one to zero to five p p m is our working range
and you can had one m l of sodium ascorbate
solution and buffer solution followed by point
five m l of potassium [thioi/cyanide] cyanide
so you can also add one m l of zincon solution
and ah after adding zincon solution you have
to add point two m l of cyclohexanone this
releases the zincon complex zinc complex from
the cyanide and that ah mixes it with zincon
so you allow it to stand for about thirty
minutes at room temperature so colour develops
and you have to measure the absorbance at
six hundred twenty nanometers further chemistry
of this reaction you can study in feagul that
is a book i have recommended for almost all
spectrophotometric procedures you can also
look up third addition of e b sandal colorimetric
methods of analysis so sample recommended
volume is an one m l and cookbook value will
be for ten micrograms in ten m l
that is one p p m should give a absorbance
of about point three four plus or minus point
zero one that is point three two to point
three six point three three to point three
five absorbance that should give you a fairly
good control over the experiment if your all
you reagents are good so this is the lambda
max of the ah spectrum of the lambda spectrum
of zincon complex and this i have recommended
six twenty nanometer so if we go back to this
ah complier to this spectrum we can see that
lambda max is around six hundred twenty or
six hundred thirty at the lambda max the peak
is fairly broad that means again ah the importance
of this situation is plus or minus two three
nanometer plus or gives you a fine nanometer
range where the absorbance will be almost
constant with respect to the zinc concentration
around six twenty so whether you are around
six twenty five or six thirty it doesnt matter
that it will give permission to work with
colorimetric method ah that is the colorimetric
instead of spectrophotometrics
so ah you can determine zinc in all your sample
it is an agriculturally important ah element
also because it is a micronutrient it is also
used in pharmaceuticals ah industries and
then ah it is very easy to determine zinc
around this linear here at the back as you
look at this curve you can see that there
is lot of we usually add excess of the reagent
and that reagent compared to when you measure
at the blank you will get a negative value
also so its not a unusual but we concentrate
on the absorbance difference that is positive
so we can determine this you can even use
this ah somewhere around four eighty that
is a negative value that is a amount of zinc
that is liberated this also you can also use
as a negative parameter but its up to it is
normally ah comfortable to use the positive
difference between the blank and sample now
i would like to tell you about the interference
of this method
so the interference studies we have conducted
earlier and by determining ten p p ms standard
that is how we do we take ten p p m solution
in ten m l that is one p p m we normally go
for thousand p p m so for t p p m we add the
interference species and then measure the
absorbance so if there is more than ten percent
of the absorbance we conclude that it is a
ah interfering element we did not find ah
any interference from cobalt nickel calcium
magnesium sulphate phosphate nitrate and then
silver fluoride chloride iodide mercury tartaric
acid citric acid and surfactants so this shows
that the method is very very robust and ah
some interference ah can be overcome by the
addition of citrate and triturate because
around p h twelve we will get only citrate
acid sodium citrate and sodium triturate and
hundred microgram of chromium molybdenum lead
antimony arsenic and triton x hundred this
is a standard surfactant we normally come
across
so hm in the rivers etcetera whenever we use
one of the surfactants which i have been telling
you and these things did not interfere up
to hundred micrograms levels that is one p
p m of zinc and hundred microgram of the interfering
element ah interfering element or anion or
complex of whatever it is but aluminum iron
[co/copper] copper and cadmium they did not
interfere up to five times that is when you
want to determine zinc in one p p m up to
five p p m ah of any of these elements will
not interfere and vanadium limit is a little
lower that is up to two p p m it can be tolerated
and e d t a and manganese did not interfere
that means with if e d t a does not interfere
we can always ah conclude that the method
will be robust because e d t a is a very strong
complexing agent with several metals that
are normally present in drinking water river
water lake water pond water and all those
things so we will continue our discussion
in next class
thank you
