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    The Chemistry Of Swimming Pools Chemistry Essay

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    Pool chemical science is the application of chemical science to keep safe and clean H2O ( Hann, 1997 ) . This is achieved by modulating legion factors which include: the pH, the measure of germicide and the buffers used to defy pH alterations. The normal pH scope of a liquid pool demands to be between 7.2 and 8.0, although a scope between 7.2 and 7.8 is more practical as some germicides such as Cl are most effectual for this scope ( Hann, 1997 ) .

    The Importance of pH

    The pH graduated table ( see figure 1 ) is a step of the concentration of H ions ( H+ ) in a solution and is a logarithmic graduated table based on 10 ( Zumdahl, 2007 ) . Furthermore, it is represented by the equation: pH = -log [ H+ ] ( Zumdahl, 2007 ) , and since the graduated table is logarithmic, this means that a pH addition of one value represents a H + concentration addition of tenfold. The pH graduated table ranges from one, which is really acidic, to fourteen which is really alkalic. A value of seven is impersonal, intending that a solution is neither acidic nor basic ( Hann, 1997 ) .

    The pH graduated table ( figure 1 )

    ( Environment Canada, 1992 )

    A strong acid dissociates wholly in H2O to bring forth H+ ions in the signifier of H3O+ ( hydronium ion ) , whereas a weak acid does non disassociate wholly, doing a lower H+ concentration to organize ( Zumdahl, 2007 ) . Since an acid dissociates in H2O to bring forth H+ ions it is known as an H+ giver and a good illustration of this is the strong hydrochloric acid ( HCl ) . This acerb easy dissociates in H2O to organize H+ and the undermentioned equation represents this dissociation.

    HCl ( aq ) + H2O ( cubic decimeter ) A® H3O+ ( aq ) + Cl- ( aq )

    Because HCl is easy dissociated, it means that equilibrium lies far to the right, favoring the merchandises. On the contrary, a weak acid does non easy disassociate in H2O, intending that a reaction affecting a weak acid would non favor merchandises or reactants. In contrast to an acid, a base dissociates in H2O to organize OH- and is known as an H+ acceptor.

    The function of acids and bases for the care of the pH of pool H2O is of import for a figure of grounds. A pH that is excessively high or low ( above 8 or below 7.2 ) will do annoyance to the tegument ( Gothard, 2006 ) . Equally good as this, germicides such as Br and Cl require an optimal pH between 7.2 and 7.6 in order to work most expeditiously ( Hann, 1997 ) . Therefore, an addition or lessening in pH will do these germicides to work less expeditiously.

    Another ground for the ordinance of the pH balance is to forestall the formation of graduated table or H2O hardness sedimentations. These sedimentations are normally composed of Mg and Ca which can go detrimental to the operation of a pool as it affects the filter system, warmer and the piping ( Hann, 1997 ) .

    Care of the pH degrees

    Factors such as, the remotion or add-on of pool H2O ; waste from swimmers such as piss ; and the add-on of chemicals, affect the pH of the H2O. In order to hold a pH degree that is desirable, specific chemicals need to be added to the H2O, although it is besides possible to make a coveted pH balance by adding excess H2O to the pool ( Hann, 1997 ) . By adding extra H2O this will do the pH concentration to lower, which helps to equilibrate the pH. However, it is non ever possible to make this as different factors affect the pH, and so pool chemicals are needed.

    Two chief chemicals are used to take down the pH of pool H2O: Na bisulfate and muriatic acid ( hydrochloric acid ) . These chemicals both have a low pH which means that they act as a “ pH reducing agent ” . A lessening in pH occurs because the chemicals react in the H2O to bring forth more H ions, therefore increasing the sourness. The pick of which chemical to utilize depends on the size of a pool. Sodium bisulfate is normally used for little pools ( about 190 000 liters ) as it is less acidic than muriatic acid, therefore a safer option ( Hann, 1997 ) . On the other manus, muriatic acid is used for larger pools to cut down the pH because it is a more acidic. Furthermore, to raise the pH of pool H2O, Na carbonate is most normally used ( Perkins, 2000 ) . It is known as a “ pH increaser ” because it produces hydroxyl ions ( OH- ) which increases the pH of the pool H2O. Chlorine besides has an consequence on the pH of H2O ; nevertheless, it is used more normally for disinfection intents.

    Chlorine for Disinfection of Pool Water

    A safe and clean pool requires the usage of a germicide to halt the spread of catching diseases ( Hann, 1997 ) . The most common germicides used are chlorine-based merchandises as Cl is comparatively cheap and is really effectual in killing bacteriums and other harmful beings ( Perkins, 2000 ) .Chlorine gas ( Cl2 ) is ne’er used on its ain for disinfecting pool H2O because it is really toxic, and so would present many hazards. Therefore, compounds incorporating Cl are used alternatively and there are three chief types: Calcium hypochlorite [ Ca ( OCl ) 2A­ ] , Na hypochlorite ( NaOCl ) and chlorinated isocyanurate.

    When these compounds are added to the H2O a reaction occurs, organizing a chemical called hypochlorous acid ( HOCl ) which is an oxidising agent and hydrochloric acid ( HCl ) . Since hydrochloric acid is formed, this will intend that the pH of the pool H2O decreases somewhat.

    Cl2 + H2O A® HOCl + HCl

    The hypochlorous acid kills the bacterium in the H2O by oxidization and the HOCl can easy go dissociated to from H ions ( H+ ) and hypochlorate ions ( ClO ) – .

    HOCl a†” H+ + ClO-

    Both the hypochlorous acid and the hypochlorite ions are considered as “ free Cl ” , which is the chemical species responsible for killing bacteriums in the H2O ; nevertheless, the hypochlorous acid is far more efficient ( Daniels, 1973 ) . The dissociation of the hydrochlorous acid is an equilibrium reaction, intending that the reaction can happen in either way. Furthermore, the pH of the H2O affects the way in which the reaction returns, therefore act uponing the effectivity of the germicide. An addition in pH would intend that there are more H+ ions which would do the reaction to continue to the right, intending that less hypochlorous acid is present in the H2O. Furthermore, a lessening in pH will do the reaction to continue to the left, intending that more hypochlorous acid is produced which increases the effectivity of the Cl as a germicide. To be able to find the pH of a solution, the usage of acid/ base indexs are needed.

    The function of acid/base indexs

    In order to maintain the pH of pool H2O in the right scope it is necessary to utilize an acid/base index to prove the pH. An acid/base index is a substance that gives an accurate indicant of the sourness or alkalinity of a solution ( Dice, 2008 ) . Besides, an index is a weak acid represented by HIn ( Zumdahl, 2007 ) and it can be written as an equilibrium look:

    HIn ( colourless ) a†” H+ + In- ( pink )

    where the In- is the basic signifier of the index. The HIn and the In- both show a different coloring material which corresponds to the pH of the solution. As an illustration, the index phenolphthalein is colorless in an acidic solution and pink in a basic solution. This means that the HIn represents the colourless molecules, whereas the In- represents the pinkmolecules. Since an index is in equilibrium, an acidic solution would do an addition in H+ concentration, therefore switching equilibrium to the left. Likewise, in a basic solution the OH- ions cause a lessening in H+ , which shifts equilibrium to the right. The equation for an index can be written as an equilibrium changeless look.

    Ka = [ H+ ] [ In- ] / [ HIn ]

    This look can be rearranged to organize an equation which is really utile in happening the terminal point of an index, which is the point at which colour alteration occurs.

    Ka / [ H+ ] = [ In- ] / [ HIn ]

    The ratio between the In- and the HIn will demo the coloring material of the index ; for illustration if there is one In- pink molecules for every one hundred colourless it means that the solution will look colourless. However, for the human oculus to observe the coloring material alteration, the coloring material alteration occurs at a pH where the ratio of In- to HIn is 1:10 for an acidic solution, whereas for a basic solution the alteration will happen at a ratio of 10:1 ( Zumdahl, 2007 ) .

    There are a assortment of indexs all of which are utile for specific pH ranges, and so it is of import to utilize an appropriate index for mensurating pool H2O pH. The following tabular array ( figure 2 ) shows four different acid/base indexs that could be perchance used to help with pool direction.

    Four Possible Indexs for Testing pH of a pool ( figure 2 )


    pH scope

    Colour shown for Acidic Solution

    Colour shown for Basic Solution


    pKa ( -log10Ka )

    Bromthymol Blue




    1.0 x 10-7


    Cresol Red ( alkaline )




    1.0 x 10-8.32


    Phenol Red




    1.0 x 10-7.9






    1.0 x 10-9.3


    From the tabular array it can be seen that phenol ruddy, cresol ruddy and bromthymol blue would wholly be utile indexs as their pH scope is rather close to the pH scope of a pool which is 7.2-7.8. Since phenolphthalein ‘s scope is 8.-10, this index would be the least effectual as is non really near to the pool scope, whereas the other indexs each have similar scopes that are within the scope. To verify the pH at which an index alterations colour, the equation from above can be used:

    Ka = [ H+ ] [ In- ] / [ HIn ]

    Sample computation for the index phenol red:

    ten 10-7.9 = [ H+ ] [ In- ] / [ HIn ]

    First, the pH at which the index will alter for an acidic solution will be found. For an acidic solution the coloring material alteration will be seeable when [ In- ] / [ HIn ] = 1/10:

    ten 10-7.9 = [ H+ ] ( 1 ) / ( 10 )

    [ H+ ] = ( 1.0 x 10-7.9 ) x 10

    = 1.26 ten 10-7

    pH = -log ( 1.26 x 107aˆ‘ )

    = 6.9

    pH = 6.9, which is near to the existent value of 6.8. This means that at this point, the coloring material alteration will be xanthous.

    The pH at which the coloring material alteration occurs in a basic solution can besides be calculated, nevertheless, the ratio of In- to HIn will be 10:1 as there must be more In- molecules for a color alteration to happen.

    ten 10-7.9 = [ H+ ] ( 10 ) / ( 1 )

    [ H+ ] = ( 1.0 x 10-7.9 ) / 10

    = 1.26 ten 10-9

    pH = -log ( 1.26 x 10-9 )

    = 8.90

    pH = 8.9, which is the point at which the coloring material alteration will be ruddy. However, the value is non precisely the same as the existent value because it is an estimate and is non exact. Phenol red ‘s pH scope is the closest to the pool ‘s scope when compared to the other three indexs, which means that it is the most appropriate for proving pool H2O. Acid/base indexs are non merely of import for finding the pH of a solution, but are besides important for happening a pool ‘s buffering capacity, as the pH of a solution must be known.

    Buffer solutions

    To help in the care of pool pH, it is necessary to utilize a buffer solution. A buffer solution is any solution which resists fluctuating alterations in pH, doing it easier to maintain pool H2O in the appropriate pH scope ( Hann, 1997 ) . The buffering ability of a pool is dependent on the entire alkalinity, which is the step of the measure of alkalic substances present in the H2O ( Hann, 1997 ) . Calcium carbonate is the chief compound which makes up the entire alkalinity and when it is dissolved in H2O it produces carbonate ions which act as a buffer.

    CaCO3 ( s ) a†” Ca2+ + CO32-

    The carbonate ions work as a buffer because they neutralise the H+ ions formed when an acid is added. To find the buffering ability of a solution, the Henderson-Hasselbalch equation can be used. This equation is derived from the equilibrium invariable for the dissociation of a weak acid which is given by the equation:

    Ka = ( [ H+ ] [ A- ] ) / [ HA ]

    where A- is the coupled base, and HA is a simple acid. By taking the logarithm of both sides and rearranging it will give the undermentioned equation:

    -log [ H+ ] = -logKa + log ( [ A- ] / [ HA ] )

    Since, -log [ H+ ] = pH and -logKa = pKa, the equation can be written as:

    pH = pKa + log ( [ A- ] / [ HA ] )

    This equation is known as the Henderson-Hasselbalch equation and can be used in respects to pool chemical science to be able to cipher the buffering capacity, which is the measure of hydroxide ions ( OH- ) that can be absorbed by the solution before a important pH alteration occurs. Additionally, the magnitude of [ HA ] and [ A- ] find the buffering capacity of a solution. The most effectual buffer is one that has a ratio of one, as this will do no alteration in pH ( Zumdahl, 2007 ) . The undermentioned computation will demo a possible application of buffer chemical science for the pool.

    pH = pKa + log ( [ A- ] / [ HA ] )

    A concentration of 0.002M Hydrochloric acid is added to a solution with a pH of 7.6 and a Ca carbonate concentration of 2.11 ten 10-8M. Hypochlorous acid is besides present in the H2O and its pKa is 3.5 ten 10-8.

    7.6 = 3.5 ten 10-8 + log ( [ 2.11 ten 10-8 ] / [ 0.002 ] )

    = 7.46

    Therefore, the pH of the solution has decreased, which means that the solution does non hold the best buffering capacity. The best buffer solution would necessitate to hold a ratio of 1:1 which would do the pH to remain about the same.


    The chemical science involved in the direction of backyard swimming pools is an of import facet and needs to be understood in order to keep a safe swimming pool environment. A pH scope between 7.2 and 7.8 is recommended and so an apprehension of how to take down and raise the pH is indispensable. Chlorine can be used as a germicide of pool H2O ; nevertheless, it needs to be known that the reaction signifiers hydrochloric acid which lowers the pH. Thus, an add-on of a pH increaser such as Na carbonate is needed as this increases the figure of OH- molecules in the H2O, which increases the pH. Furthermore, the usage of pH indexs is necessary in commanding the pH, as they give an accurate reading of the pool ‘s pH. However, each index has a different pH scope and so it is of import to take the appropriate index for the pool. Besides, a pools buffering ability is important in maintaining the pool ‘s pH in balance. Poor buffering capacity means that an add-on of an acid or a base will do the pH to dramatically fluctuate, doing it hard to pull off a pool. Entire alkalinity is the step of a pool ‘s buffering ability and Ca carbonate is frequently used to increase the buffering ability.

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