pH, BUFFERS AND ISOTONIC SOLUTIONS Sele£ted Dellnitions •
pH of the solution is defmed as the negative logarithm of the hydrogen ion concentration.
•
ButTers are compounds or mixtures of compounds which, due to their presence in solution, resist changes in
•
ButTer Action: It is defined as the resistance to a change in pH.
•
ButTer Solution:
•
Acidic butTer solution is the solution whose pH is less than 7.
pH after addition of small amounts of acid or alkali. A solution whose pH is not much altered by the addition of small amounts of strong acid (H
+ ions) or a strong base (OH- ions). •
Alkaline butTer solution is the solution whose pH greater than 7.
•
ButTer Equation:
•
ButTer Capacity:
This equation is used to calculate the pH of a buffer solution and the change in pH with the
addition of an acid or base. It is the measure of its magnitude of its resistance to change in pH in the addition of an
acid or a base. •
Maximum butTer capacity: This occur when pH = pKa
•
Osmosis:
The movement of water molecules across a semi permeable membrane from region of low solute
concentration to high solute concentration. •
Isoosmotic solutions: The solutions having same osmotic pressure as blood.
•
Isotonic solution:
The solution having the same salt concentration and hence the same osmotic pressure as
the red blood cell. •
Hypertonic
solutions: The solution having an osmotic pressure greater than that of O.9%w/v sodium chloride.
•
Hypotonic solutions: The solutions having an osmotic pressure lower than that of O.9%w/v sodium chloride.
•
Osmolality:
•
Osmolarity:
It is the number of os moles of solute in a kilogram of solvent.
•
Tonicity-
It is the number of osmoles of solute in a litre of solution.
It is the concentration of only the solutes that cannot cross the membrane since these solutes exert
an osmotic pressure on that membrane.
157
pH, Buffers and Isotonic Solutions
8.1 INTRODUCTION The term pH was first used in 1909 by Soren Peter Lauritz Sorensen. The p in pH denotes "potenz" meaning "power" in Danish and H stands for Hydrogen. Hence H is written in capital letter. Sorenso defmed pH of the solution as the negative logarithm of the hydrogen ion concentration. (eq 8.1) The concentration of the hydrogen ion is a measure of its acidity or basicity of a aqueous solution at a specific temperature. Acidic solutions have a higher relative number of H+ ions, while alkaline or basic solutions have a higher relative number of OR ions. The pH scale Acidic
/'..
r 0 I
1 I
2
I
3 I
4 I
5 I
6
I
-V-
Alkaline
/'..
7
8
I
I
10
9
11
12
13
"'14
Neutral Figure 8.1: Sorenson's pH scale
The pH scale ranges from 0 to 14. The scale starts with a zero pH indicates that the solution is strongly acidic. At the other end of the scale, pH is 14 indicates that the solution is strongly alkaline. The central point pH in the scale is 7.0 (neutral). The region with pH values below 7 is designated as a acidic and above pH 7.0 is designated as basic (or alkaline).
8.1.1 Limitation of pH scale 1.
It does not cover very high concentration or pH value and very low pH values
2.
pH is confined only to dilute aqueous solution.
3.
pH measure only the concentration of H+ ions actually dissociated in a solution and not the total acidity or alkalinity. Due to this reason the pH value changes with temperature of liquid. As the water temperature increases the dissociation into H+ and OH- ions increases that result in decrease in pH value. pH is affected by temperature ( standard temperature is 25°C)
Solved problem Exercise S.l: The hydronium ion concentration of an acid was found to be 2.24 x 10.3 M. Calculate pH of solution. pH = -log[H+]
Solution:
= -log (2.24 x 10pH = 3- log 2.24 = 2.65 pH
Ans.
3
)
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158
8.2 MEASUREMENT
Physical Pharmaceutics-I
OF pH
The pH value is measured by following methods which are 1.
pH paper
2.
Electrometric
3.
Colorimetric
method and method
8.2.1 pH paper For routine work pH of a solution is determined with pH paper. pH papers of both broad and close ranges are available. Those are dispensed in reels. The colour of the paper changes with pH. The colours produced in contact with solutions of different pH are given on the reel for comparison. Procedure: I.
Take 10 rnl of sample in a clean and dry test tube.
2.
Take a piece of pH paper with a pair of forceps and partly deep into the sample. Compare the change of colour with the colour chart and record the range of pH
Figure 8.2: pH paper
This is the quickest method used in the field. The broad range covers 1 to 5, 4 to 6, 6 to 8, 8 to 10, 10 to 12 and 12 to 14. While The narrow range covers fractions of each pH.
8.2.2 Electrometric method Electrometric 0.001 pH
Method is the most accurate of the methods. It measure accurately to 0.1 to
Principle The basic principle of the electrometric pH measurement is to determine the activity of the hydrogen ion by potentiometric measurement using a standard hydrogen electrode and a reference electrode. Electronic pH measurement 1.
Measuring electrode
2.
Reference electrode
system consists of
rs and Isotonic Solutions
159
. easuring electrode: _. {of the glass envelope having pH sensitive glass membrane at the bottom which contains ~l;;r.:Jm{ pH buffer solution. A glass electrode is made of a thin glass membrane of special cozaposition. This electrode is dipped in the measuring solution so that potential is developed at latinum electrode which is proportional to the pH of the measuring solution. This potential is t:::e;asured against a standard calomel electrode. Shielded cable
.>
..--.
Glass envelope
Platinum wire
~-""""--.
Buffer solution
Figure 8.3: Measuring electrode
., Reference electrode The Calomel electrode can be used as a reference electrode having glass envelope containing tube having calomel (mercury and mercurous chloride paste) along with platinum wire _ merged therein. This tube is surrounded by a saturated KCl solution that diffuses slowly into process liquid through the liquid junction provided by the asbestos fiber. Because of this, the re erence electrode develops a constant potential.
e-
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Physical Pharmaceutics·1
Pt. Wire
Saturated Kcl solution
Porous fibre Figure 8.4: Calomel electrode
Procedure:
pHamplifier_---.
/
pH indicator
•
•
recorder thermocompensator
,
':/
1---- beaker
containing sample
Reference electrode Figure 8.5: Diagrammatic
representation
of system for determination
of pH
pH, Buffers and Isotonic Solutions
161
A pH meter is composed of a probe, which is formed by two electrodes. This probe passes electrical signals to a meter that displays the reading in pH units. The measuring and reference electrode together form an electrolytic cell. A voltage difference occurs due to its differences in the electron mobility produced by the two electrodes. This net voltage is recorded and calibrated as a function of the pH of the measuring liquid. A pH meter essentially measures the electrochemical potential between a known liquid inside the glass electrode (membrane) and an unknown liquid on the outside. Since the operation of the electrode depends on the electrical resistivity of the glass, the change in temperature can cause an error in the pH reading. To compensate for changes in temperature of the measuring solution, a temperature compensation resistance (thermocompensator) is included in the circuit that is immersed in the solution. Advantages of pH meters include: 1. Electrometric method is more precise than colorimetric method. 2. Readings are not fluctuated by the natural color or opacity of a solution. 3. Results can be easily read. Disadvantages include: 1. Chances of damaging the electrode. Proper care of electrode should be done. 2. Expensive equipment. 3. Some chemicals other than H+may cause spurious readings. 8.2.3 Colorimetric Method The principle behind this method lies in developing colour in the sample with an indicator dye and comparing the colour of solution of unknown concentration or pH with intensity of solution of known concentration or pH, concentration of unknown solution can be determined. Colorimetric means to measure color. In the colorimetric method, chemicals are added to the sample and those chemicals produce a color change. The color indicates the pH of the sample. The color can be measured visually or electronically. ' a. Visual Method of estimation: Different kits are available to determine pH. After adding reagent, the color of unknown solution in test tube is compared with standard to determine pH value. b. Electronic method of estimation: An electronic colorimeter is used to determine pH. Take the sample in two square tubes up to same level. Put 2 to 3 drops of indicator in one tube and put it in the right hand side compartment. Place the blank (tube without indicator) in left hand side compartment. Rotate the disc till the colour developed in the right hand side sample coincides with the disc colour. Note the corresponding pH and recorded.
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8.2.4 Universal indicators Table 8.1 Following indicators may be used for aqueous medium depending on the expected pH range: Indicator
Colour
Ph range
Bromophenol
Blue
3.0-4.6
Bromocresol green
Blue
3.8-5.4
Bromocresol purple
Purple
5.2-6.8
Bromothymol blue
Blue
5.9-7.6
Phenol red
Red
6.8-8.4
Cresol red
Red
7.2-8.8
Thymol blue
Blue
8.0-9.6
8.3 BUFFERS Buffers are an important concept in acid-base chemistry. Buffers are compounds or mixtures of compounds which, due to their presence in solution, resist changes in pH after addition of small amounts of acid or alkali. The resistance to a change in pH is known as buffer action. A solution whose pH is not much altered by the addition of small amounts of strong acid (H + ions) or a strong base (OH- ions) is called buffer solution. Buffer solutions are very important in biology and medicine because most biological reactions and enzymes need very specific pH ranges to function properly. For example, the pH of blood lies at about 7.35. If this value falls below 7.0 (acidosis) the results are fatal. Also if it rises above 7.7 (alkalosis) the results are also fatal. So Blood contains a buffer system that maintains the acidity at the right level. Pharmaceutical formulations are often buffered to control pH and thus help to minimize drug degradation, improve patient comfort and compliance, or allow delivery of a sufficient drug dose. A combination of a weak acid and its conjugate base (i.e, its salt), or a weak base and its conjugated acid act as buffers. An acidic buffer solution is simply one which has a pH less than 7. For example Acetic acid (CH3COOH) + Sodium acetate (CH3COONa). An alkaline buffer solution has a pH greater than 7. An example of alkaline buffer solution is a mixture of ammonia solution and ammonium chloride solution.
pH, Buffers and Isotonic Solutions
163
8.4 BUFFER EQUATION The pH of a buffer solution and the change in pH with the addition of an acid or base is calculated by the use of Buffer Equation.
8.4.1 Buffer equation for weak acid and its salt: An acid is known as proton donor. The pH of the acid buffer can be calculated from the dissociation constant Ka of the weak acid and the concentrations of the acid and salt used. The dissociation expression of weak acid can be represented as HA~H++A
(eq8.2)
By applying law of mass action Ka=
[Hi [A1 [HA] (eq 8.3)
or [Hi = Ka [HA]
,
[A-] (eq 8.4) Where Ka is the dissociation
constant of the acid HA.
On addition of salt to acid, the dissociation constant get disturbed by the addition of the salt which provides A ion. The degree of dissociation of an weak electrolyte is suppressed by the addition of another strong electrolyte containing a common ion, this is termed as common ion effect. If acid is weak and slightly ionized, then [HA] is represented as total concentration of acid and common ion concentration [A] is considered as a result from salt. The equation is represented as [Hi = Ka. [Acid] [Salt] (eq 8.5) Taking log on both sides, we get: Log [Hi = log Ka + log [Acid] . [Salt] (eq 8.6)
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164
Multiplying
Physical Pharmaceutics-I
both sides by negative (-ve) sign, -log[Hl
=
-log Ka -log [Acid] :
(eq 8.7)
[Salt]
As -log[H+] = pH and -log Ka = pka (dissociation pH
= pka
exponent)
- log[Acid] [Salt] (eq 8.8)
pH
= pka
+ log [Salt] [Acid]
or
(eq 8.9)
This is called as Henderson - Hasselbalch equation. It helps in calculating the pH value of buffer solution, if the concentrations of acid as well as that of the salt are known.
i
8.4.2 Buffer equation for weak base and its salt: The base is considered as proton acceptor. It consists of a weak base and its salt with strong acid. Ionization of a weak base, BOH, can be represented by the equation. +
BOH~B
+OH
(eq 8.10)
(eq 8.11) OR [OH-]
Kb[BOH] .
= :.!.....'--
, [Bl (eq8.12) If base is weak and slightly ionized, then [BOH] may be considered as total concentration of base and represented as [base] and [B+] may be considered from salt. Therefore, above equation can be represented as [OK]
=
Kb [Base] [Salt]
Taking log on both sides, we get:
(eq 8.13)
pH, Buffers and Isotonic Solutions
165
log [OH1 = log Kj, + log [Base] [Salt]
(eq 8.14)
By Multiply both sides by -ve sign, -log [Oft]
=
-log Ks -log [Base] [Salt]
As -log[Oft]
= pOH & -log K, = pk, pOH
= pkj,
[Base] -log __ [Salt] "
(eq 8.15)
l
pOH = pkj, + log [Salt] , [Base]
Or
(eq 8.16)
8.4.3 Significance of Henderson - Hasselbalch "uation Henderson equation for a basic buffer will give pOH, and so pH can be calculated as; pkw=pH+pOH Or
(eq 8.17)
pH=pkw-pOH
(eq8.18)
pH=14-pOH
(eq 8.19)
Also, the dissociation constant of a weak acid (pkj) or a weak base (pkg) can be calculated by measuring the pH of a buffer solution containing equimolar concentrations of the acid (or base) and the salt.
Solved problem Exercise 8.2: Calculate the pH of a buffer solution containing 0.05 moles/litre of acetic acid and 0.3 molesllitre of sodium acetate. pka for CH3COOH is 4.57
Solution: Cone. Of acid = 0.05 M Cone. Of salt = 0.3 M So, pH = pka + log [salt] / [acid] pH = 4.57 + log 0.3 /0.05 Ans, pH = 4.57 + 0.78 = 5.35
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8.5 BUFFER CAPACITY The buffer capacity of a solution is "a measure of its magnitude of its resistance to change in pH in the addition of an acid or a base" .Buffer capacity is also referred as buffer index, buffer value, buffer efficiency or buffer coefficient. The buffer capacity represented by 'W may also be defined as: "The ratio of the amount added of strong acid or base to the small variation in pH (LlpH) caused by this addition" I1A
or I1B
~=
(eq 8.20) llpH
Where Ll,has its usual meaning, a finite change, LlAor llB represents the small increment (in gram equivalents / litre of strong acid or base added) to the buffer to bring about a pH change of LlpH. According to the above equation, a solution has a buffer capacity of 1 when one liter of it requires one gram equivalent of a strong acid or base to change the pH by one unit. Therefore, the lower the pH change in a solution when an acid or a base is added, the greater the buffer capacity and vice versa. The buffer capacity depends essentially on two factors. One is Ratio of the salt to the acid or base. Another is Total buffer concentration. The relationship between buffer capacity and buffer concentrations is given by the Koppel, Spiro and Van Slyke equation:
P =2.3C
Ka[Hp+] (Ka+[Hp+ ])
2
(eq 8.21)
Where C = the total buffer concentration (i.e. the sum of the molar concentrations of acid and salt). Buffer capacity is not a fixed value. It depend on the amount of base added. For most of the pharmaceutical solutions, Buffer capacities ranging from 0.01 - 0.1 are usually adequate. If buffer capacity must be large enough to maintain the pH of the product for a reasonably long storage time. Changes in the pH of the product may result from the interaction of the solution components with each other or with the product container (glass, plastic, rubber). On the other hand, the buffer capacity of ophthalmic and parenteral products should be low enough to allow rapid product readjustment at physiological pH upon administration. Maximum buffer capacity: Maximum buffer capacity occur when pH = pKa or [H30+] = Ka ~max = 2.303 C [H30+]2 / (2 [H30+f) Bmax = 2.303 C /4 or
Bmax
= 0.576C
(eq 8.22) (eq 8.23) (eq 8.24)
pH, Buffers and Isotonic Solutions
167
where C is total buffer concentration
O~--~r----.----r----.----~---r-123 456 7 PH
Figure 8.6: plot showing maximum buffer capacity
Solved problem Exercise 8.3: What will be the maximum buffer capacity of a buffer having total concentration is 0.20 mollL?
P max = 0.576C
Solution: Ans.
P max = 0.576 * 0.20 = 0.1
8.6 BUFFERS IN PHARMACEUTICAL
AND BIOLOGICAL
SYSTEMS 8.6.1 Biologic Buffer Systems a.
Blood: The blood is maintained at a pH of about 7.4. The plasma contains carbonic acid / bicarbonate and acidic / alkaline sodium salts of phosphoric acid as buffers. Plasma proteins, which behave as acids in blood, can be combined with bases and thus act as buffers. In erythrocytes, the two buffer systems consist of hemoglobin / oxyhemoglobin and acid / alkaline potassium salts of phosphoric acid. b. Lacrimal fluid: The pH of tears is about 7.4, with a range of 7 to 8. The lacrimal fluid or tears have good buffer capacity. Lacrimal fluid is diluted in ration of 1:15 with neutral distilled water because its pH range is slightly hidugher. c. Urine: The urine of a normal adult has a pH of about 6.0 with the range of 4.5 to 7.8 When the pH of the urine is below normal values, hydrogen ions are excreted by the kidneys.
f4J Physical Pharmaceutics-I
168
8.6.2 Pharmaceutical
Buffer
Buffer solutions are used particularly are available today. 1. 2. 3. 4.
in the formulation
of ophthalmic
solutions. Many buffers
As per Gifford when we mix various proportions of boric acid and monohydrated sodium carbonate they yield buffer solutions with pH range 5 to 9. Sorenson proposed mixture of salt of sodium phosphate for buffer of pH 6 to 8. A buffer system suggested by Palitzsch consist of boric acid, sodium borate and NaCl and used for ophthalmic solution with pH range 7 to 9. The buffers of Clark and Lubs were determined at 20 0 C and re-determined at 25 0 C.
8.6.3 Standard Buffer Solutions for various ranges between pH 1.2 and 10.0 may be prepared by appropriate combinations of the solutions described herein, used in the proportions as described in table. The volumes shown below are for 200 mL of buffer solution, except that the volumes shown for Acetate Buffer are used to prepare 1000 mL of buffer solution 1.
Hydrochloric
2.
Potassium Biphthalate,
Acid, 0.2 M, and Sodium Hydroxide, 0.2 M
[KHC6~(COOh] 3.
40.85 g of potassium biphthalate
in water, and dilute with water to 1000 mL.
Potassium Phosphate, Monobasic 0.2 M-Dissolve phosphate (KH2P04)
4.
0.2 M-Dissolve
27.22 g of monobasic potassium
in water, and dilute with water to 1000 mL.
Boric Acid and Potassium Chloride, 0.2 M-Dissolve
12.37 g of boric acid (H3B03)
and
14.91 g of potassium chloride (KCl) in water, and dilute with water to 1000 mL. 5.
Potassium Chloride, 0.2 M-Dissolve
14.91 g of potassium chloride (KCl) in water, and
dilute with water to 1000 mL. 6.
Acetic Acid, 2 N
1. Hydrochloric Acid Buffer Put 50 mL of the potassium chloride solution in a 200-mL volumetric flask. Add the specified volume of the hydrochloric acid solution, then add water to adjust volume. pH
1.2
1.3
lA
1.5
1.6
1.7
1.8
1.9
2.0
2.1
2.2
0.2 M HCI, mL
85.0
67.2
53.2
41.4
3204
26.0
20.4
16.2
13.0
10.2
7.8
pH, Buffers and Isotonic Solutions
169
2. Acid Phthalate Buffer Put 50 mL of the potassium biphthalate solution in a 200-mL volumetric flask. Add the specified volume of the hydrochloric acid solution, then add water to adjust volume. pH
2.2
2.4
2.6
2.8
3.0
3.2
3.2
3.6
3.8
4.0
0.2
49.5
42.2
35.4
28.9
22.3
15.7
10.4
6.3
2.9
0.1
M HCI, mL
3. Neutralized Phthalate ButTer Pour 50 mL of the potassium biphthalate solution in a 200-mL volumetric flask. Add the specified volume of the sodium hydroxide solution, then add water to adjust volume. pH
4.2
4.4
4.6
4.8
5.0
5.2
5.4
5.6
5.8
0.2M
3.0
6.6
11.1
16.5
22.6
28.8
34.1
38.8
42.3
NaOH mL
4. Phosphate Buffer Put 50 mL of the monobasic potassium phosphate solution in a 200-mL volumetric flask. Add the specified volume of the sodium hydroxide solution, then add water to volume. pH
5.8
6.0
6.2
6.4
6.6
6.8
7.0
7.2
7.4
7.6
7.8
8.0
0.2M
3.6
5.6
8.1
11.6
16.4
22.4
29.1
34.7
39.1
42.4
44.5
46.1
NaOH, mL
s.
Alkaline Borate Buffer
Place 50 mL of the boric acid and potassium chloride solution in a 200-mL volumetric flask. Add the specified volume of the sodium hydroxide solution, then add water to volume. pH
8.0
8.2
8.4
8.6
8.8
9.0
9.2
9.4
9.6
9.8
10.0
0.2M
3.9
6.0
8.6
11.8
15.8
20.8
26.4
32.1
36.9
40.6
43.7
NaOH, mL
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6. Acetate ButTer Put the specified amount of sodium acetate NaC2H302 . 3H20 in a lOOO-mLvolumetric flask. Add the specified volume of the acetic acid solution, then add water to volume, and mix. pH NaC2H302 3H20, ~
•
2N CH3COOH,
4.1
4.3
4.5
4.7
4.9
5.1
5.2
5.3
5.4
5.5
1.5
1.99
2.99
3.59
4.34
5.08
5.23
5.61
5.76
5.98
19.5
17.7
14.0
11.8
9.1
6.3
5.8
4.4
3.8
3.0
mL
8.6.4 Preparation of Pharmaceutical Buffer Solutions The following steps are helpful in the development of a new buffer. a.
Select a weak acid having a pKa approximately equal to the pH at which the buffer is to be used.
b. From the buffer equation, calculate the ratio of salt and weak acid required to get the desired pH. The buffer equation is satisfactory for approximate calculations within the pH range of 4 to 10. c.
Consider the individual concentrations of the buffer salt and the acid needed to obtain adequate buffer capacity.
d. A concentration of 0.05 to 0.5 M is usually sufficient, and a buffer capacity of 0.01 to 0.1 is generally adequate. e.
Other major factors important in the selection of a pharmaceutical buffer include availability of chemicals, sterility of the final solution, stability of the drug and buffer on aging, cost of materials, and freedom from toxicity. For example, a borate buffer, because of its toxic effects, certainly cannot be used to stabilize a solution to be administered orally or parenterally.
f.
Finally, determine the pH and buffer capacity of the completed buffered solution using a reliable pH meter. In some cases, sufficient accuracy is obtained by the use of pH papers.
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171
pH, Buffers and Isotonic Solutions
Solved problem Exercise 8.4 Prepare a buffer solution of pH 5 Solution Select a weak acid whose pKa is close to reqd. pH.. So here acetic acid used has pK a = 4.75
= IM, of salt required = x
Molar concentration of a~d required Molar concentration By using equation pH
= pka + log [Salt] / [Acid]
5 = 4.75 + log [x] / [1] 5 - 4.75 0.25
= log x -log
= log x -
1
0 (because log 1 = 0)
Or x = Antilog 0.25 = 1.78 Therefore the ration of salt and acid required to prepare buffer solution of pH 5, was found to be Ans. 1.78/1.
8.7 pH AND SOLUBILITY At a low pH, a base is predominantly in the ionic form, which is usually very soluble in aqueous media. When the pH is raised, a more undissociated base is formed. When the amount of base exceeds the limited solubility in water, in this case, the free base precipitates from the solution. Therefore, the solution must be buffered to a low pH so that the concentration of alkaloid base in equilibrium with its salt is calculated to be less than the solubility of the free base at the storage temperature.
8.8 APPLICATIONS
OF BUFFERS
Various application of buffers are described below
8.8.1 Buffers in pharmaceutical formulations Buffers are used in pharmaceutical solutions to control the pH of the formulated product.To find stability problems, any liquid formulation should be formulated at optimum pH. Rheology,
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viscosity and other properties also depend on the pH of the system. Buffering agent are added to ensure that drug will be stable throughout the shelf life of the product and also during their administration or usage to patient. Buffers are used to stabilize various pharmaceutical formulations. 1. Parenteral preparations:
Large deviations of pH in case of parenteral preparations
cause serious consequences. The most commonly used buffers in parenteral products (injections) are acetate, phosphate, citrate and glutamate. 2. Ophthalmic preparations: Opthalmic preparations include eye drops, eye solution, eye lotions etc. Buffers are generally used in ophthalmic preparations to maintain the pH of lacrimal fluid. Optbalmic preparations are buffered for (a) greater comfort to the eye, (b) to stabilize the formulation, (c) to enhance the drug's bioavailability and (d) to maximize preservative efficacy. The buffering agents most commonly used in ophthalmic preparations include borate, carbonate and phosphates. 3. Ointment and cream: Topical products such as ointments and creams are also buffered to ensure stability of the formulation. The most commonly used buffers in ointments and creams are citrate & phosphate.
8.8.2 ButTersto study pH stability profIle of drug at development stage pH stability studies at the developmental stage are done for selection of adequate excipients and container closure systems, to develop new method of synthesis and also to determine shelf life and storage conditions for development of a stable product. During preformulation study, drug solutions in different buffer system are prepared. This is done to ensure that the drug will remain stable upto a predefined storage period at definite pH. Many different therapeutic drugs are synthesized under strict pH conditions to ensure the stability and clinical effectiveness. This is usually achieved with buffer solutions. Buffers are used to increase drug shelf life.
8.8.3 Buffers in Fermentation: Fermentation reactions (as in alcoholic beverages or yogurt) are greatly affected by the variable pH. This means that it is essential to use buffer solutions to avoid harsh changes and to allow the fermentation to obtain maximum yield.
pH, Buffers and Isotonic Solutions
173
8.9 BUFFERED ISOTONIC SOLUTIONS Pharmaceutical solutions used for the application in the delicate membranes of the body (specially for ophthalmic and parenteral administration) should have same osmotic pressure as that of the body. The isotonic solutions do not cause swelling or contraction of the tissues and do not produce discomfort when injected into the eye, nasal tract, blood or other body tissues. The movement of water molecules across a semi permeable membrane from region of low solute concentration to high solute concentration is known as osmosis. The pressure applied to prevent this movement is called osmotic pressure. Isoosmotic solutions are the solutions having same osmotic pressure as blood. In medicine, the term isotonic is used interchangeably with iso -osmotic. If Red blood cell are mixed with a solution containing 0.9 g of NaCl per 100 mL Or0.9%w/v, the cells retain their normal size. The solution having the same salt concentration and hence the same osmotic pressure as the red blood cell is known as Isotonic solution. If the solutions have an osmotic pressure greater than that of 0.9%w/v sodium chloride, are known as Hypertonic solutions. This cause cell to shrink and become wrinkled or crenated. If the solutions
have an osmotic pressure lower than that of
0.9%w/v sodium chloride, are known as Hypotonic solutions. This cause cell to swell and finally burst, with liberation of haemoglobin. This phenomenon is called hemolysis .
• -- - ------
~-------------- -
Hypertonic
Isotonic
-
----- -
-----
~----
-
----- -
Hypotonic
Figure 8.7: Diagram representing difl'erent tonicity conditions
Osmolality is an estimation of the osmolar concentration of plasma. Osmolality is the measure of solute concentration per unit mass of solvent. Osmolality is the number of osmoles of solute in a kilogram of solvent. Osmolarity is the concentration of an osmotic solution. Osmolarity is the measure of solute concentration per unit Volume of solvent. Osmolarity is the number of osmoles of solute in a litre of solution.
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Solved problem Exercise 8.5 : Determine the amount of NaCI to make 50 ml of an isotonic solution Solution: 0.9 g /lOO ml = x/50 ml Ans. X
=
0.45 g
8.10 TONICITY It is the concentration of only the solutes that cannot cross the membrane since these solutes exert an osmotic pressure on that membrane 8.10.1 Measurement
of Tonicity
The tonicity of solutions may be determined by one of the following two methods: a.
Haemolytic Method.
b.
Colligative Method
a. Haemolytic method: In this method, different drug solutions are prepared and their effects are observed on the appearance of red blood cells when they are suspended in the solutions. Hematocrit is used for this purpose. One capillary tube (tube A) is filled with blood diluted with 5 ml of 0.9% w/v NaCI (isotonic solution). The other capillary tube (tube B) is filled with blood diluted with 5ml of test solution. Both tubes are centrifuged at high speed. The PCV (Packed Cell Volume) of test solution tube (tube B) is compared with PCV of isotonic solution tube (tube A).
If PCV of test solution (tube B) is same as that of tube A, then test solution is regarded as isotonic. If RBCs volume (i.e. PCV) of tube B is more than that of tube A, then test solution is regarded as hypotonic solution. If RBCs volume (i.e. PCV) of tube B is less than that of tube A, then test solution is regarded as hypertonic solution. Hypotonic solution releases oxyhemoglobin in direct proportion to the number of cells hemolyzed. By such mean the van't Hoff factor i can be measured and the value compared to that computed from cryoscopic data, osmotic coefficient and activity coefficient. b. Colligative method: This method is based on the measurement of the slight differences in temperature from the differences in the vapor pressure of the thermally insulated samples contained in constant humidity chamber. The freezing point of both human blood and lacrimal
rs and Isotonic Solutions
-0.52
e;
0
C. This temperature
175
corresponds
to the freezing point of a 0.90% NaCl solution,
. therefore considered isotonic with both blood as with the tear fluid.
8.11 METHODS OF ADJUSTING TONICITY AND PH are everal methods and one of them can be used to calculate the quantity of sodium .de added to solutions of drugs to render them isotonic. methods are divided into two classes.
Methods of adjusting
Tonicity
Class I method .....-
Class II methods
1
-.
.--
Sodium chloride equivalent method
ryo copic method
__
White-Vincent
1
method
-.
Sprowls method
Figure 8.8: Methods or adjusting tonicity
ethod:
In Class I methods,
zing point of the solution to -0.52
sodium chloride is added to drug solution to reduce the 0
C and also to adjust tonicity with body fluid.
in lude cryoscopic
method
odium chloride
and
equivalent
method.
methods method, water is added to the drug in a sufficient amount to form an isotonic solution. And final volume is adjusted with an isotonic or a buffered isotonic dilution solution. This method e
'hite-Vincent method and prowls method
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176
Physical Pharmaceutics-I
8.11.1 Cryoscopic method or freezinl! point depression method: For pharmaceutical solutions, osmotic pressure caused by dissolved drugs and chemicals is difficult to measure directly whereas changes in freezing point caused by these substances are determined rather easily. The fundamental expression relating freezing point depression and concentration of solute in solution is given by the equation (eq 8.2S) where L\Tfis the freezing point depression, K, is the freezing point depression constant, and c is the concentration of the solute in solution. This equation is true only for non electrolytes in dilute solution. The van't Hoff factors is taken into account with the equation (eq 8.26) where i, the van't Hoff factor, is the ratio of the colligative effect produced by a given concentration of electrolyte divided by the effect observed for the same concentration of nonelectrolyte. This expression further modified for the dilute aqueous solutions and written as (eq 8.27) where c is the molar concentration of solute in aqueous solution and Liso is equal to iKf The freezing point of human blood and lacrimal fluid is -0.S2°C. Since 0.9 % w/v Sodium chloride solution also correspond to this temperature and considered to be isotonic to blood and lacrimal fluid. In this method, amount of tonicity adjuster (sodium chloride) are added to drug solution depress the solution freezing point below 0.52°C
Adjustment to the tonicity of solutions is simplified if the freezing point of I% solution of unadjusted substance and freezing point of 1% solution of adjusting substance are known. The freezing points are usually expressed in terms of 1% solution. The quantity of the adjusting substance needed for making the solution isotonic with blood may be calculated from the general formula 01
0.S2-a
W-IO=---
b
(eq 8.28)
pH, Buffers and Isotonic Solutions
177
Where
= Amount of adjusting substance required a = freezing point of 1% solution of un-adjusted
w
solution
b = freezing point of 1% solution of adjusting substance If sodium chloride is used as adjusting substance whose LlTrof the solution is 0.58°C, then above equation will be 0.52-a w=--0.58
(eq 8.29)
Solved problem Exercise 8.6: Find proportion of sodium chloride required to make 1% solution of cocaine hydrochloride
isotonic with plasma.
Solution: Freezing point of 1% w/v solution of cocaine hydrochloride
is 0.09°C
Freezing point of 1% w/v solution of sodium chloride is 0.58°C So
A os. 0.52 - 0.09 0.58
= 0 .74m
70 0
f Sodirum c hlorid on e requir. ed
8.11.2 Sodium Chloride Equivalent Method The sodium chloride equivalent method is the most widely used method in calculating the amount of sodium chloride required to prepare isotonic drug solutions. Sodium chloride equivalent (E) of a drug is the amount of sodium chloride (in grams or grains) that is equivalent to (or have same osmotic effect) as 1 gm of the drug. The E value can be calculated from the Liso value or freezing point depression.
So
19
c=-
MW By substituting value of c in eq 8.27, we get
(eq 8.30)
W
178
= L.
I1T
f
Ig MW
X--
ISO
f
1-;.so
(eq8.31)
E
I1T =3.4x--
where 3.4 is the
Physical Pharmaceutics-I
(eq 8.32)
58.45
value for sodium chloride and 58.45 is its molecular weight.
By equating two values of f1Tf from eq 8.31 and eq 8.32, we get
(eq 8.33)
Or
E= 17 L;so MW
(eq 8.34)
Solved problem Exercise 8.7: Calculate sodium chloride equivalent of pilocarpine nitrate. (given molecular weight of pilocarpine
nitrate is 101 g and Liso of pilocarpine
nitrate is 0.23)
Solution:
= 17 L
MW
= 17 * 0.23
/101
E
Ans. E
iso /
= 0.04
pH, Buffers and Isotonic Solutions
179
Class IT Methods
8.11. 3 White Vincent Method Class II methods for calculating
tonicity involve adding water to the drugs to make an isotonic
solution. This is followed by the addition of an isotonic or isotonic buffered diluting vehicle to adjust the solution to the final volume.
White -Vincent
developed
a simplified
equation
for
calculating the volume V (mls) which can be written as (eq 8.35)
V=w*E*111.1 Where V is volume of isotonic solution (ml) prepared by mixing drug with water w is weight of the drug (gm) E is sodium chloride equivalent of drug
111.1 is a constant denotes volume of isotonic solution in millilitres obtained when 1 g sodium chloride is dissolved in water. If more than one ingredient is used in isotonic preparation,
the volume of isotonic solution V can
be calculated as V = [ (WI x El)
+ (W2 x Ez) + ---------+ (w, x EJ ] x 111.1
(eq 8.36)
olved problem ercise 8.8: Make the following prescription
isotonic with tears
Ephedrine Sulphate - 0.06 g Boric acid- 0.30 g Sterilized distilled water q.s. 100 ml for ephedrine sulphate-0.23, Solunon
E for boric acid-0.52)
V = [(0.06 x 0.23) Ans. V
+ 0.3 x 0.52)] x 111.1
= 19 ml
g are mixed with water to make 19 ml of isotonic solution and preparation issolution. o olume of 100 ml by adding isotonic dilute solution
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180
Physical Pharmaceutics-I
8.11.4 Sprowls method This method is simplification of White and Vincent method. Here weight of drug (w) is set to constant value of 0.3. So equation will become
v = 0.3 * E * 111.1 V = 33.33E
Or
(eq 8.37) (eq 8.38)
REVIEW QUESTIONS SUBJECTIVE PART VERY SHORT ANSWER QUESTIONS I.
Define pH.
2.
Answer- It is defined as the negative logarithm of the hydrogen ion concentration. Define isotonic solution. Answer- The solution having the same salt concentration and hence the same osmotic pressure as the red blood cell.
3.
Define buffer Answer- Buffers are compounds or mixtures of compounds which, due to their presence in solution, resist changes in pH after addition of small amounts of acid or alkali.
4.
Deftne buffer capacity
5.
Answer- It is the measure of its magnitude of its resistance to change in pH in the addition of an acid or a base. Define Acidic buffer solution. Answer- Acidic buffer solution are the solution whose pH is less than 7.
SHORT ANSWER QUESTIONS Q1. What are the difference between Osmolality and Osmolarity? Answer- Osmolality is the number of osmoles of solute in a kilogram of solvent. While Osmolarity is the number of osmoles of solute in a litre of solution. Q2. Why Buffer solutions are very Important In biology and medicine? Answer- Because most biological reactions and enzymes need very specific pH ranges to function properly. For example, the pH of blood lies at about 7.35. If this value falls below 7.0 (acidosis) the results are fatal. Also if it rises above 7.7 (alkalosis) the results are also fatal. So Blood contains a buffer system that maintains the acidity at the right level. Q3. Why lacrimal fluid Is diluted with distilled water? Answer- The pH of tears is about 7.4, with a range of 7 to 8. The lacrimal fluid or tears have good buffer capacity. Lacrimal fluid is diluted in ration of 1: 15 with neutral distilled water because its pH range is slightly higher.
pH, Buffers and Isotonic Solutions
181
Q4. How pH Is affected by temperature? Answer- pH value changes with temperature of liquid. As the water temperature increases the dissociation into H+ and OH- ions increases that result in decrease in pH value. QS. What is hemolysis? Answer- If the solutions have an osmotic pressure lower.than that of 0.9%w/v sodium chloride, are known as Hypotonic solutions. This cause cell to swell and finally burst, with liberation of haemoglobin. This phenomenon is called hemolysls. Q6. Determine the amount of NaCI to make 20 ml of an isotonic solution Answer- 0.18 Q7. Calculate sodium chloride equivalent of drug. (given molecular weight Is 340 g and Loo value is 0.25) Answer- E = 0.012
LONG ANSWER QUESTIONS I. 2.
What are the different methods used for measurement of pH? Refer article 8.2 Explain Henderson - Hasselbalch equation for buffers containing weak acids with its salts and weak base and its salts.
3.
Refer article 8.4.1, 8.4.2 Explain various applications of buffer in Pharmacy.
4.
Refer article 8.8 Describe in detail methods used for measurement of tonicity.
5.
Refer article 8.10.1 Explain in detail methods of adjustment of tonicity. Refer article 8.11
6.
Write note on a.
7.
Buffered Isotonic Solutions Refer article 8.9 b. Significance of Henderson - Hasselbalch equation Refer article 8.4.3 Explain in detail, the compositions of standard buffer solution used in Pharmacy. Refer article 8.6.3
OBJECTIVE PART MULTIPLE CHOICE QUESTIONS 1.
2.
In term pH, H Indicates a.
Hydrogen
b. Helium
c.
Haemoglobin
d. Half
Maximum buffer capadty (IJ max) equals to a.
0.576C
b. 2.303 C
c.
0.2303 C
d.57.6C
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182
3.
4.
S.
6.
In which method, tonicity is calculated by adding water to the drugs to make an isotonic solution. a. Sodium chloride equivalent method
b. Cryoscopic method
c. White Vincent Method
d. Potentiometric method
The tonicity of solutions can be determined by a. Colorimetric method
b. Haemolytic Method
c. Colligative Method
d. Both b and c
Cryoscopic method for adjusting tonicity and pH comes under a. Class I Method
b. Class II Method
c. Class ill Method
d. Class IV Method
The solution having an osmotic pressure greater than that ofO.9%w/v sodium chloride is called a. Hypertonic solutions c Isoosmotic solution
7.
b. Hypotonic solution d. Isotonic solution
The value 14 on pH scale indicates a. Strongly alkaline c. Neutral
8.
b. Strongly acidic d. None of the above
Which of the following methods are used to measure a. pH paper c. Colorimetric method
9.
Physical Pharmaceutics-I
pH value?
b. Electrometric method d. All of the above
The number of osmoles of solute in a litre of solution is called a. Osmolarity c. Buffer capacity
b. Osmolality d. Molarity
10. The term pH was first used by b. Louis Pasteur d. Alfard Columb
a. Soren Peter Lauritz Sorensen. c. James Kelvin 11. Maximum buffer capacity occur when a. pH =pKa
b.pH>pKa d. All of the above
c. pH<pKa
ANSWERS l.a
2.a
3.c
4.d
5.a
6.a
7.a
S.d
9.a
lO.a
Il.a