**Study the page 29 equivalents**

Methods to adjust osmotic pressure:*Freezing point depression or Cryoscopic Method -->This is based on solutions that are isoosmotic with the body tissues or cells have a freezing point of -0.52 degrees C. Goal is to adjust a solution to this value.-This is a colligative property -Osmotic pressure is a major concern in Neonates, is still a concern in adults but not as high.*Sodium Chloride Equivalent (E) method:-->Ability of a substance to lower the freezing point relative to sodium chloride. Goal is to prepare a formulation that contains the equivalent of a .9% NaCl solution. Just to solidify this, what this means is that if you were to make 100ml of an iso-osmotic solution, you would need 0.9 grams of NaCl.

Lidocaine HCl E=0.22 this means that 1 gram of lidocaine has the same colligative effects as 0.22g sodium chloride, osmotically equivalent.

ExampleEphedrine Sulfate 0.8Gm E=.20 (this means that 1 gm epedrine=0.2g NaClWater qs 100mlMake this isotonic.

What this is saying that 1g EphSulfate acts like 0.2g of NaCl in solution, therefore to find what 0.8g of EphSulfate acts like, all you have to do is solve for X. Then to make an isotonic solution, subtract X from 0.9, and this will tell you how much NaCl you must add in order to create a 100ml isotonic solution with 0.8g EphSulfate.

Suppose you wanted to use dextrose to make the solution iso-osmotic? the E value of dextrose is 0.16.*I gm of dextrose has the same colligative effects as 0.16g NaCl.

What this is saying is that you must figure out how much dextrose is needed to give the same effects as 0.74g NaCl.

*Therapeutic use of osmotic pressure differences:-Osmotic diuretics -Treat edema-Laxatives (produce a shift of fluids into the intestine)-Drug Delivery system

*Osmol-- Weight in grams of soltue that is osmotically equivalent to a gram-molecular weight of Ideal nonionizable substance. ---------------->

*Osmolality: Number of osmols per kg solvent

*Osmolarity: Number of osmols per liter of solution.

(USP sets standards for drug quality--One thing that is very unique about USP is that it is a private organization)

*However the actual USP value for this solution is 252 mOsm, not 278 because it includes a molecule of water for every molecule of dextrose. So the real value for the MW should be 198.

-If the 1000 is not in the equation then the answer would be in osmols and not milliosmols.

*Ideal osmolar concentration-0.9% NaCl Inj.-Calculated value is 308 mOsm/L-Measured value is 291.4 mOsm/L-In real life this does not totally disassociate

-The higher the concentration of ions, the more it will deviate from the ideal value.

Acid/Base and Buffers

A buffer will act to resist large changes in pH when acids or bases are added. A buffer must have TWO components must have a weak acid and itʼs conjugate base (or visa versa).

*Always use this form of henderson hasselbach: pH=pka + log (Base/Acid)-->This part is Molar concentration of each

Henderson-Hasselbach can be used to:-Estimate the pH of a buffer solution-Estimate ratio of buffer components at a given pH-Estimate pH change in system with acid or base addition.

Example: Would the actual pH values of a 0.1M solution of buffer components be the same as a 0.01M solution of the components?--Henderson-Hasselbach tells us they should be the same the actual measured concentration would be different. This is due to it not being an ideal solution. There will be more interactions in the solution that has higher concentrations, and therefore it will be further away from pH 5 and the actual measured pH will be higher than 5.--pH=-log[H3O+] therefor free hydronium ions will be higher in the solution that has the higher concentration.

*In this example you are adding HCl acid to a solution with these different buffer concentrations.*Concentration will have a big impact on how well the buffer resists changes in pH...this is a fairly common sense conclusion.

*As displayed by the example, when the ratio of Base/Acid is nearest to being equal, i.e. 1 to 1 the buffer is most effective.*Higher the concentration, the better that it resists change in pH, but the closer the ratio is to 1:1 the better the buffer.

Buffer Capacity: is a measure of a bufferʼs ability to resist changes in pH. Magnitude of resistance of a buffer to changes in pH.

Further you get away from 0, the worse the buffer is, 0 indicates a 1:1 buffer ratio.

Van Slyke equation.C=Moles/liter

EX: What is the capacity of a buffer containing 0.1 moles of each acetic acid and sodium scetate per liter?pKa for this question is 4.76 **pKa= -logKa** p mean -log

C= 0.1M acetic acid + 0.1 = 0.2Ka= 10^(-4.76)= 1.74x10^-5pH=pKa[H3O+]= (10^-4.76)=(1.74x10^-5)

*Buffer capacity has NO UNITSthey usually range b/t 0.01 and-0.25.Related to the concentration of the buffer components. Higher concentration of buffer components is going to better. Beta increases as we approach a 1:1 ratio.**Higher the concentration, the higher the beta value.**

Where you have these two Betas:0.1 and 0.01-The 0.1 is a 10X better buffer, meaning it will neutralize to times more acid than the beta that is 0.01.

Preparing a buffer system:1. Select a weak acid with a pKa as close to the pH of the buffer as possible. --WITHIN PLUS or MINUS ONE!--If an acid has multiple pKa values then it means that it has multiple H+ʼs to donate.--Three commonly used weak acids are Acetic acid. citric acid, and phosphoric acid.2. Calculate ratio of base to acid components--Use H-H equation.3. Use the van slyke equation to calculate buffer concentrations--Useful range within 0.01--0.254. Calculate molar amounts of the base and acid.5. Know toxicity of a buffer, cost, whether or not the components negatively interact. --NEVER USE BORIC ACID.

This is how you would calculate the ratio of conjugate base to acid if you knew the pH and the pKa values.

so 99% is the base.

*ANYTIME THE DIFFERENCE B/T THE PH AND PKA IS +2, THE %IONIZED IS 99%. IF BY THE SAME LOGIC THE DIFFERENCE IS -2 THEN THE % IONIZED IS 1%.

*pKa only tells you how easily the substance accepts or donates a proton, but doesnʼt tell you if it is an acid or a base. --Only way to know is by looking at the functional groups.--We associate high pKa values with weak acids.--As pKa increases Ka decreases **LOOK AT COMMON ACID AND BASE FUNCTIONAL GROUPS BEFORE THE TEST.**

*the concentration of drug in solution at any point is the same once you are beyond the diffusion layer.*We will use the dissolution rate calculations to predict what will happen with a drug.*Decreasing particle side will increase dissolution rate, but it will not effect the solubility. You can only effect solubility by changing temperature and pressure.*Reducing the particle size will increase the surface area.*Dissolution rate and solubility are not the same things

*This is a weak acid.*Remember Higher the pKa, the weaker the acid or base.*You will be able to dissolve more of the ionized form in water than you can the unionized form, ion-dipole interactions.*pH of a solution and the pKa of a compound are the only 2 things that determine which form will be most prevalent in solution.*If we exceed the solubility in either one of these forms, we will have a precipitate. *The unionized form in this example will be the one that will precipitate out. *If we saw precipitate, which way do we need to shift the pH to get rid of the precipitate?

If you make the solution more acidic by adding in H+ then you will shift the solution to the left. Which is not what we want. You need to make the solution more basic which will make the unionized form to shift to the ionized form and dissolve.

*These equations allow us to estimate the pH where a saturated solution of the unionized form of the compound would be produced.

*Top is for a weak acid, bottom is for a weak base.* Find C by dividing the mg/ml by the molecular weight.

*These equations essentially let us know if we will have precipitation at a given set of condtitions

Example of this for a weak acid on the next page.

*Final answer is pH=9.6There for what this is saying, is for a weak acid anything below pH 9.6 there will be a precipitate, and above 9.6 there will be no precipitate. This is important b/c you can not inject a solution into a pt. if there is a precipitate.

*These are so that you can estimate the concentration of a solution so that everything will be in solution at a given pH value.

QUICK REVIEW:*Solubility is the maximum amount that will dissolve under given conditions in a volume of water.--Concentration of a drug present in a saturated solution, max amount that will dissolve*Dissolution rate is the speed at which the above process will occur.*Stirring will increase a rate b/c the molecules “move through a thinner diffusion layer into the bulk”*The more soluble something is the faster it will dissolve.

*Both of these species will always be present in solution.*pKa is 7.2 and the solubility is 1g/1300ml it would appear that it is not very soluble.-->this is a value for a saturated solution, so for this molecule, 1 gram in 1300mills means that at that value the solution is saturated with solute. THIS IS WHAT SOLUBILITY MEANS.-->The pH of the solutino and the pKa of a compound are the only two things that determine how much ionized and unionized form will be present.-->The ionized form will always have higher solubility.-->We may at some point exceed the solubility of one or the other depending on the pH.

*Is this a weak acid, a weak base or a neutral compound?--we can eliminate neutral compound based on this curve.--What you need to think about when answering these questions, is what the balanced formula looks like, so if this is a weak acid, by increasing the pH you push the rxn to the right *as seen in the equation that is above*

What could be the reasons that the use of pH adjustment to increase solubility may not be appropriate for a drug?--Drug may only be active at certain pH values--We would not want exceed the bloodʼs buffer capacity--Wouldnʼt want to inject an extreme pH--Could relate to the type of buffer that needs to be used i.e. couldnʼt use a boric buffer**A way to get around these problems is to use a co-solvent**

Co-Solvents--Blend of solvents, no pharmacologic effects, no toxicity--Limited to a few solvents, propylene glycol, glycerin, sorbitol, alcohol, polyethylene glycol--This is used to improve the solubility of a solute or solutes, over just using a single solvent.--Sorbitol can have a potential problem, risk of diarrhea.

Going back to our amobarbital reaction, what effect would a blend of solvents (alcohol and water) have on this compound?--Alcohol would make the pKa value higher, decrease solubility, this doesnʼt make sense, so why do we do this?--Co-solvent will result in less dissociation(increase pKa), solvent we are mixing with is usually less polar. But the overriding effect is the solubility of the UNIONIZED is increasing to make up for the decrease in disassociation of the ionized form.

*pHp in water of a 1% amobarbital solution: 8.34*pHp in 30% ethanol: 7.5

*Notice that for the glycerin/water solution, when glycerin is at 100% it doesnʼt really improve the solubility, this is b/c glycerin is very similar to water, it is very polar. *What can we say about the point where the max solubility is occurring?--It means that at this specific blend, it is most like the phenobarbital. LIKE DISSOLVES LIKE.--Solubility is a complex phenomenon, there are many different characteristics that can influence it, hydrogen bonds, etc.

*This is FALSE*To make this a true you should say the two are equal.*If this was in water, then the answer would be true.*In this case it doesnʼt matter, b/c it is in a buffer and the pH doesnʼt change. *Doesnʼt matter if you start out with the ionized or unionized form, the result will be the same.

Factors affecting solubility.*In general smaller molecules are more soluble*Molecular shape *Substituents or functional groups: Whether they are hydrophobic or hydrophilic*Ratio of polar to non-polar groups as you can see below, the resorcinol has two polar hydroxyl groups

making it about 15X more soluble.*Position of the functional groups also has an effect.--As seen below, the molecule in the middle has the highest solubility, and this is do to the groups hydrogen bonding with each other.

*Therefore, an alcohol with a shorter carbon chain is going to be MORE soluble than one with a longer chain.*Branching of these chains however will reduce the NON-polar effect.*Higher melting point of a solid usually means that it will have lower solubility.*Cis isomers generally are more soluble than trans.

*pKa is 4 so it will be 50% ionized at pH 4.*Remember that the difference b/t pKa 4 and pH of solution at 6 is 2 so there will be 1 part ionized and 100 parts unionized. In this case it is a base, so the compound will be 100 parts in the UNIonized form. Now, if we still had a weak base with the same pKa BUT the pH in question was 2, then we would have 100 parts in the IONized form.

TEMPERATURE EFFECTS: *Can have a wide range of effects, it could increase solubility, decrease solubility, or have no change.*So if the solute solvent interactions, exceed the solute-solute, and solvent-solvent interactions, then there is going to be excess energy that is given off as heat--If heat is given off, putting , more heat in will not help the solubility, and will decrease the solubility.

*this is a case where call of these drugs decrease in solubility when the temperature is raised. This can be a big problem b/c if lidocaine for instance was mixed at room temperature in the pharmacy and then injected into someone, the solubility would be reduced because of the patientʼs body temperature.

*Size and shape of a molecule is generally not a huge factor although it does play a role.*Effect of additives on solubility--Salting in: This is where the additive increases the solubility of the drug molecule.--salting out: Additive reduces the solubility of the drug molecule, sometimes causes a precipitate--Complexation (covalent or non-covalent): Drug and some additive have an interaction, but it can increase OR decrease the solubility, it can be hard to predict, which is why there will not be a question on the test over this. A classic example is antibiotics and antacids (results in the complex having almost NO solubility in water, causes the drug to not be absorbed into the body. Caffeine and benzoate can be used to make a complex which will make the caffeine water soluble.--Interaction of ions can result in a precipitation. **COMMON ION EFFECT results in a decrease in solubility and occurs when an ion that occurs in one compound interacts with the same ion when another compound is added.*For this antimalarial drug to the right, this is a problem especially when it comes to the hydrochloric acid that is in the stomach interacts with it. However this is a good drug an we want to use it. Below is a table of what happens when you simply use a different salt.*Also notice here that for structurally similar salts, the higher the melting point, the lower the solubility of the salt.