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Pharmaceutical Calculations

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Information about Pharmaceutical Calculations
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Published on February 16, 2014

Author: pcteidf

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PowerPoint Presentation: Pharmaceutical Calculation PowerPoint Presentation: METROLOGY formerly and according to its strictest signification meant the science of measures Systems of measure The metric system Apothecaries ’ or Troy System Avoirdupois Systems or Imperial System PowerPoint Presentation: Metric System The metric system which is federally mandated and appears in the official listing of drugs in the United State Pharmacopoeia (USP), is a logically organized system of measurement. It was first developed by the French. In the metric system , the three primary or fundamental units are:   the meter for length the liter for volume the gram for weight.   PowerPoint Presentation: Apothecaries' System This system was used by the physicians in writing prescriptions and by the pharmacists in dispensing and retail trade. Historically , grains (gr) were the standard unit of weight (originally based on the weight of one grain of barley) and formed the base unit of the apothecaries’ system. Scruples , drachms (pronounced ‘drams’) and apothecaries’ (or troy) ounces were all multiples thereof Grains Scruples Drachms Apothecaries’ or troy ounces 20 1     60 3 1   480 24 8 1 Relationship between grains, scruples and apothecaries’ (troy) ounces PowerPoint Presentation: Imperial weights (avoirdupois) Imperial system of weights and measures is commonly known as Avoirdupois system and was formerly recognized by the British Pharmacopoeia. 'Avoirdupois ' is derived from the French words ' avoir ' and ' pois ' meaning 'to have weight '. The imperial system was introduced for bulk counter sales and was based on an imperial pound of 7000 grains. Further units of avoirdupois ounces were also used. The relationship between these units and grains is shown Grains Avoirdupois ounces Avoirdupois pounds 437.5 1   7000 16 1 Relationship between grains, avoirdupois ounces and avoirdupois pounds PowerPoint Presentation: Volumes Within the imperial system, minim(m ) were the standard unit of volume . Fluid drachm ( fl dr ), fluid ounces( fl oz ) and pints ( pt ) were all multiples thereof   Minims Fluid drachms Fluid ounces Quart Pints Gallon 60 1     480 8 1   9600 160 20 1 40 1 160 4 8 1 Relationship between minims, fluid drachms, fluid ounces and pints PowerPoint Presentation:  Historical weighing and measuring units and symbols used in pharmacy Symbol Meaning gr Grain m or min Minim dr Drachm fldr Fluid drachm floz Fluidounce oz(apoth) Ounce (apothecaries) oz Ounce (avoirdupois) lb Pound (imperial) pt Pint PowerPoint Presentation: Conversions to Memorize Weight 1 gram (g) 15.4 grains (gr) 1 grain (gr) 65 milligrams (mg)/ 0.065 grams (g) 1 pound (lb) 454 grams (g) / 0.454 kilograms (kg) 1 kilogram (kg) 2.2 pounds (lb)/ (1000/454 = 2.2) 1 ounce (oz) 28.4 grams (g)/ (454 grams/16 = 28.4 grams) Volume 1 fluid ounce 30 milliliters (mL) 1 pint ( pt ) 16 fluid ounces/480 mL 1 gallon 3840 milliliters (mL) PowerPoint Presentation: Household Measurements 1 drop 0.06 milliliters (mL) 1 minim 1 teaspoonful (tsp) 4 milliliters (mL) 1 fluid drachm 1 desert spoonful 8 milliliters (mL) 2 fluid drachm 1 tablespoonful ( tbsp ) 15 milliliters (mL) 4 fluid drachm 2 tablespoonful ( tbsp ) 30 milliliters (mL) 1 fluid ounce 1 wine glassful 60 milliliters (mL) 2 fluid ounce 1 teacupful 120 milliliters (mL) 4 fluid ounce 1 tumblerful 240 milliliters (mL) 8 fluid ounce PowerPoint Presentation: Iso-osmoticity and Isotonicity Osmosis Osmotic Pressure Solutions containing the same concentration of particles and thus exerting equal osmotic pressures are called iso -osmotic. The term isotonic, meaning equal tone, is sometimes used interchangeably with the term iso -osmotic. A 0.9% solution of NaCl (Normal Saline) is iso -osmotic with blood and tears. PowerPoint Presentation: It is desirable to match the tonicity of body fluids as closely as possible when administering a drug into that body fluid. Solutions which are not isotonic with the body fluid produce a painful stinging sensation when administered. Isotonicity is most commonly taken into account in the preparation of parentral solutions, eye drops and nasal solutions. To ensure isotonicity , you will need to be able to equate everything administered into the body to sodium chloride, since sodium chloride is the major determinate of blood and body fluid osmolarity . So, when faced with an isotonicity problem you will need to know: a ) the amount of drug in whatever amount of solution you’re making b ) the sodium chloride equivalent [E] for that drug* PowerPoint Presentation: Calculation for adjustment of Isotonicity Percentage w/v of adjusting substance needed = 0.52 – a/b a = freezing point depression of unadjusted solution b = freezing point depression of a 1% w/v solution of adjusting substance PowerPoint Presentation: Calculation to prepare isoosmotic solutions Comparison of depression in freezing point , a colligative property, between solutions is used. Non electrolyte 1 gm molecular weight of a non electrolyte in 1000gm of water causes depression in freezing point of 1.86 0 C of pure water. Lachrymal fluid and blood have freezing point of - 0.52 0 No. of gms (X)of Non-electrolyte per 1000gms of water used to form a isotonic soution is given by Example: Boric acid – Mol. Wt. = 61.8 gms X = 17.3 gms i.e 17.3 gms of boric acid in 1000gms of water should produce a solution isotonic with tears and blood PowerPoint Presentation: Electrolyte More is the dissociation, less is the quantity of electrolye required to produce same osmotic pressure . Calculation involve a dissociation factor ( i ) Following ( i ) values are used generally: Substances that dissociate into 2 ions = 1.8 Substances that dissociate into 3 ions = 2.6 Substances that dissociate into 4 ions = 3.4 Substances that dissociate into 5 ions = 4.2 No. of gms (X) of Electrolyte per 1000gms of water used to form a isotonic soution is given by Example: Nacl – Mol. Wt. = 58.5 gms; (i) = 1.8 X = 9.09 gms i.e 9.09 gms of sodium chloride in 1000gms of water should produce a solution isotonic with tears and blood PowerPoint Presentation: What is a sodium chloride equivalent? The relationship between the amount of drug that produces a particular osmolarity and the amount of sodium chloride that produces the same osmolarity is called the sodium chloride equivalent what amount of sodium chloride the drug you are adding is equivalent to Quantities of two substances that are tonic equivalents are proportional to the molecular weights of each multiplied by the (i) value of the other. Example : Atropine sulphate Mol. Wt. of Nacl = 58.5, (i) = 1.8 Mol. Wt. of Atropine sulphate = 695, (i) = 2.6 X = 0.12g of sodium chloride represented by 1g of atropine sulphate . PowerPoint Presentation: An example . A prescriber orders tobramycin 1% ophth sol, 10ml. You have tobramycin 40mg/ml preservative-free injectable solution on your shelf, as well as some sodium chloride in crystalline form. The Merck Index tells you that tobramycin’s MW = 468, so you calculate the E to be 0.07? 1. 90mg NaCl or equivalent needed for this product 2. 7 mg of NaCl equivalent accounted for by the tobramycin 3. 90mg - 7mg = 82mg NaCl needs to be added to the product. 4. Drawn 2.5ml of the tobramycin 40mg/ml stock solution, weigh out 82mg of NaCl crystals, qs to 10ml with water, and place the contents into an eye drop container PowerPoint Presentation:  R X Atropine Sulfate 2%  NaCl qs   Aqua. dist. q.s . ad. 30 ml The amount of NaCl to make 30 ml of an isotonic solution = 0.27 g The amount of atropine sulfate to make 30 ml of an solution = 0.6g E atropine sulfate = 0.13 the contribution of atropine sulfate to the NaCl equivalent = 0.6g X 0.13 = .078  the amount of NaCl to add to make the solution isotonic = 0.27g – 0.078 = 0.192g PowerPoint Presentation: Alligations If the desired percentage concentration of a solution, ointment, or cream is unavailable, it may be compounded by mixing a more concentrated and a less concentrated formula to obtain the desired strength. To determine the number of parts or proportions of each the alligation method may be used. Alligations are sometimes referred to as “Tic-Tac-Toe” problems because the formula has a resemblance to the game’s pattern. PowerPoint Presentation: A prescription order requires 35% dextrose solution in 1 liter (1000ml), and the pharmacy stocks 70% and 20% dextrose solution; how much of the 70% solution will you use to fill the order ? To solve the problem, begin by drawing your “tic-tac-toe” grid: Insert the larger formulation in the top left corner; the smaller in the bottom left corner, and the desired amount in the center square. Percentage   Parts 70       35   20     Subtract the center square from each corner box. So, 70% minus 35% equals 35, and 20% minus 35% equals 15 (even though it is a negative number, only use positives with allegations.) This gives how many parts of each formula needs to use to get the appropriate amount. Percentage   Parts 70   15 Parts of the 70% Soln   35   20   35 Parts of the 20% Soln PowerPoint Presentation: Add the two right-hand boxes together and you have the number of total parts Now that the proportional parts have been determined, the quantities of the higher and lower concentrated solutions may be calculated by setting each equal to X divided by the total quantity of the preparation to compounded. Cross multiply and divide to find X . To compound this prescription order, 300ml of 70% dextrose is added to 700ml of 20% dextrose to make a total of 1 liter of a 35% dextrose solution. Percentage   Parts 70   15 Parts of the 70% Soln   35   20   35 Parts of the 20% Soln     50 Total parts Quantity of Higher Parts of Higher = X Total parts Total Quantity to be Compounded         Quantity of Lower Parts of Higher     = X Total parts Total Quantity to be Compounded 70% Dextrose 15 = X X = 300 ml 50 1000           20% Dextrose 35 =   X X = 700 ml 50 1000

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