LAB 2-Measurements-2010

LAB 2-Measurements-2010 - PHCY 6101- Dose Form Lab Fall...

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Unformatted text preview: PHCY 6101- Dose Form Lab Fall 2010 LABORATORY 2 PHARMACEUTICAL MEASUREMENTS Objectives Introduction to the use of prescription balance class III (formerly class A) Learn how to determine the sensitivity requirement of a prescription balance Practice the use of the prescription balance and use of the Aliquot method Practice measurements of liquid chemicals: correct use of graduate cylinders, pipettes, and calibration of a medicine dropper. References: Remington's (18th edition): pp 69-82. Pharmaceutical Dosage Forms and Drug Delivery Systems: H. Ansel, N. Popovich and L. Allen, 8th edition (2005), Appendix C, pp 685-695. Measurement of Solids CHARACTERISTICS OF THE PRESCRIPTION BALANCE Although the mortar and pestle are commonly used as a symbol of pharmacy, the ever-present (required by law) prescription balance could as appropriately represent pharmacy. As the prescription balance is used daily to weigh vital medicinal agents, it is imperative that the pharmacist completely understands its characteristics and correct usage. Prescription balances are termed class III (formerly class A) balances, which meet the prescribed standards of the National Institute of Standards and Technology. The sensitivity of a balance is usually referred to as the sensitivity requirement (SR), which is defined as the maximum change in load that will cause a specified change, one subdivision on the index plate, in the position of rest of the indicating element of the balance. Prescription balances have a SR of 6 mg with no load and with a load of 10 g on each pan. The smaller the weight required moving the indicator one subdivision, the more sensitive is the balance. Most class III prescription balances have a maximum capacity of 120 g and bear a statement to that effect. If no information is given, the nominal capacity of a prescription balance is assumed to be 15.5 g. The prescription balance class III comes with a set of analytical weights, which meet the National Bureau of Standards’ specifications for analytical weights. These weights LAB 2: Pharmaceutical measurements 1 PHCY 6101- Dose Form Lab Fall 2010 consist of one 50-g, one 20-g, two 10-g, one 5-g, two 2-g, one 1-g, one 500-mg, two 200-mg, one 100-mg, two 20-mg, one 10 -mg, and one 5-mg weight. Balances with similar SR of a prescription balance should be used for all weighing operations required in prescription compounding. The USP directs that to avoid weighing errors of 5% or greater, which may be due to the limit of accuracy of the prescription balance, one must weigh a minimum of 120 mg of any substance in each weighing (5% of 120 mg being the 6 mg SR, or error inherent with the balance). If a smaller weight of material is desired, it is directed that the pharmacist mix a larger calculated weight of the ingredient (120 mg or more), dilute it with a known weight of an inert dry diluent (as lactose), mix the two uniformly, and weigh an aliquot portion of the mixture (again 120 mg or more) calculated to contain the desired amount of agent. The class III balance with a capacity of 120 g should be used for all weighing required in prescription compounding. Electronic balances are available in various sensitivities. The one most commonly used in prescription compounding has a readability of 0.001 g; consequently, the least amount that can be weighed is 20 times that, or 20 mg. Electronic balances are much faster and easier to use than prescription balances. The digital readout is easy to read and the balance is quite versatile and easy to clean, and it has a relatively small footprint. LEVELING OF THE BALANCE The prescription balance should always be kept in a well-lighted location, placed on a firm, level counter approximately waist-high to the operator. The area should be as free from dust as possible and in an area that is draft-free. When not in use, the balance should always be kept with the weights off and the beam in the fixed or locked (arrested) position. Before weighing the balance must be made level. This is accomplished with the leveling screws on the bottom of the balance. The arrest knob must be released (unlocked) and the balance adjusted to obtain equal swings of the pointer level to the right and left or up and down (depending on the type of balance) or to obtain stoppage or rest with exact alignment of the pointer. The rider on the graduated beam or the indicator of the dial on the front panel (depending on the type of balance) must be at its zero position at all times during operations. • Arrest knob turned right = locked or arrested position • Arrest knob turned left = unlocked or released position DETERMINATION OF THE SENSITIVITY REQUIREMENT The National Formulary (NF) calls for a sensitivity requirement of 6 mg for a prescription balance. This could be determined by the following procedure: 1. Level the balance and determine the rest point. The balance lid is to be closed to prevent drafts from affecting the oscillations. 2. Place a 6-mg weight on the right pan. The rest point should not shift less than one division on the indicator scale. 3. Level the balance and determine the rest point with a 10-g weight in the center of each pan. 4. Place a 6-mg weight on the right pan. The rest point should not be shifted less than one division. Since few, if any, set of weights include a mass of less than 10 mg, the following LAB 2: Pharmaceutical measurements 2 PHCY 6101- Dose Form Lab Fall 2010 practical procedure is suggested to determine the sensitivity requirement in the laboratory: 1. Level the balance and determine the rest point. The balance lid is to be closed to prevent drafts from affecting the oscillations. 2. Record the number of scale divisions the index pointer is shifted by a 20-mg and a 10-mg weight. In some balances you may have to use a combination of 10, and 5-mg weights. 3. Plot the weight against the number of scale divisions shifted by each weight on a graph paper. 4. Draw a straight line through the points, and from the straight line read the weight that will cause a shift of one scale division. 5. Repeat with a 10-g weight on each pan. USE OF THE PRESCRIPTION BALANCE The pharmacist must not only maintain a proper prescription balance, but he must employ the correct weighing technique and be conscious of the limitations of the balance for the particular quantity of drug to be weighed. Medicinal substances are weighed on powder papers or weighing dishes/boats. The paper/dish protects the pans from chemical action and eliminates the need for repeated washing of the pans. A new paper/dish for each item prevents contamination. Another advantage to utilizing a paper/dish is that it serves as a transfer funnel. Weighing papers should have a glazed surface or should be made of aluminum foil so that no appreciable amount of the drug will adhere to the paper. This is especially important when small quantities are to be weighed. A paper should be chosen so it is of reasonable size giving a maximum weighing area without touching any part of the balance except the pan. The papers should be creased diagonally from one corner to the opposite corner or the edges turned up (aluminum foil) before placing on the pans. If desired, tared watch glasses or small beakers may be used on the balance pans instead of papers. WEIGHING TECHNIQUE In using a prescription balance, neither the weights nor the substance to be weighed should be placed on the balance while the beam is free to oscillate. Usual guidelines to weighing technique are: 1. Before weighing, powder papers of equal size should be placed on both pans of the balance and the equilibrium of the balance tested by releasing the arresting knob. If the balance is off because of differences in the weight of the powder papers, small tearings of powder paper must be added to the light pan. Powder papers taken from the same box can vary in weight by as much as 65 mg. If balance equilibrium is not established after the papers are placed on the pans, an error of more than 30% can result in weighing 200 mg of material. 2. When balanced (index pointer is at zero), the balance is placed in the arrested position, the lid is opened and the desired weights added to the right-hand pan and/or the weigh beam. 3. An amount of substance considered to be approximately the desired weight is LAB 2: Pharmaceutical measurements 3 PHCY 6101- Dose Form Lab 4. 5. Fall 2010 carefully placed on the left hand pan with a spatula and the lid closed. The arrest knob should then be slowly released to check the equilibrium. The arresting knob is most conveniently operated by the left hand. If the substance is in excess, the balance is arrested again and a small portion of the substance removed with the spatula. The same procedure is used if more substance is needed. A pharmacist should always remember that when a stated quantity of a substance was weighed, this includes not only the true weight but also the possible excessive or deficient weight by virtue of the sensitivity of the balance. Knowing the sensitivity of the balance, one may calculate the percent of possible error for a given amount to be weighed. For example, assume that 5 mg produce a change of one index plate division in the rest point and 200 mg of drug is to be weighed, the actual weight could be between 195-205 mg. The percentage of possible error is calculated by the ratio, 5 mg X% = 200mg 100% producing a value of X = 2.5%. Thus, the inherent percentage of error for any given situation may be expressed as, Percentage of Error = Sensitivity Requirement X 100 Quantity Desired The same relationship may be used to calculate the smallest amount that can be weighed within a particular permissible percentage of error. Assuming the sensitivity requirement of a balance is 5 mg and the permissible percentage of error is 5%, one may calculate the smallest quantity that can be weighed within this limit or error. A ratio is set up as before, 5mg 5% = xmg 100% and will produce a value of X = 100 mg. Thus, the smallest amount that can be weighed with a certain permissible percent or error may be expressed as Smallest quantity to be weighed = _____SR X 100_______ Permissible Percent of Error In order to achieve 99% accuracy, that is, to permit a 1% error, the amount weighed must be at least 100 times as great as the sensitivity requirement of the balance. To achieve 90% accuracy or to allow a 10% error, one must weigh at least 10 times as much as the sensitivity requirement of the balance. LAB 2: Pharmaceutical measurements 4 PHCY 6101- Dose Form Lab Fall 2010 ALIQUOT METHOD When the amount of drug is too small to be weighed directly on the prescription balance (<120 mg), the aliquot method of weighing is used. An aliquot part is defined as any part that is contained a whole number of times in a quantity; e.g. 2 is an aliquot of 10, since 10/2 = 5, 2 is referred to as the fifth aliquot of 10. Also, 2 mg of drug uniformly dispersed in exactly 98 mg of diluent represents a fifty-fold dilution. 1. 2. 3. To use the aliquot method in compounding, follow three steps: Weigh a multiple of the quantity desired (prescription balance). Dilute the multiple in a suitable manner. Weigh or measure an aliquot of the final mixture (dilution). Example 1: Suppose the smallest amount of drug that we can weigh on a Prescription Balance is 100 mg, but we need only 20 mg. Procedure is to weigh out 100 mg of drug (which is multiple of 20 mg). We then dilute it with 900 mg of lactose to make a total of 1000 mg (1 g). This 1000 mg of mixture contains 20 mg of drug in each 200 mg. We can now weigh out 200 mg of mixture (fifth aliquot of 1000 mg) and it will contain 20 mg of the drug. Example 2: A pharmacist needs 60 ml of silver nitrate solution 1:5000. For the above preparation we need 1000 mg x 60 ml = 12 mg AgNO 3 5000 ml The procedure is to weigh out 120 mg AgNO3 and dissolve it in distilled water. The volume is made up to 100 ml. This will give a 1.2 mg/ml solution. 10 ml of this solution should be measured and diluted to 60 ml. Measurement of Liquids In pharmacy, measuring of liquids should be done by selecting the measuring device most appropriate to the volume of liquid to be measured and the desired degree of accuracy. With liquids, the more narrow the column of liquid, the more accurate is likely to be the measurement. Many types of devices are used in this operation, depending on the kind and quantity of liquid to be measured. The three most important devices used in liquid measurement in pharmacy are graduated cylinders (or cylindrical graduates), pipettes (pipets) and droppers. Cylindrical graduates are generally calibrated in metric units and are available in a wide variety of capacities, ranging from 5 to 1000 mL or more. In measuring small volumes, less than 1.5 mL, the pharmacist should use a pipet and a pipet filler (pipet helper), which will also allow accurate delivery of the liquid. The pharmacopeial LAB 2: Pharmaceutical measurements 5 PHCY 6101- Dose Form Lab Fall 2010 medicine dropper is used for measuring small quantity of liquid. The dropper is constricted at the delivery end to a round opening having an external diameter of 3 mm. A vertically positioned pharmacopeial medicine dropper delivers a drop of water weighing between 45 and 55 mg. In using droppers the pharmacist must realize that a drop is not a unit of volume as the volume of a drop delivered from the same dropper depends on different factors. For a given dropper, the volume of a drop varies as a function of the pressure applied and the temperature, and as the function of the density, viscosity, and surface tension of the liquid delivered. To maintain accuracy, one must remember the position of the dropper while pouring the drop. Errors in measuring may be made by the operator if he tilts the measuring device or fails to align the graduation mark and the bottom of the meniscus (meniscus = concave or crescent shaped surface of liquid that bulges downward). This is known as parallax effect. Parallax is the apparent displacement of an object that follows a change in the position from which it is viewed. This phenomenon may cause measuring errors larger than 5%, which are unacceptable from a pharmaceutical point of view. To avoid this error, the graduate/other device is raised and held straight up so that the graduation point to be read is on a level with the eye, keeping the brown surface (if any) towards the back, and the liquid is measured. LAB 2: Pharmaceutical measurements 6 PHCY 6101- Dose Form Lab Fall 2009 In using graduates, the pharmacist pours the liquid into the graduate slowly, observing the level. In measuring viscous liquids, adequate time must be allowed for the liquid to settle in the graduate, as some may run slowly down the inner sides of the graduate. It is best to attempt to pour such liquids toward the center of the graduate, avoiding contact with the sides. In emptying the graduate of its measured contents, adequate drain time should be allowed. When pouring liquids from bottles, good pharmaceutical technique is to keep the label on the bottle facing up; this avoids the possibility of any errant liquid running down over the label as the bottle is righted after use. The bottle orifice should be wiped clean after each use. Prescriptions consisting of several liquids should not be compounded in prescription bottles, but they must be accurately measured in a suitable graduate and then transferred into graduated or un-graduated prescription bottles. The graduations in the prescription bottles are usually very inaccurate. LAB EXERCISES Exercises using the torsion/prescription balance: A. Obtain a box with a standard set of weights (in a drawer of the lab center island) with a number that matches your balance number. Determine the sensitivity requirement (SR) of your balance twice. 1)Using a 20-mg and a 10mg weight. Record the number of scale divisions the index pointer is shifted. Plot the weight against the number of scale divisions shifted by each weight on a graph paper (provided). Draw a straight line (as best as possible) through both points, and from the straight line read the weight that will cause a shift of one scale division, if needed. 2) Repeat the same procedure and graphing with a 10-g weight on each pan. B. Fill one medium size weighing boat with charcoal and one with lactose and bring to your bench. Using the aliquot method or the least weighable amount method (consider USP-approved error and SR) prepare the following charcoal dilution (trituration), have it checked by the instructor or TA and discard the mixture into trash can. Return unused charcoal and lactose to original containers. Do not mix spatulas and try to not contaminate one chemical with the other. ℞ Sig. Charcoal 25 mg Lactose q.s. 2g Mix and make trituration Instructions: 1. Use glass mortar and pestle and plastic spatulas (to help fluffing the powders as you mix them). 2. Prepare a dilution to allow an aliquot to contain the required amount of charcoal in lactose, avoiding excessive waste of unused dilution (trituration). 3. In preparing the dilution to obtain the required aliquot, start with weighed amount of charcoal. When mixing a small amount of drug (mg level) with a large amount LAB 2: Pharmaceutical measurements 7 PHCY 6101- Dose Form Lab 4. 5. 6. Fall 2009 of a second ingredient, the drug is placed in the mortar with equal bulk of the other ingredient. The two ingredients are triturated until intimately mixed. Then, an equal bulk of the second ingredient is added to the mixture, and the powders are triturated until intimately mixed. The material is triturated by geometric addition until all gone. This method is known as geometric dilution. Get dilution checked by instructor/TA for homogeneity. Weigh out the aliquot and the additional lactose required to complete the Rx amount prescribed. After final Rx is completely mixed, get it checked by instructor/TA for homogeneity and discard into trash cans. Exercises using electronic balances: C. Perform the following weighing operations using electronic balances. i. Weigh 0.5 g (gram) of dextrose. Ask instructor/TA to check the amount weighed. Return weighed powder to original container. ii. Weigh 50 mg of dextrose, by instructor/TA, return to container. Exercises using glass cylinders: D. Fill a large beaker available in your locker with tap water. Perform on your bench all of the following measurement operations and get all measured volumes checked at the same time by instructor/TA. Discard checked volumes in sink. i. Using 10-mL cylinder measure 2.5 mL of water. ii. Using 25-mL cylinder measure 12 mL of water. iii. Using 50-mL cylinder measure 28 mL of water. iv. Using 100-mL cylinder measure 56 mL of water. Calibration of a prescription bottle: E. Obtain one 3-ounce prescription bottle and fill with tap water to the 90-ml mark. Transfer the contents of the bottle to a 100-ml cylindrical graduate. Record the volume. Observe the difference between both measurements. Return the prescription bottle separated from cap to the container on the counter. Exercises using pipettes: F. Using pipettes, pipette helpers, and liquids provided on top of center island in lab, perform the following measuring operations and get the measured volumes checked by instructor/TA. i. Using a 5-mL pipette measure 2.7 mL of water, by instructor, discard into large container provided. ii. Using a 10-mL pipette measure 4.5 mL of water, by instructor, discard into large container provided. iii. Using a 10-mL pipette measure 5.7 mL of alcohol, by instructor, discard into the bottle provided. Exercises using droppers: G. Obtain one disposable “dropper” (transfer pipette) and get your 10-mL cylindrical graduate. Using droppers, and liquids provided on top of center island in lab, perform the following measuring operations: i. Holding a medicine dropper vertically, count and record the number of LAB 2: Pharmaceutical measurements 8 PHCY 6101- Dose Form Lab Fall 2009 drops of water required to fill a 10-ml cylindrical graduate to the 3-ml mark. ii. Repeat the previous measurement holding the dropper at a ~20-30° angle with the horizontal line. iii. Holding a medicine dropper vertically, count and record the number of drops of alcohol required to fill a 10-ml cylindrical graduate to the 3-ml mark. H. Measure glycerin using droppers attached to the glycerin containers. Holding a medicine dropper with a slight inclination, count and record the number of drops of glycerin required to fill a 10-ml cylindrical graduate to the 3-ml mark. Discard into sink by rinsing the graduate thoroughly (glycerin is water-soluble and harmless). I. Prepare a 1% solution (10 ml) of sodium lauryl sulfate (use mask or handle powder slowly while weighing to prevent inhalation). i. Holding the dropper vertically, count and record the number of drops required to fill a 10-ml cylindrical graduate to the 3-ml mark. ii. Heat the surfactant solution to 70° C (use thermometer to monitor the temperature, not to stir the solution). Using this warm solution, repeat the previous measurement. CALCULATIONS/RESULTS (to be submitted at the end of lab) At the end of lab submit on a sheet of paper all results, calculations, graphs, and performed measurements to the instructor/TA for grading (lab assignment = BONUS). Include your name, section (TUE, WED, THU), locker number and balance number. 1. 2. 3. 4. 5. Submit the graph paper with SR determinations. Include your name, section (TUE, WED, or THU), locker number and balance number. Submit the measurements (number of drops = gtt) performed with droppers (exercises 9, 10 and 11 above). Calculate the percent of error that would be incurred in measuring 50 mg of atropine sulfate using the balance on your work bench. If the permissible error of your torsion balance were 15%, which would be the minimum quantity of a drug that you could weigh accurately using it? Show calculations for the following compounding situation. A pharmacist needs 0.15 ml of flavoring oil (orange oil) to prepare an oral liquid. He has alcohol to be used as a solvent and only a 2 mL pipette with 0.2 ml graduations. How could this volume of orange oil be obtained? LAB 2: Pharmaceutical measurements 9 ...
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