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Lab 5 Pharmaceutical Measurement Percentage of error In pharmacy its important to keep measurement as accurate as possible. Pharmacist must know the limitations of the measuring instruments. When using a torsion prescription balance the percentage error must be calculated to determine whether the error is allowed or not. Percentage of error Percentage error : it’s the maximum potential error multiplied by 100 and divided by the quantity desired. × % = This formula is valid only if the error and the quantity desired are expressed in the same denomination. Percentage of error Example: when the maximum potential error is ± 4 mg in a total of 100 mg, what is the percentage error? ×% = % Percentage of error If certain % of error is not to be exceeded, and the maximum potential error of an instrument is known, its possible to calculate the smallest quantity that can be measured. () = Percentage of error Example what is the smallest quantity that can be weighed with a potential error of not more than 5 % on a balance sensitive to 6 mg? × = Aliquot method of measuring weighing When a degree of precision in measurement is required that is beyond the capacity of the instrument in hand, the pharmacist may use the aliquot method of measuring. Aliquot part: its any part that is contained a whole number of times in a quantity. Thus, 2 is an aliquot part of 10; and since 10 / 2 = 5, 2 is called the fifth aliquot of 10. Again, 4 is an aliquot part of 16 “the 4th aliquot of 16”. Aliquot method of measuring Procedure: Step 1 Select some multiple of the desired quantity that can be weighed with the required precision. This is done by calculating the smallest quantity of the substance that can be weighed with the required precision. Step 2 Dilute the multiple quantity with an inert substance (diluent) that is compatible with the given preparation. Step 3 Weigh the aliquot part of the dilution that contains the desired quantity. Aliquot method of measuring Aliquot method of measuring Aliquot method of measuring Measuring volume Its identical in principle to the aliquot method of weighing. Procedure Step 1 select multiple of the desired quantity that can be measured with required precision. Step 2 dilute the multiple quantity with compatible diluent to an amount evenly divisible by the multiple selected. Step 3 measure the aliquot of the dilution that contains the quantity originally desired. Aliquot method of measuring Least weighable quantity method This method may be used as an alternative to the aliquot method of weighing to obtain small quantities of a drug substance. Least weighable quantity method Procedure: Step1 weigh an amount of the drug substance that is equal to or greater than the least weighable quantity. Step 2 dilute the drug substance with a calculated quantity of inert diluent. Step 3 weigh the predetermined quantity of the drug- diluent mixture will contain the desired quantity of drug. Least weighable quantity method Example: if 20 mg of a drug substance are needed to fill a prescription, how you would obtain this amount of drug with an accuracy of ±5% using balance having a sensitivity requirement of 6 mg. use lactose. = The smallest amount to be weighed on this balance × = × = 120 mg of drug substance is weighed. Drug-diluent mixture must be equal 120 mg or greater. ( 150 mg of drug-diluent mixture is selected) Least weighable quantity method The total amount of diluent to use is: 20 mg → 150 mg (drug-diluent mixture) 120 mg → X (total amount of drug-diluent mixture) X = 900 mg 900 – 120 = 780 mg of diluent (lactose ) to use. Practice problems 1. 2. A pharmacist attempts to weigh 0.375 gm of morphine on a balance of dubious accuracy. When checked on highly accurate balance, the weight is found to be 0.4 gm. Calculate the percentage of error in the first weighing. A prescription balance has a sensitivity requirement of 0.006 gm. Explain how you would weigh 0.012 gm of atropine sulfate with an error not greater than 5%, using lactose as the diluent. Practice problems 3. A 10 mL graduate weighs 42.745 grams. When 5 mL distilled water are measured in it, the combined weight of graduate and water is 47.675 grams. By definition, 5 mL of water should weigh 5 grams. Calculate the weight of the measured water and express the deviation from 5 grams as percentage error. Practice problems 4. A pharmacist failed to place the balance in equilibrium before weighing three grains of codeine. Later he discovered that the balance was out of equilibrium and that 20% error was incurred. If the balance pan on which he placed the codeine was heavy, how many grains of codeine did he actually weigh? Practice problems 5. 6. On a prescription balance having a sensitivity requirement of 0.012 gm, what is the smallest amount that can be weighed with a maximum potential error of not more than 5%? A prescription balance has a sensitivity requirement of 6.5 mg. Explain how you would weigh 20 mg of substance with an error not greater than 2%. Practice problems 7. A prescription calls for 0.2 mL of clove oil. Using a 5 mL graduated calibrated in units of 0.5 mL, how would you obtain the required amount of clove oil using the aliquot method and alcohol as the diluent? Home work 1. 2. A torsion prescription balance has a sensitivity requirement of 0.002 gram. Explain how you would weigh 0.008 gram of substance with an error not greater than 5 % In preparing a certain ointment, a pharmacist used 28.35 gram of zinc oxide instead of 32.3 grams called for. Calculate the percentage of error on the basis of the desired quantity.