Weights and measures

• Weighing and measuring


  The two most fundamental practical skills practiced by a compounder
• During compounding, ingredients will need to be either weighed or measured and the accuracy of the compounder's technique will have a great bearing on the accuracy and efficacy of the final product
• Types of balances encountered in a pharmaceutical environment

  • Standard dispensing balance (the Class B balance or electronic equivalent)
  • Sensitive electronic balances
  • Balances for weights greater than 50 g
• Capacity

  • The maximum weight, including containers and tares that can be placed on a balance pan
• Sensitivity

  • The smallest weight that gives a perceptible change in the indicating element
• Readability


  • For electronic balances, the smallest weight increment that can be read on the digital display of the balance
• Precision


  • The agreement between repeated measurements of the same quantity under the same conditions
• Accuracy


  • The closeness of the displayed weight, as measured by the balance, to the true weight
• Standard dispensing balances


  The balance most commonly found in pharmacies, either a traditional "Class B" dispensing beam balance or a modern electronic equivalent
• Class B beam balances were designed to weigh up to 50 g in 10 mg increments. They had a nominal minimum weighable quantity of 100 mg, but in practice a higher limit was preferred for potent substances (e.g. 150 mg)
• Class II balances are similar to older Class B balances and use the same weighing techniques, but the maximum weight for class II is 25 g
• Electronic pharmacy balances

  The scale pan is usually made of metal rather than glass, so the ingredient to be weighed is not placed directly onto the scale pan
• Sensitive electronic balances


  Used to weigh amounts below 150 mg, with an accuracy of 1 mg
• Balances for weights greater than 50 g


  Used to measure out bulk ingredients
• Triple beam balance


  Consists of three beams to measure from 0-10 g, 0-100 g, and 0-500 g, with a capacity of 610 g and a minimum weighable amount of 100 mg
• Rules in Weighing

  • Keep the balance in a draught-free environment, clean and free from debris
  • Before use, the balance must be set up at a particular location and used on a solid, level, firm surface
  • It is important that it is not moved while in use as this will affect the accuracy of the instrument
  • Zero the balance before weighing
  • Solids should be size reduced or sieved (if required) before weighing
  • Don't split quantities as this will increase inaccuracies
  • If a quantity less than the legal minimum is required, it is necessary to weigh the minimum weight allowable and make an excess of the product or prepare it by trituration
• Measures


  Any vessel that is used to measure accurately an ingredient must comply with the current Weights and Measures Regulations and should be stamped accordingly
• Main types of vessel used within pharmacy for measuring liquids

  • Conical and cylindrical measures
  • Pipettes
• Conical measures


  • Easier to fill without spilling liquid on the sides above the required level
  • Easier to drain out the preparation
  • Easier to rinse out the residue left after draining viscous liquids into the preparation
  • Easier to clean after use
• Conical measures vs cylindrical measures

  • Harder to read the meniscus accurately
  • More difficult to estimate volumes between graduations
• It is not good practice to use a conical measure to measure a volume that is smaller than half of the total volume of the measure
• Rules in measuring

  • Never use more than one measure as it increases error
  • Select the smallest measure that will hold the desired volume
  • Ensure that the measure is thoroughly drained
  • The bottom of the meniscus should be in line with the desired graduation mark
• Household measuring system

  Inaccurate system of measurement using household devices like teaspoons, tablespoons, etc.
• Solution

  A homogeneous liquid preparation that contains one or more dissolved components
• Advantages of pharmaceutical solutions

  • Drug available immediately for absorption
  • Flexible dosing possible
  • May be designed for any route of administration
  • No need to shake container
  • Facilitates swallowing in difficult cases
• Disadvantages of pharmaceutical solutions

  • Drug stability often reduced in solution
  • Difficult to mask unpleasant tastes
  • Bulky, difficult to transport and prone to container breakages
  • Technical accuracy needed to measure dose on administration
  • A measuring device needed for administration
  • Some drugs poorly soluble
• Excipients used in pharmaceutical solutions for oral administration

  • Vehicle (purified water)
  • Co-solvents (propylene glycol, glycerin, alcohol, PEG)
  • Preservatives (parahydroxybenzoate esters, boric acid, sorbic acid)
  • Antioxidants (sodium sulphite, ascorbic acid)
  • Sweeteners (sucrose, glucose, sorbitol)
  • Rheology (viscosity) modifiers (methylcellulose, sodium carboxymethylcellulose)
  • Colours (tartrazine, amaranth)
  • Flavours (raspberry, mint, anise)
  • Buffers (citrate buffer, sodium phosphate)
• Excipients in pharmaceutical solutions

  • Vehicle
  • Co-solvents
  • Preservatives
  • Antioxidants
  • Sweeteners
  • Rheology (viscosity) modifiers
  • Colours
  • Flavours
  • Buffers
• Vehicle

  • Purified water
• Co-solvents

  • propylene glycol
  • glycerin
  • alcohol
  • PEG
• Preservatives

  • Parahydroxybenzoate esters (methylhydroxybenzoate and propylhydroxybenzoate)
  • boric acid and borate salts
  • sorbic acid and sorbate salts
  • phenolics
  • double strength chloroform water
  • benzoic acid and salts
  • sorbic acid and its salts
  • alkyl esters of parahydroxybenzoic acid
  • a 9:1 combination of methyl and propyl parahydroxybenzoates
• Colours

  • Compound tartrazine
  • amaranth
• Buffers

  • Acetates (acetic acid and sodium acetate)
  • Citrates (citric acid and sodium citrate)
  • Phosphates (sodium phosphate and disodium phosphate)
• Aromatic waters

  Saturated solutions of volatile oils in water used mainly for their flavouring properties
• Vehicle (for oral solutions)

  Purified Water USP, prepared by distillation, ion exchange or reverse osmosis. It is non-toxic, non-irritant, tasteless, relatively cheap and many drugs are water-soluble.
• Co-solvents

  Used to increase the solubility of the therapeutic agent within the formulation
• Glycerol

  An odorless, sweet liquid that is miscible with water and has similar co-solvency properties to ethanol
• Alcohol USP

  Contains between 94.9 and 96.0% v/v ethanol and is commonly used as a co-solvent, both as a single co-solvent and with other co-solvents
• Propylene Glycol USP

  An odourless, colourless, viscous liquid that contains two hydroxyl groups and is used as a co-solvent, generally as a replacement for glycerin
• Poly(ethylene glycol) (PEG)

  A polymer composed of repeating units of the monomer ethylene oxide, with lower-molecular-weight grades (PEG 200, PEG 400) preferred as co-solvents