This section is from the book "Practical Materia Medica And Prescription Writing", by Oscar W. Bethea. Also available from Amazon: Practical Materia Medica and Prescription Writing.

This is found to be one of the hardest problems in prescription writing, to impress upon the student. Some grasp it with such facility and it seems so simple withal that an apology would seem necessary for laying any stress upon it; but in almost any senior medical examination, where every student thinks the matter either mastered or too simple to be worthy of study, less than half of the exercises will be found correctly written. Some methods of calculation might be illustrated, as follows:

The quantities for a four-fluidounce prescription for a 5 per cent. solution of liquefied pnenol in glycerin.

If the complete prescription contains 4 fluidounces or 32 fluidrachms, 1 per cent. would be 1/100 of that amount and found by pointing off two decimal places (0.32 drachm) 5 per cent. would be five times as much, or 0.32 X5 = 1.60 fluidrachms, or 96 minims.

Or, 5 per cent. is equal to one-twentieth; therefore, 1/20 of the total bulk of the prescription must be liquefied phenol, the prescription containing 4 fluidounces. 1/20 of 4 fluidounces equals 4÷20= 4/20 = 4/20=1/5 of a fluidounce. One fluidounce being 480 minims, 1/5 would be 480 ÷ 5 = 96 minims.

Or, the prescriber soon remembers that 4.8 minims are 1 per cent. of a fluidounce; therefore, 5 per cent. of a fluidounce is 4.8 X 5 = 24 minims; then 5 per cent. of 4 fluidounces would be 24 X 4 = 96 minims.

In making a correct percentage solution all items must either be weighed or measured. It is not correct to weigh the shot and measure the feathers.

Water at standard temperature, etc., weighs 456.37 grains to the fluidounce of 480 minims; therefore, 4.8 grains of cocaine hydrochloride with water to make a fluidounce is not strictly speaking a 1 per cent. solution by either weight or measure. There should be 4.56 grains of the cocaine salt. The prescriber seldom finds such exactness necessary, but may well carry in mind 4.5 grains of the solid as the 1 per cent. quantity of a 1 fluidounce aqueous solution of solids.

The metric system being a decimal one, the matter of percentage solutions is simplicity itself:

Phenolis Liq., | ||

Glycerini............................................................. | .q. s. 120 |

120 being the total quantity, 1 per cent. is found by dividing by 100, that is, pointing off two decimal places = 1.20; 5 per cent. by multiplying 1.20 by 5 = 6.00:

Phenolis Liq................................................................ | 6 | |

Glycerini................................................................... | 120 |

In, say, a 25 per cent. solution, 25 per cent. is one-fourth of the total; therefore, 1/4 of the total quantity (120) is 30, which would be the quantity of the liquefied phenol for a 25 per cent. solution.

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