The making of solutions is a phase of nursing procedure which necessitates exact knowledge and careful technique. It is essentially a pharmaceutical process and could be taught to good advantage in the hospital drug room by the pharmacist, since here only is to be found the requisite combination of expert knowledge and suitable equipment which makes for efficient teaching. By actually handling drugs, and by observation under expert supervision of exact methods of weighing and measuring, the nurse will more readily grasp the underlying principles and realize the necessity for caution and accuracy.
A solution is a liquid in which has been dissolved particles of a solid, a gas, or another liquid, so finely divided that the resultant mixture appears clear and homogeneous and the dissolved substance cannot be seen.
A saturated solution is a fluid which contains as much of the solid as it is capable of dissolving. If more of the solid be added, it will not be dissolved but will remain as a sediment. The solubility of various substances varies widely: some substances, such as sodium iodide, dissolve readily in a relatively small quantity of water; others, such as salicylic acid, require a large proportion of water and dissolve very slowly. The solubility of a substance is affected chiefly by the following factors, viz.: (a) The nature of the sub-stance to be dissolved. (b) The nature of the dissolving medium. (c) The temperature of the dissolving medium.
The first point has already been explained. To illustrate the second point, it may be stated that substances which will not dissolve in water at all will dissolve readily in alcohol (ex. camphor), and on the other hand, magnesium sulphate, which in water dissolves with the utmost ease, remains undissolved in alcohol.
As regards the third factor it may be stated that, as a general rule, the higher the temperature of the medium, the higher the saturation point - e. g., the more of the solid may be dissolved in it. The saturation point of alum in cold water is only 10 %, whereas if boiling water be used this is increased to 80 %.
By the strength or concentration of a solution is meant the quantity of particles of a solid, a gas, or a liquid which are dissolved in a given quantity of that solution. This may be expressed in terms of percentage - that is to say, by stating the presence of so many parts of dissolved substance in every hundred parts of solution. Thus: a 5% solution of boric acid would contain five parts of the solid drug to every hundred of water. Solutions decompose very quickly, and the appearance of a cloudy growth shows that they are no longer fit to use.
Sometimes the strength of a solution is designated by stating that a given quantity, such as an ounce, contains so many grains. Thus: atropine solution grs. iii. ad. 3 i. The quantity of the solid drug here used is indicated by arbitrary proportion and not by percentage.
On these two arithmetical procedures, viz., percentage and proportion, are based the following methods of working out typical problems encountered in the making of solutions.
To estimate the amount of a drug which must be added to a given quantity of the dissolving medium in order to make a solution of a given percentage.
Let the solution called for be atropine 5% one ounce. Reduce quantity needed to lowest unit, viz.:
℥ i = 480 e.
Multiply result by the rate per cent.
480 x 5 = 2400. and divide by 100. 2400 % 100 = 24.
The result, viz., 24 grains, represents the amount of atropine which must be added to I ounce of water to make a solution of 5%.
A convenient practical rule for diluting a stronger solution to one of any given weaker strength is the following:
Take number of units indicated by solution desired, add water to bring the bulk up to number of units indicated by original strength. Thus, to get a solution of 75% from one of 95%, take 75 units of the 95% solution and add 20 units of water. The result will be of the concentration desired, viz. 75%.
To find the amount of a stock solution of known strength to use in making a given quantity of a solution of known different strength.
One quart of bichloride of mercury solution 1:3000 is desired. The stock solution on hand is 1:25.
Reduce quantity called for to lowest units. I quart = 15000 e. (Approx.) Multiply result by the weaker proportion.
15000 x 1/3000 = 5.
Divide the result by the stronger proportion:
5 % 1/25 = 5 X 25/1 = 125.
The result, viz., 125 e, represents the quantity of the 1:25 solution necessary to make the quantity called for, viz., I quart.
To make a solution of known strength from tablets of a different known strength. This problem occurs especially in the preparation of a drug to be given hypodermically when the stock tablets on hand require division in order to procure the dose desired.
Atropine gr. 1/120 desired. Atropine gr. 1/100 on hand. Calculate what proportion of 1/100 is contained in 1/120. Thus:
1/120 % 1/100 = 1/120 * 100/1 = 5/6.
The resulting fraction gives the proportion of the stock tablet (gr. 1/100) required, e.g. 5/6.
Since the most accurate method of dividing the tablet is to dissolve it in a known quantity of water and take 5/6 of the resultant solution, we may proceed thus: Add to stock tablet gr. 1/100, e xxx of water:
5/6 x 30 = 25.