ALLIGATIONS

Purpose Used when you need to prepare (compound) a percent strength that you do not have in stock. To make the strength you need to use TWO other strengths to attain the correct one.

EXAMPLE If a doctor orders 20% KCL, but you only have 10% and 50% on hand, you would have to calculate the amount of each solution to attain a 20% solution. You can only do this if at least one of the two strengths is less concentrated than the final solution. This means that you cannot make a 20% solution from a 5% and a 10% solution. Water can be used as another element for a solution and water is considered 0%.

PROBLEM You have in stock 70% solution and a 20% solution. How much of each do you need in order to create 1L of 40% solution? We will work it together coming up…. Please have a scrap piece of paper available.

Use a simple TIC-TAC-TOE diagram following these rules: Draw a tic-tac-toe board. Place your desired strength in the middle square. 40%

Use a simple TIC-TAC-TOE diagram following these rules: Put your higher strength solution in the top left square. 70 70 .40 .40 20 Put your lower strength in the bottom left square. If you are using water, you will place a zero in this square.

Tic-tac-toe diagram (Continued) Subtract the lower number (top left or middle) from the lower number (top left or middle) and put that # in the bottom right square. Then do the same with the middle and bottom left squares and place that # in the top right square. .70 .20 .70 .20 .40 .40 .20 .30 .20 .30 Then add the right top and bottom numbers. Divide the top right by this new #. Then, divide the bottom right by this new number. .20 / .50 = .40 and .30/.50 = .60

Tic-tac-toe diagram (Continued) Multiply both numbers (individually) by the total volume that you need. This will be how much you will need of each solution. .40 X 1000ml (1 L) = 400 and .60 X 1000ml = 600 Check your answer by adding the two parts. They should equal the total volume. 400 + 600 = 1000ml (1 L)

Practice problem Prepare a 20% KCL 500ml from your stock of 5% and 70% KCL. Answer: 70% = 115ml 5% = 385ml