Isotonicity Calculation - Help needed

[norx]

I took this sample question straight from the Pharmaceutical Calculations textbook to study for a test. The reason why I didnt get the correct answer is because part of the ingrediant is already isotonic so that threw me off.

Tetracaine Hydrochloride 0.5% (0.18 NaCl Equivalent or E Value)
Sol. Epinephrine Bitartrate 1:1000 10 (0.18 NaCl Equivalent or E Value)
Boric Acid q.s.

Purified Water ad 30.0
Make isoton. sol.
Sig. Eye drops.

The solution epinephrine bitartrate (1:1000) is already isotonic. How many grams of boric acid should be used in compounding the prescription?


Answer: 0.294g

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[norx]

My Incorrect Method:

STEP 1
0.5% --> 0.5g/100ml * 30ml = 0.15g Tetracaine HCl
0.18 x 0.15g = 0.027g of Sodium Chloride represented by Tetracaine HCl

1/1000 * 10g = 0.01g Sol. Epinephrine B.
0.18 x 0.01g = 0.0018 of Sodium Chloride represented by Epinephrine B.

STEP 2
30 ml x 0.009 (or 0.9%) = 0.27 g of Sodium Chloride in 30ml of an isotonic sodium chloride solution.

STEP 3
0.27g (from Step 2)
-0.027g (from Step 1)
-0.0018g
--------------
0.241 g of sodium chloride to be used.

Therefore, convert to Boric acid --> 0.241g / 0.52 (sodium chloride equivalent of boric acid) = 0.464 g

 

[norx]

Question 2.

How many grams of anhydrous dextrose should be used in preparing 1 liter of a 0.5% isotonic ephedrine sulfate nasal spray?

FYI:

anhydrous dextrose "E- value" = 0.18
ephedrine sulfate "E-value" = 0.23

*E values are otherwise known as sodium chloride equivalents

 

[MountainPharmD]

Probably a good idea to ask one of your fellow classmates who did not miss the problem. You could always ask your instructor to explain it as well.

Honestly, the only time you do those problems is in Pharmacy school.

I of course could dust off my copy of Remingtons and my old calculations book and figure it out. Uhhhh...no thanks.....

 

[genesis09]

Just subtract out the 10 mL of Epinephrine bitartate, and do the isotonicity calculation for tetracaine only. Use 20ml and not 30ml, in short.

 

[Kristen Willard]

does anyone know how exactly to tell if a solution is isotonic, hypertonic, or hypotonic? I have a problem that says...
"1500ml of Dextrose 5% (E=0.18) in sterile water" is it hypo hyper or iso tonic?
-->i know that the answer is isotonic but i don't know how to figure that out!
Thanks!!

 

[drhenPharmD]

does anyone know how exactly to tell if a solution is isotonic, hypertonic, or hypotonic? I have a problem that says...
"1500ml of Dextrose 5% (E=0.18) in sterile water" is it hypo hyper or iso tonic?
-->i know that the answer is isotonic but i don't know how to figure that out!
Thanks!!

Dextrose 5% in water is always isotonic. It does not matter what the volume is.

Same with 0.9% saline solution. It is always isotonic no matter the volume.

 

[rxlea]

I appreciate your use of the search function, but SDN is not for homework help.