That short question requires a long answer! The answer is, it depends on what you are diluting with. For our purposes I assume you are asking if diluting with Normal Saline or D5W. Effective dilution requires a good buffering agent, both NS and D5W have poor buffering ability. A practical example of this would be Vancomycin.
Most Vanco dosing in 0.9% NS has a ph around 3.9. D5W has slightly better buffering ability but the ph still ranges from 2.5 to 4.5, range is dose dependent. Premixed Vanco, which is optimally buffered, ranges from 3-5. Someone once told me to get Vanco from 3.5 to 4.5 requires 10 liters of NS, but I don't have a official reference for that. I often hear from pharmacists "No that drug is fine for a PIV I can dilute it". A great comeback is " Can you dilute it so I can meet the standard of practice?"
Since the pH of water is 7, it WILL change the pH of a basic or acidic solution when it is added, but probably not enough. For example - if you mix one ounce of a solution that has a pH of 3 and one ounce of water (pH=7) the resulting solution will not have a pH of 5. It will still be close to 3. It's important to remember that the pH scale is a logarithmic one. A change of 1 in pH reflects a 10-fold change in the concentration of H+ and a change of 2 in pH reflects a 100-fold change in H+ concentration, etc. I haven't heard of 10 liters of NS to change Vanco pH from 3.5 to 4.5 before...but don't find it impossible to believe!
That short question requires a long answer! The answer is, it depends on what you are diluting with. For our purposes I assume you are asking if diluting with Normal Saline or D5W. Effective dilution requires a good buffering agent, both NS and D5W have poor buffering ability. A practical example of this would be Vancomycin.
Most Vanco dosing in 0.9% NS has a ph around 3.9. D5W has slightly better buffering ability but the ph still ranges from 2.5 to 4.5, range is dose dependent. Premixed Vanco, which is optimally buffered, ranges from 3-5. Someone once told me to get Vanco from 3.5 to 4.5 requires 10 liters of NS, but I don't have a official reference for that. I often hear from pharmacists "No that drug is fine for a PIV I can dilute it". A great comeback is " Can you dilute it so I can meet the standard of practice?"
Stephen Harris RN, CRNI, VA-BC
Chief Clinical Officer
Carolina Vascular Wellness
Great reply Stephen -
Since the pH of water is 7, it WILL change the pH of a basic or acidic solution when it is added, but probably not enough. For example - if you mix one ounce of a solution that has a pH of 3 and one ounce of water (pH=7) the resulting solution will not have a pH of 5. It will still be close to 3. It's important to remember that the pH scale is a logarithmic one. A change of 1 in pH reflects a 10-fold change in the concentration of H+ and a change of 2 in pH reflects a 100-fold change in H+ concentration, etc. I haven't heard of 10 liters of NS to change Vanco pH from 3.5 to 4.5 before...but don't find it impossible to believe!
Kathleen Mazza, RN, MBA
Chief Operating Officer
Advanced Care, Inc.
Kathleen is correct - dilution of any drug does not alter its pH enough to be clinically relevant. I would recommend you read
1. Stranz M. Adjusting pH and osmolarity levels to fit standards an practices. Journal of Vascular Access Devices. 2002;7(3):12-18.
Marc is a pharmacists that has published and presented numeorus times on this issue. Lynn
Lynn Hadaway, M.Ed., RN, BC, CRNI
Lynn Hadaway Associates, Inc.
126 Main Street, PO Box 10
Milner, GA 30257
Website http://www.hadawayassociates.com
Office Phone 770-358-7861
Great Question. Where is a Chemist when you need one?