Yes. Just dilute it in the correct ratio to bring it to the required concentration. The purity of the water is not particularly important. (If it's drinkable, it's perfectly OK to use. Go ahead and use tap water if you want.)
The formula is simple: 'Concentration Required' divided by 'Concentration Supplied' multiplied by 'Final quantity Required'. That will give you the amount of 91% isopropyl alcohol you need to measure out, then dilute to the 'Final quantity required' with your tap water. (Or better still, use Distilled Water or De-Ionized Water.)
So: You want 500 mls of isopropyl alcohol 70% and you have isopropyl alcohol 91% on hand -
Amount of isopropyl alcohol 91% needed = 70 / 91 x 500 = 384.6 mls
Dilute that 384.6 mls to 500 mls with water and there's your 500 mls of isopropyl alcohol 70%.
Of course, you don't need to be quite so finicky when it comes to biological measurements. You'd round off the 384.6 mls to 385 mls (or even 380 mls or 390 mls) without any appreciable loss of effectiveness.
The formula is simple: 'Concentration Required' divided by 'Concentration Supplied' multiplied by 'Final quantity Required'. That will give you the amount of 91% isopropyl alcohol you need to measure out, then dilute to the 'Final quantity required' with your tap water. (Or better still, use Distilled Water or De-Ionized Water.)
So: You want 500 mls of isopropyl alcohol 70% and you have isopropyl alcohol 91% on hand -
Amount of isopropyl alcohol 91% needed = 70 / 91 x 500 = 384.6 mlsDilute that 384.6 mls to 500 mls with water and there's your 500 mls of isopropyl alcohol 70%.
Of course, you don't need to be quite so finicky when it comes to biological measurements. You'd round off the 384.6 mls to 385 mls (or even 380 mls or 390 mls) without any appreciable loss of effectiveness.