# Conversion help for making a solution - (Jan/20/2012 )

I'm trying to make a solution off a protocol that basically reads:

2% A (liquid)

1% B (solid)

.1% C (solid)

.1% D (liquid)

.01 M PBS, pH 7.2

etc. So if I choose to use 40 mL of PBS, then is it correct to calculate the amounts of the other reagents by:

liquid

40 mL PBS * .02 = .8 mL A

solid (the one I'm rather confused on)

40 mL PBS * 1.85 g/mL (sd of .01 M PBS) = 74 g

74 g PBS * .01 = .74 g B

?

Thanks in advance for any help.

When calculating the ratio of liquid to be added to make a solution, one usually uses volume: volume (v/v), and when expressing the amount of solid, it is usually weight:volume,so yes, your calculations are essentially correct. Note that the density of PBS is very close to that of water (http://www.proscitech.com.au/cataloguex/msds/apbs-1l.pdf and http://www.hdscientific.com.au/media/msds/cp%20l%20salipbs%20-%20phosphate%20buffered%20saline.pdf for reference), so its safe to use 1.0g/ml. I am not sure from where you got a specific gravity of 1.85g/ml.

A: 0.02 * 4ml = 0.8ml

B: 0.01 * 40g = 0.4g

C: 0.001 * 40g = 0.04g

D: 0.001 * 40ml = 0.04ml

So you would add 39.16ml of PBS, and then the above reagents (hopefully the solids don't affect the volume dramatically, if they do, then just add the above to, say, 30ml of PBS, then top up to 40ml in a measuring cylinder while stirring)

Thanks for your reply lamaksha77. I was accidentally looking at the specific gravity of 1% PBS-TBN which is 1.85. Thanks for clearing this up for me!