Mash Acidification: Diluting Phosphoric Acid by Weight

[SenorWanderer]

It's been a while since high school chemistry, so just wanted to check my math with the hive mind.

I bought a bottle of 85% food grade phosphoric acid from dudadiesel for mash acidification. I'd like to dilute it to 25% for use and the most accurate measurement tool I have is a scale.

I found a useful calculator for dilution, and I calculated that adding 100ml of 85% acid to 240ml of water will yield a 340ml solution at 25%.

Now all i need is the density of phosphoric acid. Google seems to agree that the density of 100% phosphoric acid is 1.87. Great. So how do I get to the density of the 85% acid? 85% of 1.87 is 1.59. But that's not the right answer. the density of the 15% water is 1, not 0.

I did the math for a weighted average (1.87 x 85)+(1.00 x 15)/100=1.74. That sounds pretty reasonable. Is there something else affecting it I'm not considering?

For some reason (because the interwebs), there is actually a calculator out there for this, and it says that an 85% solution at 20C is 1.70. The calculator is based on % by weight. Could the difference be in that I'm doing a volume calculation?

Bottom line for dilution: if I presume the density is 1.7 (g/ml) then the 100ml of acid I need will weigh 170g. The 240ml of water needed will weigh 240g!

Anyone who cares to chime in is welcome!

 

[jstaas]

If you can measure volume accurately I would suggest using: Concentration(start) x Volume(start) = Concentration(final) x Volume(final)

For a 100ml of 25% solution:

.85 x volume = 0.25 x 100
x = 29.4ml

For 100ml of 25% solution combine 29.4ml of your 85% with 70.6ml water.
I hope this helps.

 

[doug293cz]

It's been a while since high school chemistry, so just wanted to check my math with the hive mind.

I bought a bottle of 85% food grade phosphoric acid from dudadiesel for mash acidification. I'd like to dilute it to 25% for use and the most accurate measurement tool I have is a scale.

I found a useful calculator for dilution, and I calculated that adding 100ml of 85% acid to 240ml of water will yield a 340ml solution at 25%.

Now all i need is the density of phosphoric acid. Google seems to agree that the density of 100% phosphoric acid is 1.87. Great. So how do I get to the density of the 85% acid? 85% of 1.87 is 1.59. But that's not the right answer. the density of the 15% water is 1, not 0.

I did the math for a weighted average (1.87 x 85)+(1.00 x 15)/100=1.74. That sounds pretty reasonable. Is there something else affecting it I'm not considering?

For some reason (because the interwebs), there is actually a calculator out there for this, and it says that an 85% solution at 20C is 1.70. The calculator is based on % by weight. Could the difference be in that I'm doing a volume calculation?

Bottom line for dilution: if I presume the density is 1.7 (g/ml) then the 100ml of acid I need will weigh 170g. The 240ml of water needed will weigh 240g!

Anyone who cares to chime in is welcome!

Assuming the 85% solution is 85% by wt, then 100 g of the original solution will contain 85 g of H3PO4. 85 g in a 25% by wt solution gives a total solution wt of 85 g / 0.25 = 340 g. So you need to add 100 g of 85% solution to 240 g of water to create 340 g of 25% solution.

Brew on

 

[SenorWanderer]

Thanks for the replies.

The point of this was to be able to use my .001g scale to measure the acid, as it's much more accurate than the various graduated cylinders I have that are +/- 5% accuracy.

I finally got around to needing some acid this morning so I followed the procedure I outlined above and it worked out great.

98.6g of 85% phosphoric acid, added to 142ml (g) of distilled water yielded a 200ml solution. I have to presume then, that the figure of 1.7 g/ml for 85% phosphoric acid is accurate, at least in this context.

Don't for get to ADD ACID to the water when mixing chemicals!

 

[brewkinger]

Don't for get to ADD ACID to the water when mixing chemicals!

Agreed. Safe lab practices will always benefit you.