My curiosity has gotten the best of me.

[bobwysiwyg]

Why, when you read the directions for mixing D76 for example, does it suggest doing so in 800ml of water and then adding 200 more to make 1000ml? Why not just mix the powder in 1000ml to start with? Surely the 200ml added won't reduce the mixing temp that much?

 

[David A. Goldfarb]

Does it really say to add 200ml, or does it say to add water to make 1 liter? Usually the instructions would say to add water to make 1 liter to account for the added volume of the chemicals in solution.

 

[bobwysiwyg]

David,

You are correct. The instructions say mix in 800ml then bring it up to 1000ml. I took liberties there. Still has me wondering though, unless I have forgotten my high school physics, a true solution does not increase volume does it?

 

[Ole]

You did physics and not chemistry, I guess? Some salts will decrease the volume, some will increase it. Most will increase the volume by something very close to the volume of the dry chemicals.

 

[bobwysiwyg]

Thanks, curiosity now satisfied.

 

[Aurum]

The instructions are a very standard method used by all chemists making standard solutions. Dissolve the solids in a reasonable amount of water, then once its all gone in make up to volume.
Its more accurate, and easier to adjust, and allows any changes in volume or SG to be ignored.

The real fun comes when you're mixing up bulk amounts of exothermic chemicals. I've used crushed ice made from distilled water to do this if I'm in a hurry

 

[Ian Grant]

An additional reason for mixing in 700-800ml is you can use fresh water to dissolve any residual crystals that haven't dissolved before topping up to 1 litre.

Ian

 

[titrisol]

Volumes are rarely additive, and when making solutions some water can be formed by chemcal reactions.
Thus it is safer to top up to 1000 later.