Sunday, March 20, 2011

Do I Really Need A Scale?

As a follow up to Erik's posting of my experience with Phenidone / Vitamin-C / Borax developers, a theme that comes up often is the issue of accurate measurement.  There has been, and still is, considerable debate over the methods and accuracy of how we measure the quantities of chemicals we use in our developers.  Ultimately you cannot beat an accurate scale but for what we're doing, is a scale really necessary?  Volumetric measurement (e.g. teaspoons) of dry ingredients is a source of constant debate so we'll leave that for another day and examine a different approach ... percentage solutions.

For the sake of this discussion, percentage solutions contain a certain amount of ingredient X g in a quantity of liquid Y mL (e.g. 10g of Sodium Bicarbonate in 100mL water is a 10% Sodium Bicarbonate solution).  We will also assume general room temperature (20C) as funny things can happen when you significantly decrease/increase temperature of solutions.  Advantages of percentage solutions are that you don't necessarily need a scale, measurement accuracy is significantly improved, they are easy to handle, and can be easy to prepare.  There are two approaches that I use to avoid the scale, the first is to purchase pre-measured and convenient quantities of ingredients, the second is to use saturate solutions.

Is it possible to accurately measure 0.15g of phenidone with a teaspoon ... the answer may surprise you.  Using pre-measured quantities is almost cheating but it works so we'll use it.  The most common example from my own work is measuring phenidone.  It's used in such small quantities that even "good" scales can have a hard time.  I learned from various forums that phenidone, among other things, is soluble in propylene glycol.  What is particularly convenient is that you can easily get 10g of phenidone and 1L of propylene glycol.  Both quantities are pre-measured so all you need to do is dissolve the entire 10g bottle of phenidone in the 1L of propylene glycol and you now have an accurate 1% solution.  When you need 0.15g of phenidone just measure 15mL of this 1% solution.  Incidentally, 15mL is exactly 1 standard tablespoon or 3 teaspoons so to answer the question about accurately measuring 0.15g of phenidone with a teaspoon, the answer is a definitive YES!  Another common example is sodium hydroxide.  A 100g container of the stuff in 1L of water makes an accurate and convenient 10% solution.  As in life, nothing is free and there is a catch ... not everything can be put in solution without consequence.  For example, adding phenidone to water will quickly oxidize it and make it useless.  Before making a solution make sure that the components are compatible.

Saturate solutions leverage simple chemistry to ensure a consistent quantity of ingredient in a quantity of solution.  Many dry ingredients have different states of hydration that makes accurate measurements difficult.  Often you do not know how much of what you weigh is the actual ingredient and how much is water.  Saturate solutions can eliminate the water variable and facilitate accurate measurement.  Using borax as an example, I learned that 4.71g of borax dissolves in 100mL of water at 20C ( and any excess simply remains undissolved.  If we add 10g of borax to 100mL of water and let as much dissolve as possible you will get a clear solution of 4.71% borax on top with excess at the bottom.  Use the same math as before to make whatever quantity you need using the solution.  As you use the solution add water to replace what you have used and ensure there is excess borax at the bottom.  This will ensure a constant 4.71% solution as the replaced water will dissolve more borax.  As in the percentage solutions, make sure the components are compatible (e.g. don't put phenidone in water).

Do percentage and saturate solutions eliminate the need for a scale ... sometimes.  In the case of the PCB developer I use in the previous post I can accurately mix the entire developer without a scale.  Even if you have an accurate scale (I do), mixing solutions is much less tedious and quicker than dealing with dry ingredients (try mixing borax in water ... you'll be there for a while).  These methods won't necessarily work for all the ingredients we use but when we can use them the general convenience is appreciated.


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