TEK - Liquid Measurement of Research Chemicals

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Hello all - I'd like to post a comprehensive guide to liquid measurement of RCs. I did a search and didn't find a post with this content. Please don't be shy if you have any suggestions.

It is assumed you possess a scale accurate to at least 0.01 grams. While a scale accurate to 0.1g may be used, you will see in the math later that other logistical issues pop up that make a a more accurate scale desirable.

Scales accurate in this range may be purchased for roughly 50 bucks or so. A scale is essential - what this liquid measurement method does is quantify the error present in your entire measurement method and provide a method to manage it. It does not remove the requirement for a scale.

First off - we must define a few variables.

1. The individual dose desired (D).
2. The allowable error in measurement for that dose (Ed).
3. The known error in measurement from a scale (Es).

Suppose I seek to consume 15mg of NaCl in solution (D=15). I wish for this to be accurate within 2 mg either way worst case (ie 13-17 mg is acceptable, Ed=4). In all likelyhood I will be more accurate, but these are the established boundaries for this experiement. My scale is accurate to 0.01g or 10 mg (Es=10).

We assume a worst case scale error - that the measurement is off by 2 times the accuracy of the scale (ie worst case rounding errors. This is highly conservative).

We must then solve for the total number of doses to be divided up so that our accuracy is maintained. We must equate to our allowable error and solve for number of doses N.

Ed/100 = (ND + 2E)/(ND) - 1

0.04 = (15N + 2(10))/(15N) - 1

Now solve for N, N=34. Therefore 34*15=510 mg of NaCl is weighed out.

This assumes your measurement from this point forward is accurate. 340 mL of distilled water is measured precisely and added to the measured compound. This provides a solution with a concentration of 15 mg/10 mL, which is easily dispensible by a liquid syringe. The solution itself should be kept chilled in amber glass for optimum stability.

There is a very strange perception in the RC community that current measurement methods are far more accurate than what is actually experienced. I hope this guide helps, and makes one realize the importance and rigor of proper measurement.

And in closing, to get back to the 0.1 scale - your N would be 10 times that of this example, ie 340 doses. Unless you have a very large research facility, this is not practical.

Reputation Comments on this post:

	solid tutorial
	This is a fantastic post. Serious, worthwhile contribution to the community as a whole. My hats off to you :)