Please help with blue dye question!

[Chemshavon]

Hi! I am doing the project listed on this site as Make a Colorimeter to measure the concentration of blue dye in various liquids. I have built my breadboard and was just starting to make my solutions when I realized I may not have the correct dye for the project. I have liquid food coloring- McCormicks from the grocery store which I did make sure contains FD&C Blue #1. But now I am wondering if I am supposed to be using the concentrated, powdered form? Or is my "regular" food coloring ok? Thanks in advance for any help.

[deleted-71840]

Hello,

Let's dig into this Project Idea so that we can come up with a good path forward for your project. My apologies if this reply is long, but you've pointed out a very valid question and I want to make sure that I do it justice.

I reviewed the project idea that you're looking at, and you pose an excellent question. The objective of the Project Idea is to use known concentrations of food coloring from a commercial sample of blue dye to determine the concentration of blue dye in an unknown sample (such as a sports drink). The key thing here is having a standard or reference sample - that is, a sample of known food coloring concentration that you can compare your unknown samples against.

The question you raised is a very good one - which type of food coloring should be used, and what is the concentration of this dye? After all, the commercial food coloring you obtained will be your reference against which you measure your unknown samples - what is its concentration? Does it vary by brand / type?

I agree that there is ambiguity in this Project Idea procedure. Part 2, Instruction 6(a) of the procedure states:
The concentration of blue dye in the commercial package is approximately 0.026 mol. After dilution (1/8 tsp in 1 cup = 1:384 ), the concentration is 68 micromolar (µM) .

I interpret this to mean that the Project Idea author has done some research and concluded that the concentration of off-the-shelf liquid food dye (such as the McCormick's brand that you have bought) has a concentration of 0.026 moles per liter. I do not believe the author's intention was for you to use pure, powdered FD&C Blue #1 food coloring - if that were the case, it would not make sense to define a "concentration" of the pure substance.

(As an aside, it is actually not correct for the Project Idea procedure instructions to use "0.026 mol" as a concentration - it should state "0.026 molar", which means 0.026 moles of dye per liter of solution. "mol" is an amount; "molar" or "M" is a unit of concentration which is defined as "1 mole of substance per liter of solution". The Project Idea does later use the correct units when defining the diluted concentration as 68 micromolar (µM).)

The ingredients of the McCormick's Blue Food Dye you procured probably list something like: "Water, Propylene Glycol, Blue Dye #1, Propyparaben" as ingredients - the exact ones may vary by brand. Ingredients listed on food products don't give concentrations of each substance - only what materials are present in the food. Exact values per serving of certain substances are required to be listed by the FDA (for example, sodium or sugar), but unfortunately for us, FD&C Blue #1 is not among these!

I agree that we are left with a bit of ambiguity - and you are right to raise the question you did. Questioning things rather than simply accepting them is a hallmark of a good scientist!

Based on all this information, we are left with a couple options:

1) You can follow the Project Idea through using the assumption that the author's concentration of "0.026 M" applies to undiluted McCormick's Blue Food Coloring. If you go this route, be sure to use the Project Idea as your reference so that people know where this number came from. My recommendation is to do the experiments using "1/8 teaspoon of McCormick's Blue Food Coloring dissolved into 8 fl. oz of water" as your definition for your full strength solution, rather than the molar value given in the instructions. Then, add a footnote to your project. Indicate in this footnote that the original, undiluted concentration of McCormick's Blue Food Coloring is 0.026 M according to the Science Buddies Project Ideas website, which would mean that your "full strength" solution of 1/8 tsp diluted into 8 fl. oz of water has a concentration of 68 µM, and finally emphasize that you have not been able to experimentally verify that number.

2) If you have the time and resources, you could expand upon this Project Idea by using a known concentration that you yourself have created, and experimentally verify the concentration of McCormick's Blue Food Coloring. You could obtain a pure sample of FD&C Blue #1, use an accurate scale to measure a precise amount of powder, and then dilute it with pure water so that you yourself have verified a precise concentration. Or, devise some other way to validate the concentration of McCormick's dye.

The essential purpose of this experiment is to measure an unknown dye sample against a reference dye sample. The Project Idea's instructions on diluting the reference concentration into full, 1/2, 1/4, 1/8, 1/16, 1/32, and 1/64 strength are valid regardless of whether you know the original concentration in units of "moles of FD&C Blue #1 per liter" or in units of "fractions of McCormick's Blue Food Coloring solution, with the solution concentration defined as 1/8 tsp McCormick's Dye into 8 fluid ounces of water". My recommendation is to go through with the project with the latter phrase as your reference, measure the sports drink against that, and then in the footnotes describe what that concentration translates to in moles per liter according to the Science Buddies Project Ideas website. If you are feeling ambitious, expand upon your experiment and find a way to verify the Science Buddies Project Ideas number.

Please feel free to respond with any followup questions you may have, and good luck!