## Question:

I enjoy your specific, accurate responses to questions, even when the subject is out of our “field of view.” I’m the part-time environmental/safety manager for a small custom coater. We use both solvent and water-reducible coatings. While our suppliers provide VOC data, we still have to derive the VOC content of the coatings as applied since we may add solvent and/or water to the coatings. Part of our job is to file monthly coatings VOC reports with our state environmental authority. These are being done in a timely, thorough, but wholly by rote fashion using spreadsheets developed by my predecessors. The underlying chemistry of the various calculations, such as “VOC minus water” and “weighted average,” remains a mystery to me. Can you shed some light on this matter? Thanks for any assistance. T.L.

## Answer:

T.L., hopefully these explanations will part the cloak of VOC mystery. The term “pounds of VOC minus water” means how much volatile organic compound (VOC) does the coating contain if you took the water out of it. For example, suppose you have a coating that contains 1 lb VOC/gal and 40% water by volume. If you took out the water, you would now have 1 lb VOC in 0.6 gal of coating or 1.67 lb VOC/gal minus water. The reason that EPA makes this distinction is to prevent coating suppliers and users from “diluting” their coatings with water in order to achieve compliance with VOC content limits. We have found the easiest way is to calculate the VOC content with water so you can more easily calculate emissions (lb VOC/gal x gal = lb VOC) and then remove the water as above to verify compliance.

“Weighted average” means that the VOC concentration is weighted based upon usage and is calculated as follows:

(C_{1} x Q_{1}) + (C_{2} x Q_{2}) + ...(C_{n} x Q_{n})/(Q_{1} + Q_{2} + ... Q_{n})

where C_{1} = VOC concentration of a coating, and Q_{1} = usage of the coating with its respective VOC concentration.

In contrast, a strictly “average” concentration would be equal to (C_{1} + C_{2} + ...C_{n})/n.

A few other terms in calculating VOC content of coatings are density and specific gravity. Density is usually defined in term of weight per unit volume (for example, lb/gal). Once in a while people call specific gravity, “density.” Specific gravity is the ratio of a material’s density as compared to water. Since water density is 8.34 lb/gal, a specific gravity of 1.0 means the material’s density is also 8.34 lb/gal. Most solvents are lighter than water with a specific gravity of, let’s say, 0.8 or a density of (0.8 x 8.34) 6.7 lb/gal.

Last, knowing weight and density or specific gravity, you can calculate volume of solvents. Let’s say that the VOC content is 3 lb/gal and the solvent has a specific gravity of 0.8 or 6.7 lb/gal. Take (3 lb/gal) / (6.7 lb/gal) = 0.45 or 45% by volume. I hope this helps.