Appendix C. Measuring and Mixing

One of the most important skills any chemist must master is the ability to accurately weigh out chemicals without contaminating them. You really need two balances[1] for the projects in this book, one for weighing big things and one for weighing little things. The big balance we'll call a gram balance because it can weigh things to the nearest gram. These jobbies usually have a capacity of 2 kg, which means you can load them down with 2 kg of stuff before they max out. The little balance we'll call a centigram balance; it can weigh to the nearest 0.01 g and usually has a capacity of 200 g. You'll use the centigram balance whenever the thing you are weighing weighs less than 200 g and the gram balance whenever the thing you are weighing weighs between 200 and 2000 g.

You might think weighing things out would be simple; just put a container onto a balance and spoon stuff into it until it reads the right amount. There are two problems with this approach. Let's say that you are spooning silica and plaster into a plastic bag. You spoon in the right amount of silica but you get a little too much plaster. What do you do? You reach in and spoon out a little plaster. But plaster and silica look pretty much the same, so how do you know which one you took out? The second problem is that you might be tempted to put the extra "plaster" back into your container of plaster. After all, why waste it? But chances are you just spooned a little silica back into your container of plaster. Over time all those little bits of contamination will add up and pretty soon all your chemicals will be contaminated. There's a simple method for avoiding these problems. It's called weighing by difference.

Let's go back to the example. You have a container of silica and a container of plaster. Your goal is to get, say, 20 g of each into a bag. First place the container of silica on the balance and press the button marked "tare."[2] The balance will now read zero. Spoon some silica from the container into the bag. The balance will now read, say, -2 g because the container has 2 g less silica than it had when you tared the balance. Suppose you keep spooning and with the last spoonful the balance reads -22 g. What do you do? The spoon is still sitting over the container on the balance. It hasn't been anywhere to get contaminated. Just flick it with your finger until the balance reads -20 g and then dump the last spoonful into the bag. If you flick too much and get -19 g, just reach in and grab a little more until you get it just right. Now clean off your spoon, take the container of silica off the balance, put the container of plaster on the balance, and press the tare button again. It reads zero, of course. Spoon some plaster into your bag. Suppose that with the last spoonful the balance reads -23 g. The spoon still hasn't been anywhere to get contaminated. Just flick it with your finger until it reads -20 g and dump the last spoonful into the bag. Your silica container still contains silica and your plaster container still contains plaster. Your bag contains exactly 20 g of each. Weigh by difference and you will keep all of your chemicals as pure as the day you made them.

It may seem strange to you that I had you weighing things into a bag, but that's a great way to mix dusty materials. With silica and plaster, for example, you can seal the plastic bag and then knead it, shake it, massage it, and turn it end for end until the silica and plaster are all mixed up and you won't get dust everywhere. If the powders are flammable, I like to use those anti-static plastic bags like circuit boards come in. That way, you won't have static electricity accidentally set it off.

I buy silica in 50-pound bags and you might wonder how I can weigh it out if the balance has a capacity of only 2 kg. I use 1-pint plastic tubs like they use at Chinese restaurants to store each of the chemicals I use frequently. I label each container and its lid so that the silica container never gets used for anything else. These containers are just the right size for a gram balance. I keep large amounts of chemicals in 5-gallon pails, like paint comes in. I always take chemicals from a big container into a smaller container, from the 5-gallon pail to the 1-pint tub, from the 1-pint tub to the plastic bag. As long as I keep this in mind I will never contaminate more than a little bit of chemical.

You can weigh liquids by difference just as you can solids. Just pour the liquid into a cup, place the cup on the balance, tare it, and use a medicine dropper to transfer liquid from the cup to wherever it's going. But we often measure liquids by volume rather than by weight. You can use a graduated beaker or measuring cup for large volumes, graduated a graduated cylinder for smaller ones, and a graduated pipette for really small volumes.



What most people call a "scale," chemists call a balance. You may use either mechanical or electronic balances.


On some balances the tare button is marked "zero."