Weighing Small Quantities of Compounds With Digital Scales

Weighing out defined amounts of materials is a key step in most methods in which chemistry is involved. From baking a cake, to baking a batch of high temperature superconductor, it is necessary to have exactly the right amount of each component if you want the product to come out right. While volume measurements may be good enough for some tasks, such as baking bread, even then using exact weights results in more consistent and reliable results. For more formal applications of chemistry, they are essential. So much so that there is a term for calculating these measures: stoichiometry. But calculating how much is needed is only the first step, one needs to know how to physically weigh out these materials.

Furthermore, to minimize use of expensive reagents and need for large pieces of expensive glassware, it is common to perform very small scale experiments using at most a few grams total material, with some components used in amounts well under 1 gram. Sometimes only a few tens of milligrams. While those training in academic labs or formal classwork will usually be taught how to weigh out such small quantities using an analytical balance, this tends to be technique passed along word of mouth and even then rarely described for less expensive and more available scales. Opportunity to learn these important skills may be even less available to those engaged in home science for STEM enrichment or classes before the final years of high school or introductory college chemistry. This instructable aims to bridge that gap in opportunity to provide methods for weighing out very small amounts of material on inexpensive digital scales. It was created as a companion instructable to one describing making a synthetic pigment, but since the method is more general and not previously described, I chose to make it separate so that it was not buried inside a larger protocol. And while this is geared towards weighing materials for chemistry, it is not specific for it.

You can find everything described here on Amazon, eBay, Etsy, and direct from China sites like Ali Express or Banggood.

1. Digital scale: you will need one suitable to the maximum weight and sensitivity you will be measuring to. Scales listed as 100 x 0.01 have a maximum weight of 100g and a sensitivity of 0.01g. If you need to measure quantities such as 2.36 grams, you need at least a 0.01 sensitivity. If you are measuring weights under 1 gram with any frequency, I recommend getting a 0.001 sensitive scale. These are no longer outrageously expensive, reliable models can be found for about US$20. They do however tend to have a small maximum weight, usually 10grams. 0.01 sensitivity scales tend to have higher maximums such as 100g or 200g and cost less, typically under US$10. High sensitivity scales also having high maximum weight will be more expensive. It may cost less to get an inexpensive 10 x 0.001 scale and a inexpensive 200 x 0.01 scale over a 100 x 0.001 scale.

2. One or more standards. These are weights that are carefully prepared to weigh an exact amount, such as 5.00g. Being able to weigh an item known to be 5 grams and seeing your scale show 5.00 or 5.000 grams is very confidence inspiring. They may come as sets or single weights. Search for "calibration weight" or "calibration weight set" on any of the usual recommended sites to find options for pretty much every budget, I recommend getting at least one standard. Many scales come with one that can be used to calibrate the scale according to the maker's instructions. Typically one puts the weight on the scale and put the scale into calibration mode. It will then use the measured weight to set the scale. Even if your scale cannot do this, if you know that when weighing 1.00g, your scale always shows 1.05g, you can take your measured 0.90g and call it 0.85g. Though if it starts to show increasing deviations or constantly drifts, it is time for a new scale, especially given the low cost of many scales. Be aware that cheap weights such as those provided with inexpensive scales are usually made of steel with solder cores and any rust will alter its weight. Higher quality weights are usually made from brass. Keep all weight standards dry and away from corrosive chemicals for longest life.

3. Weigh boats and glassine papers or similar items to weigh on: These include plastic weigh boats for weights 0.3g or so and higher, as well as glassine papers for smaller amounts of materials or objects. Small disposable plastic or paper disposable cups may be used in place of weigh boats and parchment paper for cooking, or most very glossy papers may be used in place of glassine papers. However, both of these tend to have greater issues with material sticking to them. To make the glassine paper easier to handle and pour from, make a crease in it by folding it in half and opening it back up again.

Regardless of the exact container or paper used, they all serve 3 purposes. First, to contain what is being weighed out , second to permit easy transfer off the scale, and third to minimize contamination of the the scale. When you put the paper or weigh boat on the scale and hit tare, it 'zeros' the scale by subtracting the weight of the weigh boat from the measured weight.

4. Material transfer items: these include small spatulas, "scoopulas," and painter's spatulas. These are items you can pick up a little of a powdered or granular substance for transfer. In some cases, disposable wood items like coffee stir sticks, popsicle sticks and tongue depressors may also be useful.

A. Weighing pad: this is where you put what you want to weigh, and whatever you are weighing it on such as a weigh boat or glassine paper. Be gentle when placing items here.

B. Display: displays the weight and current units. Often will display if tare function is active.

C. On/Off: self explanatory. Most digital scales will automatically tare when they are turned on. That is, they will assign zero weight to what ever is on the weighing pad when they start up.

D. Units or Mode: this will change between different units, typically grams and ounces and other more rarely used units of weight such as grains or carats.

E. Tare: this will assign a weight of zero to whatever is on the weighing pad.

F. PCS button: this is a counting function that determines how many items of a given weight are being weighed. We will not be using this function and it is not found on all scales.

G. Light: found on some scales to turn the display backlight on and off.

IMPORTANT NOTE ON SCALES RE-ZEROING THEMSELVES: many scales after taring will continue to reset to zero if only a very small weight is applied. This can cause serious errors as it can happen several times in row so long as each addition is too small to be recognized by the scale as something being weighed. The scale may re-zero itself after each addition. One way to avoid this problem is to make the first addition of the substance being weighed relatively large, several times the sensitivity (if the scale has a sensitivity of 0.01, try to add >0.1 grams as the first addition). A more reliable approach especially when weighing out less than 1 gram is to place a small easily removed item (such as a coin or small calibration weight) to the weighing platform. This forces the scale out of autotare mode so it will not re-zero itself. All or some of the material being weighed may then be added. Removing the small item reveals the true weight, and if necessary, additional material can be added to complete weighing out the material.

I recommend use of a standard 1 to 5g weight for this purpose. The true weight can be found without multi-digit calculations simply by ignoring the weight of the standard. If a 2g standard (with a measured weight of 2.000g) was used, a display weight of 2.075g can be read as 0.075g. This makes it simple to complete the weighing process before removing the standard. I still recommend doing so to double check that the weight remains correct.

When weighing very small amounts, it can be challenging to transfer material from a bottle to a weigh boat or paper. There are two general methods, pouring the material out and using a tool for transfer. Pouring out has the advantage of avoiding potential contamination of stock bottles. In general, good lab practice dictates that contamination of stocks should be avoided. However it may be difficult to transfer from large bottles, as well as transfer very small amounts of material. When pouring is difficult, the use of transfer tools is more appropriate. In many cases, as long as tools are kept clean between use with different materials, the risk is small and there is little chance for enough cross contamination to cause problems. In this case, use of transfer tools to remove material from stock jars poses little concern. If it is a concern given the purpose the weighed material is intended for, one solution is to pour some material from the stock bottle into a secondary container and use the tools only in this secondary stock. Unused material in the secondary container can be tossed out if inexpensive, or kept for the next time this material needs to be weighed out. If kept, be sure to properly mark the container's contents

Pouring out powders and granulated solids: unfortunately, trying to simply pour out powders as one does with liquids usually results in nothing happening until an avalanche comes out all at once making a huge mess. The method I have found works best is to slowly tilt the bottle while tapping on the rim of the bottle. This helps prevent the avalanche. As the material approaches the lip of the bottle slow or stop tilting and tap a little harder. Typically with each tap a little powder or a few granules will come out. This method tends to work best when weighing out quantities in excess of one gram, and is best when several grams are to be weighed out from a stock bottle or jar. This technique is shown in videos below entitled "tap from bottle" and "tap from large jar."

Using transfer tools: holding the tool in one hand scoop up a small amount of material onto the end of the tool. When the amount scooped up is less than the amount to be transferred in total, just dump the scoop into the container or onto the paper. Try to adjust how much you scoop up to allow you to transfer all the material scooped up. Eventually it will be necessary to transfer just a little from the scoop . At this point, holding the tip of the tool over the container or paper on the weigh platform, lightly tap the tool with a finger near where the other hand is holding it. If too much is consistently dropping off, try scooping up less material and tap lighter on the tool or tap on your hand rather than the tool. This method works well with powders using both flat tools and spoon-like ones, while granular materials usually work better with spoon-like ones. It is often possible to add only a few milligrams at a time this way.

To illustrate this method, a video ("tap15mg") is provided as a supplementary file below demonstrating the use of a transfer tool and tapping technique to weigh out 15mg of manganese (III) oxide onto a glassine paper. Note that it also shows the technique of adding a weight (a 1 gram standard) to the scale after tare in order to ensure that the scale does not try to re-zero the scale after addition of only a few milligrams. I strongly recommend doing so any time you are weighing out less than a gram, although once you have added a hundred milligrams or so, you can remove the weight standard at any time and the scale will not attempt to re-zero itself.

To practice and get familiar with these techniques, try weighing out flour or powdered sugar for powders, and table salt for granular substances.

Please note that although I show use of paper cups in some examples, this is due to photos on hand and time constraints. I recommend use of weigh boats and especially glassine papers for weights under 1 gram of material.

1. Place the scale on a stable flat surface such as a table or countertop. Because most digital scales autotare, you can place the weigh boat or glassine paper on the weighing pad before you turn it on. If you placed the weigh boat or paper on the pad after turning on the scale, press the tare button to zero the scale. When weighing out 1 gram or less, I strongly recommend adding a small standard weight before first addition of the material to be measured in order to prevent problems with scales re-zeroing themselves as described in step 1, parts of your scale.
2. Add some of the substance being weighed into the weigh boat or onto the paper, a little at a time using one of the methods described above. If the display continues to read zero, or turns back to zero, see "IMPORTANT NOTE ON SCALES RE-ZEROING THEMSELVES" in step 1, parts of your scale. This should not be a problem if you followed the advice of adding a weight standard to force the scale out of auto-zero mode. If you placed such a weight and a very small addition does not increase the displayed weight, it means the addition was less than the sensitivity limit. This is not a problem, keep adding more material and the display will read correctly.
3. As you approach the desired weight, add less and less trying not to go over the desired weight. If you do go over, just use a transfer tool to scoop up a little of the material transferred and remove it. DO NOT RETURN MATERIAL TO STOCK BOTTLES, it may be returned to a secondary container or disposed of. You will likely need to remove enough that it will be necessary to again add a little at a time as you again approach the desired weight.
4. When you have the desired weight of material weighed out, remove any weight you added to prevent the scale from re-zeroing itself. Confirm that you have the correct amount of material weighted out. Then remove the weighing container or paper from the weigh platform and set it aside where it will not be disturbed and is available for its intended use.
5. Clean tools by wiping off all powder with a clean paper towel or cloth before and after next each use. For best results you may wish to wipe first with a dry wipe, then a wet one, then dry again.
6. When all weighing is complete, clean off scale using a brush to whisk away any powder that spilled. This can be done either directly into a waste container or onto a sheet of scrap paper that can then be placed into waste.

Having precisely weighted out a quantity of a substance, you will want to be able to transfer all of it, or at least as much as you can, to wherever it needs to go. This is another reason to favor weigh boats, glassine papers, and other purpose made weighing containers. Because they are made for weighing, they are also made to minimize the amount of material that sticks to them. Simple pouring is usually sufficient to moving material from weigh boat or glassine paper into another container. This process is shown in short videos below entitled "pour from glassine" and "pour from weigh boat". 

Note that folding glassine papers before weighing materials on them makes a crease in the paper. This makes handling the paper without spilling what is on it easier. It also helps to funnel material when pouring it off the paper. With weigh boats, if the amount of material is significant enough to pile up, it is advisable to use the tapping method show previously to prevent an avalanche. Just tap slightly on the end of the weigh boat opposite where it is being poured from. This method, along with using a transfer tool to nudge a little of the material off at a time is also useful when pouring into containers with narrow necks. A wide necked funnel may also be useful though there is a risk of material sticking to it as well.

If there is a visible amount of material a stuck to the weighing container after trying to pour it all off, there are several methods to dislodge more. The frst is to flick the outside bottom of the container with your finger while pouring. You may also try scraping lightly with a transfer tool, followed by pouring out the dislodged solid material. Avoid scraping so hard as gouge out material from the weighing container, and be mindful of material sticking to the transfer tool. These methods are shown in videos below entitled "tap and pour" and "scrape and pour".

In a few cases, distilled water can be used to help move any residual material out of a weighing container. The first and most common case is when the material being weighed is going to be dissolved in water as the next step. For example, if you are making a solution of the weighed material in water, adding a little water to the weighing container will dissolve and/or loosen any stuck solids. These may then be poured off into the container used to dissolve the rest of the material. However, you must be mindful to account for the water used in transfer if a fixed amount of water is to be added at the dissolution step. A further rinse of the weighing container with distilled water can capture all traces of the substance, and is a common technique in analytical chemistry called quantitative transfer.   

A second case is when material being weighed is not water soluble, does not react with water, and the intended use is compatible with it being heated to remove the water by evaporation. For example, if the weighed material is being transferred into a crucible in order to heat it in a furnace, any distilled water added will evaporate leaving nothing behind. Distilled water is added using a squirt bottle sprayed on the retained material to help dislodge it. This can be done while in pouring position so the water and suspended solids pour off. Alternatively, the water swirled to help dislodge and keep the particles suspended, then poured off in one continuous motion.

In all cases where water is used. be sure to USE ONLY DISTILLED WATER. Deionized water will also work but is much less available outside academic and industrial settings. Additionally, DO NOT ADD WATER TO CHEMICALS THAT REACT VIOLENTLY WITH WATER. This includes adding water, especially small amounts, to concentrated strong acids or bases. The heat of dilution can cause the water to instantly boil and spatter acid. Always add concentrated acids or bases to water, never water to concentrated acids or bases.

Unless water has been used, it is often possible to determine how much has been retained on the weighing container. Just place the weighing container back on the weigh platform. The weight shown is the weight retained. This works only if the scale has not turned itself off, and you have not hit tare after removing the weigh boat. Scales typically will not shut themselves off as long as the weight being measured is changing. Usually 30 to 60 seconds of no change is required before auto shut-off. Check the manual that came with the scale. Placing a weight on the weigh platform after removing the weighing container, followed by lifting and replacing it before the shutoff timer runs out, resets the timer and can be done repetitively. Knowing how much material has been retained can help you decide if further recovery efforts are worthwhile.

Good laboratory practices should be used at all times when working with chemicals. These include not drinking, eating, or smoking/vaping while working in order to reduce chances for accidental ingestion. Wear eye protection to safeguard against damage to your vision. Wash hands frequently and after any possible skin exposure. Consider use of disposable gloves especially with toxic, caustic, or corrosive materials. Eye and skin protection are especially important when working with such materials. Use of highly toxic or reactive chemicals by people with limited lab experience and facilities is an especially bad idea and a recipe for disaster. I strongly suggest consulting more experienced chemists before even considering such efforts. Be responsible and always follow all laws in regards to possession, use, and disposal of chemical reagents.