A classroom suitable method to make electrolyte solutions of any strength for al-air or microbial fuel cells if a heat source is available. Along the way we will learn about calculating ratios, percentages and solubility of inorganic chemicals in water. Plus these solutions can be used with other projects to experiment with the role electrolytes play in fuel cells and batteries.
Our approach is very simple. It's trivial to make a fully saturated (100%) water solution. While we are using salt the same approach may be used with other soluble compounds to produce of variety of chemical dilutions.
Once you have a bottle of saturated solution, you can take a portion of it and dilute it down with distilled water (or even tap water if it's not important to be too exact) to any percentage ratio you desire from 1% to 99%, but you need to start with 100% solution in order to mix an accurate dilution.
While I will focus on common table salt the same techniques can be adapted to any of the inorganic compounds in the later step.
Step 1: Bill of Materials
Step 2: Make a Saturated Solution
A saturated solution is one in which the solvent (water) has achieved equilibrium between the dissolved and undissolved solvent. What does that mean? The water has absorbed all of the chemical which it can hold.
The hotter the water the more chemical it can hold. So what we're going to do is heat up the water (I suggest 100 degrees but substantially above room temperature will do boiling is definitely not necessary ). I found that tap water did not stay hot long enough and it was necessary to use a pan and the stove.
Pour about 500-700 mL of water into a pan, basically you want enough to fill your bottle with a reasonable margin of error (I used about 1 1/2 bottles). If you're a purist or doing real chemistry distilled water can be used, otherwise tap water will probably be fine. Soft water should be avoided as it already has dissolved salts (of a different type) and this may impact the proposed use.
Once the water has heated up add some salt and stir until fully dissolved. If you use a kitchen scale measure you can measure out about 100g as a starting point otherwise just pour some in and stir. Repeat this until the salt no longer dissolves and wet salt crystals appear on the bottom of the pan (see picture). There is no particular danger in adding too much but no need to overdo it.
Remove the pan from the heat and allow to cool to room temperature. Some folks filter the solution, I was able to carefully pour off 500 mL of solution into a measuring cup without getting any surplus salt. Then carefully pour that into one of the bottles. Close the bottle and label it "Master Solution" or "Saturated Solution". Actually I marked mine "Saturated Saline" since I frequently have more than one type of solution on hand.
The next step will talk briefly about the solubility of various household chemicals such as lye ( Sodium Hydroxide ) or Epsom salts and then we'll look at making solutions of various strengths.
Step 3: How Much Chemical Do I Need ( Solubility )
As you can see from the graph most of the chemicals under consideration are more soluble the higher the temperature. A substance that is retrograde soluble is one whose solubility decreases with increasing temperature. One such substance Cerium sulfate, Ce2(SO4) is plotted on the graph to demonstrate this characteristic.
The table below gives basic data for some common inorganic compounds. The chemical is listed on the left and the amount of substance which can be absorbed by 100mL water at various temperatures is shown. So, for instance sodium chloride at 40oC ( about 100 degrees ) will hold 36.6 g salt per 100 mL or about 183g ( about .4 lb ) in 500mL. Most of these are readily available, often at your local hardware store or hobby store with chemistry supplies.
|KI, potassium iodide||127.5||136||144||152||160||168|
|KCl, potassium chloride||27.6||31||34||37||40||42.6|
|NaCl, sodium chloride||35.7||35.8||36||36.3||36.6||37|
|NaHCO3 , sodium bicarbonate||6.9||8.15||9.6||11.1||12.7||14.45|
|NaOH, sodium hydroxide||-----------||-------------||109||119||145||174|
|MgSO4• 7 H2O, epsom salts||------------||23.6||26.2||29||31.3||-------------|
|magnesium sulfate heptahydrate|
Step 4: Parts Is Parts - Calculating Dilutions
So to make a dilution we'll need to figure out correct ratio of saturated solution to "pure" water and mix them together.
The notion of 'one part' refers to using the same measure for the saturated solution and the "pure" water. Let's say you're holding a shot glass in your hand and you fill it with the master salt solution, you now have 'one part' of the master solution. You dump that into a jar and then you fill the same glass with water and add it to the jar. You have now mixed 'one part' master salt solution to 'one part' distilled water (or 1:1 mixing ratio). This results in a 50% reduction in concentration or a 50% solution. If you take one part master solution and add two parts water, you'll wind up with a 33.3% salt water solution (1:2 ratio). If you take one part of the master solution and add three parts of distilled water, you'll have a 25% salt water solution (1:3 ratio). If you take one part master solution and add four parts of distilled water, you now have a 20% salt water solution (1:4 ratio).
Using the 'parts' method, if you merely add the two numbers of the 'parts' ratio together and then divide that into 100, it will give you the final dilution percentage. For example, the 1:4 ratio adds up to "5". Five divided into 100 gives you 20% . What if you want a 10% salt water solution? Simple. What two ratio numbers (beginning with '1' of course) add up to 10? One plus nine or 1:9 mixing ratio. One part master solution plus 9 parts distilled water will give you a 10% salt water solution.
Here's a chart that show the more commonly used ratios. Use 1 part saturated solution and the appropriate number of measures of water to obtain the desired solution strength.
Step 5: Making a 10% Solution
Now let's make a 10% solution. We know from the previous step that a 1:9 ratio of saturated solution to water will produce a 10% solution. We also know that we want 500 mL of the solution ( in this case ) so we just need to calculate the size of each "part".
Because we know that we are using a 1:9 ratio there will be 10 "parts" but what size should each part be? If we divide 500 mL by 10 we can see that each part will be 50 mL.
So first we measure out 1 "part" of saturated solution or 50 mL of the master or saturated solution and pour this into the second bottle. Next we measure out 9 "parts" of water. Since 9x50=450 we know that we need 450 mL of water in our measuring cup. Pour this into the second bottle, cap and label 10% Saline.
Step 6: Recovering Excess Salt
Depending on how much salt you added there may be a considerable bit left in the bottom of the pan. This salt can easily be recovered by pouring off the excess liquid. I used two measuring cups to pour off as much liquid as possible.
Then I carefully poured the remaining wet salt onto a cookie sheet. I placed it on a slight incline and blotted up the excess water that collected. I allowed to dry for a day or two. The dry salt may be gathered up and used in a variety of ways. Its very damp and should be stored with a saltine cracker or one of those "DO NOT EAT" packets to dry it out.
I would not mix this back in with other salt, trust me lumps will form. However the salt is edible and has suffered no harm.
DON'T BE AN IDIOT. KEEP THE CHEMICALS AND SOLUTIONS OUT OF REACH OF CHILDREN.