Greetings fellow nerds.
I've been doing a few things this month but I didn't seem to have enough material to make separate videos
so I'm assembling them all together here into a lab notes video.
The first thing I was working on was nitric acid.
In a previous video I made nitric acid using sodium nitrate and sodium bisulfate.
The advantage of this process being that it didn't use sulfuric acid, which is available to some amateurs, but not others.
Now one important question that came up was how sodium bisulfate reacts with calcium nitrate.
Calcium nitrate and calcium ammonium nitrate are somewhat easier to obtain than sodium nitrate and potassium nitrate
so there is some interest in the amateur community to use them for making nitric acid.
But when used with sulfuric acid, calcium nitrate forms calcium sulfate and nitric acid.
While this does produce nitric acid, the major issue for the amateur is the calcium sulfate produced.
Calcium sulfate is an insoluble rock and is in fact used as plaster.
If it's stuck inside a flask it needs to be drilled out and for this reason calcium nitrate and sulfuric acid are almost never mixed directly.
Instead, they are first mixed separately in water and then mixed and the calcium sulfate precipitates out as a powder that can be filtered.
The resulting dilute nitric acid is then purified by distillation.
But if we used sodium bisulfate, would the calcium sulfate still be a problem, or would the sodium sulfate stop it from forming into a hard solid?
Could we still use the dry process of making nitric acid and avoid the labor issues of the wet process?
To find out I ran the experiment with 49g of commercial calcium ammonium nitrate decahydrate mixed with 150g of sodium bisulfate
and directly heated like in the dry process.
Nitric acid was distilled off and the yield was measured at a good 85%.
But more importantly the residue of sodium bisulfate, sulfates and calcium sulfate was partially soluble.
While it wasn't easy to get out, and did require some shaking and stirring with water, it did eventually dissolve into a slurry that could be easily poured out.
No labor intensive drilling or possible flask breaking.
This actually makes sodium bisulfate a superior acid to sulfuric acid when used with calcium nitrate as we avoid the formation of insoluble bulk calcium sulfate.
I think the sodium sulfate gets between the crystals of calcium sulfate and prevents them from connecting.
Anyway, I recommend using sodium bisulfate over sulfuric acid when used with calcium nitrate or calcium ammonium nitrate.
Especially when using the dry process for making nitric acid.
For thoroughness I also tried the wet process and mixed the sodium bisulfate and calcium ammonium nitrate with 50ml of water before distilling.
The yield was somewhat higher at 95%, but at the cost of producing dilute nitric acid that required additional fractional distillation to concentrate.
Personally, since both reagents are cheap and my labor is expensive, I would use the dry process with additional chemicals.
But if the reagents are expensive and your labor is cheap then the wet process will give you better yields.
So that was my research so far in making nitric acid.
I'm hoping to remake my complete guide to nitric acid as the old one from ten years ago is no longer up to modern standards.
Anyway, I wanted to go further but I was stopped by a breakdown in a critical piece of equipment.
My hotplate stirrer stopped working.
Now a few years ago I was having huge problems with the heating elements
and I was able to replace them with some very robust elements I purchased from china.
That same element that i put in those years back is still working.
We've now reached the point where something else is breaking inside the hotplate stirrer.
So what exactly is the problem? When I turn it on the temperature reading is almost 400 celsius.
Clearly this isn't accurate as the plate has been sitting at room temperature for a while
and I can directly attest with my sense of touch that this is not 400 celsius.
Now if this were the only problem then I could still use it.
But when I turn on the heating element the hotplate stays cold.
A hotplate that doesn't get hot is useless.
The main problem is that the hotplate uses a feedback system for temperature control 
so if it believes it's too hot, it won't turn on the heating element.
This is different from other hotplates in that they will apply power regardless of temperature.
While having a feedback loop is good for safety, it means we have to fix the sensor.
So let me open it up and see if we can find it.
And here they are, sandwiched between two mica sheets.
And there are two sensors as seen here.
They seem to be resistance temperature detectors or RTDs.
I think there are two sensors because one controls the heating element while the other is a safety backup sensor that tells if the hotplate is too hot.
I'm now checking the resistance of the sensors and seeing if they're still within specification.
If they are, then maybe the circuit board is broken.
But if they're not then we know to replace the sensors.
And it seems the first sensor is still working with a resistance of about 1100 ohms.
This hotplate seems to use PT1000 RTDs which have a resistance of 1000 ohms at zero celsius.
The reading of 1100 ohms is because we're at room temperature.
Now the other sensor seems to be broken, I'm not getting a reading for it.
This makes sense that the hotplate is reading 400 celsius because RTDs work by increasing resistance as temperature rises.
If the sensor is broken, the infinite resistance appears as infinite temperature to the circuit board.
In my hotplate the circuit board only measures up to 400 celsius and that's what it displayed.
I'm actually somewhat relieved that this is the problem because that means i can fix it.
I'm not so confident I can fix a broken circuit board.
And here are the sensors.
It's very fortunate that this had two sensors so i could measure the working one and know which one to buy.
If this didn't, i would have to insert various dummy resistors until the hotplate gave a corresponding reading.
Anyway, I'm now going to order the PT1000 sensor and they should get here in a week.
Here I am a week later, and I've already started bending out the broken sensor.
Here is the replacement I've purchased and I'll see if I can get it in there.
And here we are.
I've used copper foil to connect the leads because i can't use solder at these temperatures.
Most solders would just melt again when the hotplate turns on.
The previous leads were spot welded but i don't have that capability.
Hopefully this foil fix will work and if it doesn't, the sensors will fail open and the hotplate will turn off.
So this won't fail to an unsafe condition and start a fire.
Here I am testing the sensors.
The repaired sensor looks good, so this should work.
I'm also checking the other sensor to make sure i didn't accidentally break it when i was doing my crappy repair job.
It seems they both work now.
Let me reassemble that back into the hotplate.
And now for the major test.
And we're getting an accurate reading for room temperature.
Now let me turn on the heating.
And it seems to be working.
It's definitely heating up, I can feel it.
So it looks like the hotplate is back in service.
Now I'm not going to complain about quality this time because it was working for several years before finally failing.
With the kind of abuse i put these plates through i can't really expect more than that.
But at least it was easy to fix and didn't require that I contract out a specially made part like the heating elements.
So i'm happy with the result.
Hopefully I can get back to making more videos.
Thanks for watching this month's lab notes.
Special thank you to all of my supporters on patreon for making these science videos possible
with their donations and their direction.
If you are not currently a patron, but like to support the continued production of science videos like this one,
then check out my patreon page here or in the video description.
I really appreciate any and all support.
