The last couple of weeks were all about water samples, but the plan is to sample soil as well. That’s why I bought a nice instrument, allowing me to access several depths, without digging. It’s a kind of hollow drill, called a “soil sample probe”.
Actually I still have to practice, but something else was also important. Soil is not a solution, so how do we measure the pH? Adding water is an option indeed, but does it affect the amount of acid or base? I read a lot about the different options. Some take fresh samples and mix them with water. Others wait until the soil is air dry. The amount of water differs as well between the labs. Some take an equal mass of water to mix with the soil sample, which seems quite reasonable. Others use five or ten times the volume.
Then I encountered several articles talking about Calcium Chloride (I don’t mention the formula yet, because that’s the issue for this post). Adding it would change the pH, but provide more reliable values. The article used says: “Soil pH measured in water is the pH closest to the pH of soil solution in the field (this is true for soils with low electrical conductivity and for soils that are not fertilized), but is dependent on the degree of dilution (the soil to solution ratio). Measuring soil pH in a matrix of 0.01 M CaCl 2 ,as opposed to water, has certain advantages, but the addition of the salt does lower the pH by about 0.5 pH units compared to soil pH in water (Schofield and Taylor 1955; Courchesne et al. 1995).“.
This means I should give it a try, so I decided to get some. Of course it is possible to purchase expensive, high quality chemicals, but here we are talking about mixing it with soil. Calcium Chloride is well known as a de-humidifier and that’s the cheap way to lay hands on it. The advice was to make a stock solution of 1 M and dilute it to 0,01 M (10 mM) and so I did.
The substance is hygroscopic, so I wanted to dry it well before weighing the right amount – 111 grams per mole according to my own calculation – confirmed by a couple of websites to be sure. After 20 minutes in the oven at 75 degrees Celcius or so, created my stock solution (111 grams of CaCl2 filled with demineralised water to 1 liter in total, to get 1M – or at least that’s what I thought) and took 10 ml of it, diluting it to 1 liter to get the 0.01 M.
The article (it’s actually more like a manual) mentioned the conductivity bandwidth (between 2.24 and 2:40 mS cm-1). It seemed a bit superfluous, but I gave it a try. Physics is fun as well, isn’t it? The setting was rather primitive, but sufficient for the purpose. I took about half a meter of and electric PVC pipe and put a plug at one side, pierced with metal screw. The other side I put on a clamp and connected my Ohm meter. First I used a digital one, but I still get confused and measured again using my old analogue one. After some calculations (the diameter of the pipe was 13.5 mm, so the resistance had to be multiplied by 1.43 and divided by 0.45 (length of the tube). Then the resistance was converted to conductivity, by taking the reciprocal. It was about 25% too low (1.7 mS cm-1)!
After some thinking and reading I realised that my pearls – whether heated in the oven or not – were more like CaCl2.2H2O. Otherwise it would have been a powder rather than pearls.
Adding 2 times a water molecule makes a mole: 111+36 = 147 gram. This meant my stock solution was actually 0.755 M and the diluted 0.01 M was actually only 7.55 mM – lacking 25% of the salt. The latter was corrected easily by adding some additional stock solution, but the next time I should take 13.25 ml of stock solution and fill it to 1 litre, to get the 10 mM after all.
I will also investigate if tap-water will do, because in the end it’s cheaper and saves a lot of shopping (I already used ten litres of demineralised water).
Next time we will get back to water samples again.