What is the mystery liquid in a soil pH testing kit?

I'd like to try and buy the testing chemicals in bulk rather than the kits, which feel very overpriced.

The kits contain two liquids - one called "barium sulphate" which I have read is a "flocculant" and its purpose is to clump suspended particles together to clarify the water to get a better colour reading.

The other chemical is unlabelled. My first thought is that it is universal indicator fluid, but I'm unsure.

Does anybody know with confidence?

Many thanks in advance.

I can't say for certain, but from my classes at college, I think I would guess it is either a universal indicator or a mixture of multiple indicators.

I think a good diy alternative to make is a red cabbage pH indicator.

Probably https://en.wikipedia.org/wiki/Phenolphthalein

There are other buffers that change colors due to ring structures.

Most pH "kits" will have a bottle of universal indicator included which is considered a "rough" test.
What that means is that it is going to get you in the ball park but it isn't going to be necessarily anywhere near accurate.

The caveat is that unless you are willing to spend a minimum of 60 dollars for a "stick type pH tester" you are not going to get accurate results.
In soil labs we use calibrated pH testers that usually cost around 200 minimum  and up to around 500 dollars by the time you have the tester, the probe, the calibration fluid the washing bottle and the neutral storage fluid and bottle to hold the probe in the Neutral solution.

Kits tend to look to me like pool testing kits, you will get "close enough" results but you might be out by a full point.

Wikipedia pH page
This url is a good explanation about pH testing.

Redhawk