DIY: Make your own Soda Ash cheaply
You can buy baking soda anywhere. Supermarkets, Walmart, even the Dollar Store. A box costs $0.99 and is perfectly safe for your reef. Baking Soda is sodium bicarbonate which can be used for baking, cleaning, and deodorizing. And we hobbyists use it to buffer the alkalinity level of our aquariums, too.
Sodium bicarbonate (baking soda) will raise alkalinity while depressing pH, while Sodium carbonate (Soda Ash) will raise both. By baking the baking soda in your oven, the heat will drive off the trapped CO2 in the powder that would decrease pH levels and turn it into sodium carbonate. We want to use Soda Ash, and making your own is easy and inexpensive. You can use Arm & Hammer, or any other brand sold where groceries are available. Here's one I bought at Aldi. Spead the contents of the box evenly on a pizza / cookie sheet.
Preheat your oven to 300°F, and wait for the oven light to turn off.
Once the light turns off, the oven is at the proper temperature. Don't put the tray in the oven until the temperature has been reached first, so you don't burn the product while the oven is heating up.
Place it on the rack in the middle of the oven.
Set the timer for 60 minutes. Let it bake. No reason to open the oven during this process. Once the timer chimes, remove the pan and allow it to cool off.
It should still be white. If you burned it and it turned brown, toss it out and make a new batch.
Once cooled, transfer the soda ash into a storage container, like this disposable Ziploc one.
How to use it:
If you want to mix up a gallon of alkalinity liquid solution for daily dosing, combine 1 gallon of RODI water with 2 cups of soda ash. Stir it very well for 30 seconds, then give it a few minutes to mix. Once clear, it's ready to hook up to a dosing pump.
You can also make smaller amounts for quick adjustments. For example, if you mixed up a batch of new saltwater and the pH or alkalinity is low, a couple of teaspoons of soda ash mixed in a cup of RODI water may be just the amount necessary to make that saltwater batch measure correctly. You can fine-tune what you need, just experiment and test thoroughy until you have learned what amount works best for you. Also, just cause you mixed up a cup of soda ash and water doesn't mean you have to pour it all into your barrel. Pour in a little bit, let it mix into the saltwater, and measure alkalinity or pH. Add a little bit more, mix and test until you hit the desired level.
Remember: Alkalinity should always be dosed in an area of high flow, trickled in very slowly rather than being dumped in all at once.