So let us focus on only two of these so called grades. The first one is "Technical Grade", with the second being "Photo Grade". "Photo Grade" is not a USA Government chemical grade at all..., but an Industrial Specification Rating.
The first one, we know a great deal about it. It is "Technical Grade": and it is a US Government standard grade. But "Photo Grade", does not exist. It is not a US Government approved chemical grading, it is an Industrial Specified Standard. In fact, it is a ANSI standard, set up by the American National Standard Institute so that product manufactures can protect their products, their business, and maintain consumers confidence in the products in which they purchase and use, will meet or exceed the purpose that they were intended to perform.
All of ANSI Standards, are voluntary in nature. Manufactures do not have to use them, unless that ANSI Standard, has been adopted by the United States Government in governing the manufacture and operation of their product.
ANSI Standards..., are standards that the manufacture of that product must meet or exceed in order to qualify for ANSI approval. Such as the "Good House Keeping Seal".
You will discover, that the "Photo Grade" rating, could have a chemical purity range from 77 to 98%; with the lowest purity bulk chemicals being sold only to companies, and not to the general public.
Listed below is the definition for Photo Grade.
Most products made in the USA, often exceeds ANSI Standards, and on average..., they run from poor to excellent.
One of the highest chemical grades that the general public can obtain, is a Food Graded chemical. They run normally 97 to 99% in purity; if they are slated for human consumption. Yet the word "consumption" is very tricky in its use, because it is used even for products that are not slated for consumption internally. Basically just having physical contact with human skin is good enough. Such as products like hair dye, body oils, and such.
I use a 60/40 purity mix of chemical graded chemicals in my formulas. I use swimming pool chemicals, which are 100% Food Graded products that are used in the upkeep of ones swimming pool. I use chemicals used in the making of wine and beer products, or home canning. I use baking soda, which I cook into sodium carbonate. The rest of my chemicals are Technical Grade.
Most chemicals that you will likely come across will be Food Graded chemicals. Which is one of the highest US grading. It is even higher than Lab Grade chemicals. Many companies purchase thousands of pounds of bulk Food Graded chemicals, and then later..., sell off their left over supplies to reclaim revenue shares. You will discover that many of the chemicals that you come in contact with are food or technical grade chemicals. Which far exceeds the "photo grade" labeling or ANSI standard.
You, the photographer, will be very hard press to obtain chemicals who's purity level is less than 89%. Which by the way..., is very good for any photo chemical work. Your biggest problem with photo chemical work, will be the purity of your water. In hope of it having fewer metal solids, ceramic metals, and glass.
Most US manufactures, perform additional filtering of all their bulk chemicals in order to reach the level of purity required for their finish product that they are producing. You will not be able to purchase many chemicals with a purity level less than 89% in bulk form, without a government license, because of DEA rules and EPA laws . Any chemical sold to the general public (persons without license or permits) must be 100 pound loads or less, and filtered to a purity level of at lease 89% or greater.
Example: If you need to feed your three lot area lawn, and you require 800 pounds of Scott lawn feed. You must buy 16, 50 lbs bags, because 800 pounds of bulk lawn feed can not be sold to the general public, it is a special order and requires permits, or may not be available unless you are a farmer with permits. Also, such large orders of that nature, will draw the attention of the DEA and ATF. Cement is also an item that you must buy in 100 lbs bags or less. Bulk cement is a special order and you must have a permit and show a means of storing such large bulk of cement.
If you obtain a chemical that is less than 89% pure. It was or is being sold illegally to the general public and you should pass on that purchase, unless you have a permit/license, plus a means of filtering the chemical to a higher purity level. Because those chemicals will contain a high metal content and other impurities.
The CDC and the FDA has set levels, standards, and laws to protect the general public, and our water and food supplies from accidental or intentional dumping of chemicals into our environment. By limiting the size amounts of chemicals being sold to the general public..., to smaller percentages. This action enables them to monitor the illegal chemical dumping performed by companies and private persons, that fail to properly dispose of their chemical products.
So do a little research, and hone up on your chemicals purity studies. Your understanding of it will make you feel much more safe and informed.