Used in calibrating many Myron L Instruments. It is the best choice when measuring boiler and cooling water samples, city water supply, lakes, wells, etc. "442" refers to the combination of salts mixed with deionized water to make this standard: 40% sodium sulfate, 40% sodium bicarbonate, 20% sodium chloride. A combination of standard salts is necessary, since natural water salt type and concentration can vary greatly by location. After much research, the 442 Standard was developed by Myron L Company more than 35 years ago. It remains the world's most accepted such standard.Specifications: 300 PPM Additional Details ------------------------------ Product description: Used in calibrating many Myron L Instruments. It is the best choice when measuring boiler and cooling water samples, city water supply, lakes, wells, etc. "442" refers to the combination of salts mixed with deionized water to make this standard: 40% sodium sulfate, 40% sodium bicarbonate, 20% sodium chloride. A combination of standard salts is necessary, since natural water salt type and concentration can vary greatly by location. After much research, the 442 Standard was developed by Myron L Company more than 35 years ago. It remains the world's most accepted such standard.Specifications: 300 PPM