1. Active component (1-hydroxyethylidene-1 1-diphosphonic acid) content.
1.1 Oxidative decomposition method
The active component of the finished product, 1-hydroxyethylidene-1 1-diphosphonic acid, and impurity phosphorous acid. Under appropriate oxidative decomposition conditions, both are converted to n-phosphonic acid.
Then measure the data in a corresponding method. The total amount of n-phosphonate represents the amount of phosphinate which is present in the finished product to be tested and the amount of phosphinate which represents the phosphinic acid content. The content of the active component 1-hydroxyethylidene-1 1-diphosphonic acid expressed as a mass fraction is converted.
1.2 Complexometric titration
In a slightly acidic solution (with hexamethylenetetramine as a buffer). The 1-hydroxyethylidene-1 1-diphosphonic acid can be selectively reacted with the titrant DCTA or EDTA.
If yellow xylenol orange is used as an indicator. When all of the 1-hydroxyethylidene-1 1-diphosphonic acid has been reacted. Xylenol orange is incorporated to form a reddish-purple soluble dinuclear ternary complex.
This is the endpoint of the titration. Based on the concentration and consumption of the titrant, the percentage of 1-hydroxyethylidene-1 1-diphosphonic acid in the finished product can be calculated.
This test method can also be carried out according to industry-standard HG 5-1525-85.
That is, the solution mixed with EDTA, hexamethylenetetramine, xylenol orange, hydrochloric acid, and the sample to be tested is firstly dropped from orange-yellow to reddish-purple with a cerium nitrate standard solution. The sample solution is then back titrated until the reddish-purple color turns orange again.
There is also a method of measuring the content of 1-hydroxyethylidene-1 1-diphosphonic acid using nitric acid (zirconium) as a titrant.
2. The content of the impurity phosphinate is determined by the iodometric method.
The principle is as follows. The phosphinic acid is oxidized by iodine to phosphoric acid at a pH of 6.5-7.2. The excess iodine was titrated with a sodium thiosulfate standard solution. Thus, the content of the phosphinate was measured.
3. The content of impurity chloride is determined by precipitation method.
The principle is as follows. In an acidic medium, chloride and excess silver ions form a silver chloride precipitate. Methyl violet was used as an indicator. The excess silver ions were back-titrated with a sodium chloride standard solution.
The content of the impurity acetic acid is determined by a colorimetric method as necessary.
The sample was dissolved in methanol. It is acidified with concentrated sulfuric acid. Add hydroxylamine hydrochloride solution. It is then neutralized with sodium hydroxide.
Calcium hydroxide (to increase the selectivity of the assay) was filtered. A hydrochloric acid/ferric chloride colorant was added to the filtrate. And colorimetric with a known concentration of 1-hydroxyethylidene-1 1-diphosphonic acid standard solution.