Determination of zinc ions by zinc reagent spectrophotometry

I’m not sure if the pH is within the required range?

The pH is around 9, which is also within the acceptable range

What is this zinc reagent? What are the specific steps? Provide a detailed description; maybe other sea friends will reply

Determination of zinc ion content – Method 1: Zinc reagent colorimetry (for low zinc levels) I. Principle of the method: In a solution with pH≈9.5, a zinc reagent reacts with zinc ions to form a blue complex, and its concentration can be determined by colorimetry. This method is suitable for the determination of trace zinc ions in natural water and circulating cooling water, with a detection range of 0.4–5.0 mg/L. II. Instruments and Reagents: 1. Instruments: Visible spectrophotometer 2. Reagents: (1) Borate buffer solution, pH=8.8–9.0: Weigh 37.3 grams of potassium chloride, 31 grams of boric acid, and 8.34 grams of sodium hydroxide; dissolve them in water at 60–80°C, then cool and dilute to 1 liter ； (2) Zinc reagent solution: Weigh 0.2 grams of zinc reagent and dissolve it in 2.50 mL of ethanol; let it stand overnight until it is completely dissolved. Store it in a brown bottle, where it can remain stable for one month ； (It can also be prepared by dissolving 0.2 grams of zinc reagent in 2 mL of 1 mol/L sodium hydroxide solution, diluting the mixture to 100 mL with zinc-free water, and storing it in a brown bottle; it remains stable for a week. If the color of the solution changes from red to purple, it indicates that it is losing its effectiveness.) ) (3) Zinc stock solution: Accurately weigh 0.5 g of ultra-pure zinc from which the oxide film has been removed, or 0.622 g of reference zinc oxide, and place it in a beaker. Add 20 mL of 1+1 hydrochloric acid; dissolve the substance at low temperature. Once it has cooled slightly, transfer the mixture to a 1-liter volumetric flask using zinc-free water, dilute to the mark, and mix well. This solution contains 500 μg of zinc ions per 1 mL ； (4) Zinc standard solution: Transfer 20 ml of the stock solution into a 1 L volumetric flask, dilute to the mark with zinc-free water, and mix well. This solution contains 10 μg of zinc ions per 1 mL. III. Procedure: 1. Preparation of the standard curve: Accurately transfer 0, 1, 2, 3, 4, and 5 mL of zinc standard solution into 6 50 mL colorimetric tubes, then add zinc-free water until the volume reaches approximately 30 mL ； Add 10 mL of borate buffer solution and 5.0 mL of zinc reagent respectively, then dilute to the mark with zinc-free water. Shake well, allow to stand for 10 minutes, and measure the absorbance at 620 nm using a 2 cm cuvette, with the reagent blank as a control. Plot a standard curve with absorbance on the horizontal axis and the amount of zinc ions in milligrams on the vertical axis. 2. Sample testing: (1) For water samples containing organophosphorus, take two 100-milliliter conical flasks; to one of them, add 10 mL, 20 mL, or 25 mL of the water sample depending on its zinc content, while the other flask is left without any water sample, serving as a reagent blank ； Add 0.5 mL of sulfuric acid with C(1/2H2SO4) = 1 mol/L and 1 mL of a 0.4% persulfate solution to each, then add zinc-free water to bring the volume to about 30 mL ； Boil on an electric stove for 5 minutes, remove, and cool to room temperature ； Then add 1 drop of methyl orange solution each, and adjust the sample to turn yellow using sodium hydroxide solution with C(NaOH) = 1 mol/L ； Add 10 mL of borate buffer solution and 5 mL of zinc reagent respectively ； Transfer it into a 50 mL colorimetric tube. Wash the conical flask with zinc-free water, then dilute it to the mark with zinc-free water, shake well, and let it stand for 10 minutes. Measure its absorbance at 620 nm using a 2 cm cuvette on a spectrophotometer, with the reagent blank as a control. (2) Water samples free of organophosphates: Directly take 10, 20, or 25 mL of the water sample into a 50 mL colorimetric tube; no water sample is added to another colorimetric tube. Then add 10 mL of borate buffer solution and 5 mL of zinc reagent to each tube respectively, dilute to the mark with zinc-free water, mix well, and allow it to stand for 10 minutes before conducting the colorimetry. Note: ① When determining the total zinc ions in the water sample, the sample is not filtered; first, the precipitate is dissolved using 1+1 hydrochloric acid, and then the pH is adjusted to a value at which the methyl orange indicator turns yellow using 1 mol/L sodium hydroxide solution. When only soluble zinc in the water sample is to be measured, the water sample is filtered. There is no need to add hydrochloric acid to dissolve the precipitate, nor is it necessary to adjust the pH with NaOH. ②When the pH value of the water sample is less than 4, for the determination of soluble zinc, use one drop of methyl orange as an indicator and adjust the sample to yellow with 1 mol/L sodium hydroxide. IV. Calculation: The zinc ion concentration X (mg/L) in the water sample = K·A/V × 1000, where: K is the coefficient of the zinc standard curve; A is the absorbance of the measured water sample ； V — volume of the water sample, in milliliters.

Let’s see what other sea friends have to say on this issue. And even if the color is red, is the gradient of colors varying with different concentrations noticeable? I haven’t used this method, so I can’t reply.

If the zinc reagent is prepared as an ethanol solution, it turns red when the zinc content is low, and blue only when the content is high. When we used to measure the zinc content in circulating water, it was red