How to determine the levels of zinc containing corrosion inhibitors

The principal purpose of zinc analyses is to maintain proper treatment levels of zinc containing corrosion inhibitors. A very common inhibitor system uses a combination of two anodic inhibitors: phosphate and chromate. This combination gives a very significant reduction in overall corrosion and tuberculation. A considerable economic benefit is achieved with this combination with the further addition of zinc, corrosion is reduced still further with lower levels of inhibitor treatment through a cathodic anodic type of protection. Zinc function as a cathodic inhibitor by forming a film of zinc hydronide or zinc phosphate at the cathodic site and causes localized deposition of zinc phosphate or zinc hydroxide, stifling the cathodic reaction. Proper levels of zinc are essential for obtaining optimum corrosion protection.

 

This test is based on the reaction of zinc with xylenol orange indicator to form a red complex  at  pH of 6.2, xylenol orange does not react specifically with zinc and interfering ions must be masked by means of a complexing agent.

 

Filter photometer complete with assorted laboratory glassware.

 

Bromithymol Blue Indicator

Sulfuric Acid, 1.0N

Zinc Reagent P1

Zinc Reagent P2

Zinc Reagent P3

Zinc Standard

 

This procedure uses a wavelength of 580 nm and an optical cell with a light path of approximately 5mm. Filter 40 ml of sample using filter paper with a 0.45 micron pore size. Discard the first 15 ml of filtrate.

Using the 50 ml graduated cylinder, add 25 ml of sample to the 125- ml Erlenmeyer flask. Add  two drops of bromthymol blue indicator to the sample and swirl the solution. If a blue color appears, add 1.0N sulfuric acid, one drop at a time, until the blue color disappears.

Pipet 1.0 ml zinc reagent P1 into the flask and again swirl the solution. Wait exactly 30 seconds, then pipet 0.5 ml clear zinc reagent P2 into the flask and swril the solution. (Pipet the clear portion of the zinc reagent P2 solution. Do not mix settled precipitate. Shake the stock solution bottle thoroughly and allow most of the crystals to settle before refilling a smaller bottle with clear supernatant.

Transfer a portion of this prepared solution into the optical cell. Using 580-nm filter, set the photometer at zero absorbance. Then pour the contents of the cell back into the flask containing prepared solution and adjust its temperature to 70 to 75F. Carefully pull apart one zinc reagent P3 capsule and empty its contents into the flask. Swirl the solution to dissolve this powdered reagent and wait one minute, measured from the time the capsule contents are added to the flask. Measure the absorbance on the photometer.

 

The zinc in parts per million as Zn is obtained by reference to the zinc calibration curve. The calibration curve is prepared by plotting the absorbance versus the concentration of zinc from 0 through 3 ppm.

 

To 49 ml of distilled water, add 1.0 ml of zinc standard. Swirl the solution and follow the above test procedure. Measure absorbance on the photometer. Test results should equal 2.0 ppm.

 

Care must be taken that results fall slightly below the upper limit of the calibration curve. Concentrations of zinc above the 3-ppm limit produce very little differences in absorbance.

Concentrations of ions normally present in cooling water do not interfere with this test method. No interference effects were found with the following ions up to the concentrations (in ppm) noted: calcium (as CaCO3 ) 400; magnesium (as CaCO3 ) 100; cupric, 2.5 ferric, 2.0; chromate, 50;

phosphate, 50; and aluminium, 4.0.