Analytical Procedure Molybdate, Low Range Pocket Colorimeter II Ternary Complex Method (0.10 to 5.00 mg/l MoO4 ) Tips and Techniques The Pocket Colorimeter II is designed to measure solutions contained in sample cells. DO NOT dip the meter in the sample or pour the sample directly into the cell holder. The results can be expressed as mg/l molybdenum (Mo) or mg/l sodium molybdate (Na 2 MoO 4 ) by multiplying the mg/l molybdate (MoO 4 ) result by 0.600 or 1.287, respectively. After adding reagent, a green color will develop if molybdate is present in the sample. Analyze samples immediately after collection. Filter turbid samples through a 0.45-micron filter membrane apparatus before analysis. (The membrane filter apparatus consists of: Code L6485 filter, Code L773 filter holder, and Code L775 plastic syringe.) Wipe the outside of the sample cells before each insertion into the instrument cell holder. Use a damp towel followed by a dry one to remove fingerprints or other marks. Highly buffered samples or extreme sample ph may exceed the buffering capacity of the reagent and require sample pretreatment. For increased accuracy, determine a reagent blank value for each new lot of reagent. Follow the procedure using deionized water in place of the sample. Subtract the reagent blank value from the final results. Wear safety glasses when working with chemicals. Results outside the working range of this procedure are invalid. Sample temperature must be between 20 25 C (68 77 F) for accurate results. Find a contact near you by visiting www.ge.com/water and clicking on Contact Us. * Trademark of General Electric Company; may be registered in one or more countries. 2008, General Electric Company. All rights reserved..doc Aug-08
Procedure 1. Fill a 25-mL mixing graduated cylinder to the 20-mL mark with sample. Note: Filter turbid samples. See Tips and Techniques for details. 2. Add the contents of one Molybdenum TC1 Reagent Powder Pillow to the cylinder. Stopper the cylinder and invert or gently shake until all particles are dissolved. 3. Split the sample by filling two 10-mL sample cells to the 10-mL mark. Cap one of the cells. (This is the blank.) Note: Exact measurement is not critical for the blank. The other cell must contain 10 ml of sample. 4. Using the calibrated dropper, add 0.5 ml of Molybdenum TC2 to the sample in the uncapped 10- ml cell. Cap the cell and swirl to mix. (This is the prepared sample.) 5. Wait two minutes for full color development. Note: A green color will develop if molybdate is present in the sample. 6. Press the POWER key to turn the meter on. The arrow should indicate the low range (LR). Note: See the Pocket Colorimeter II manual for information on selecting the correct range. 7. Wipe the sample cells free of liquid and fingerprints. 8. Place the blank in the cell holder with the diamond mark towards the keypad. Cover the blank with the instrument cap. Page 2
9. Press ZERO/SCROLL. The meter will show ----, then 0.00. Remove the blank from the cell holder. Note: If the meter reads 0.0, see the Pocket Colorimeter II manual for information on selecting the correct range. 10. Place the prepared sample in the cell holder with the diamond mark towards the keypad. 11. Cover the sample cell with the instrument cap. 12. Press READ/ENTER. The display will show ---- followed by the results in mg/l Molybdate (MoO 4 ). Note: If the display flashes, the result is above the working range of the test. Dilute the sample and repeat the test. Page 3
Interferences Interference studies were conducted by preparing a molybdate standard solution (3.34 mg/l MoO 4 ) as well as a solution of the potential interfering ion. When the standard solution concentration changed by ± 5% with a given ion concentration, the ion was considered an interference. Tables 1 3 list the details of these studies. Table 1 Substances that Cause a Negative Interference Interfering Substance Alum Aluminum AMP (Phosphonate) Bicarbonate Bisulfate Borate Chloride Chromium Copper Diethanoldith-iocarbamate EDTA Ethylene Glycol Iron Lignin Sulfonate Nitrite Orthophosphate Phosphonohydroxyacetic Acid Interference Levels and Treatments Greater than 7 mg/l Greater than 2 mg/l Greater than 15 mg/l Greater than 5650 mg/l Greater than 3300 mg/l Greater than 5250 mg/l Greater than 1400 mg/l Greater than 4.5 mg/l* Greater than 98 mg/l Greater than 32 mg/l Greater than 1500 mg/l Greater than 2% (by volume) Greater than 200 mg/l Greater than 105 mg/l Greater than 350 mg/l Greater than 4500 mg/l Greater than 32 mg/l In system waters, the presence of quaternary ammonium compounds can cause a negative interference based on the level of the compound. If the presence of a quaternary ammonium compound is suspected, or a green color develops after adding Molybdenum TC1 Reagent: 1. Add two brass dippers (Code L113) of Molybdenum Interference Eliminator Powder (Code L1974) to 20 ml of deionized water. Mix thoroughly. Quaternary Ammonium Compounds 2. Add 0.5 ml of this solution to 25 ml of sample. If Spectrus* BD1507 is being used, add 1 ml of the prepared solution to a 25 ml sample to counter interference from up to 50 ppm BD1507. 3. Shake the solution for two minutes. 4. Filter the 25 ml of treated samples through a 0.45 micron filter membrane apparatus (Codes L6485, L773, and L775). HEDP (Phosphonate) 5. Use the filtered sample as the test sample in step 1. The presence of the phosphonate HEDP at concentrations up to 30 mg/l will increase the apparent molybdate concentration reading by approximately 10% (positive interference). Multiply the value obtained in step 12 on page 2 by 0.9 to obtain the actual MoO4 concentration. Sulfite Greater than 6500 mg/l * Read the molybdate concentration immediately after the 2-minute reaction period. Page 4
Table 2 Substances that Cause a Positive Interference Interfering Substance Interference Levels and Treatments Benzotriazole Biocides Carbonate Highly buffered samples or extreme sample ph Morpholine Phosphonate HEDP Silica Greater than 210 mg/l Large interferences are caused by some biocides used in cooling tower samples. Test the ternary complex procedure on molybdate standard containing the specific biocide in use to determine if the ternary complex method will work with these samples. Greater than 1325 mg/l May exceed the buffering capacity of the reagent and require sample pretreatment. Use a ph meter (Code L6606) or ph paper (Code L1255) to measure ph. Adjust the sample ph between 3 and 5 by adding (by drops) an appropriate amount of 1.0 N Sulfuric Acid Standard Solution (Code L283.1000) or 1.0 N Sodium Hydroxide Standard Solution (Code L255.1000). If significant quantities of acid or base are used, a volume correction should be made. Divide the total volume (sample + acid + base) by the original volume. Multiply the test result by this number. Greater than 6 mg/l Positive interference of about 10% up to 30 mg/l. As the concentration increases above 30 mg/l, a decrease in the molybdate concentration reading occurs (negative interference). Greater than 600 mg/l Table 3 Non-interfering Substances Interfering Substance Bisulfite Calcium Chlorine Magnesium Manganese Nickel PBTC (phosphonate) Sulfate Zinc Interference Levels and Treatments 9600 mg/l 720 mg/l 7.5 mg/l 8000 mg/l 1600 mg/l 250 mg/l 500 mg/l 12,800 mg/l 400 mg/l Page 5
Accuracy Check To prepare a 3.34 mg/l molybdate standard solution (as MoO 4 ): 1. Pipet 10 ml from a 10-mg/L Molybdenum (16.7 mg/l MoO 4 ) Standard Solution (Code L2038.0500) into a 50-mL graduated mixing cylinder (Code L2631). 2. Dilute the cylinder to a final volume of 50 ml with Deionized Water (Code L243.4000). Mix thoroughly. 3. Use the standard in place of the sample in the procedure. Note - Use a safety bulb (Code L1575) when pipetting. Method Performance Precision Standard 95% Confidence Limits of Distribution 3.34 mg/l MoO 4 3.27 3.41 mg/l MoO 4 Description Quantity/Test Units Code Required Chemicals* Molybdenum TC1 Reagent, 20-mL sample size 1 100/pkg L2358 Molybdenum TC2 Reagent Solution 0.5 ml 50 ml L2359 Required Apparatus Cylinder, graduated mixing, 25-mL 1 each L2377 Sample Cells, 2.5-cm, 10-mL, with cap 2 6/pkg L2379.0006 Pocket Colorimeter II, Molybdate Kit 1 each L2888 * Wear safety glasses when working with chemicals. Page 6