Iodometric titrations, a class of oxidation-reduction (redox) titrations, are commonly used to analyze sodium hypophosphite in electroless nickel baths, chromic acid in various plating baths, and sodium or potassium dichromate in passivation and sealing baths. In these titrations, iodine, often supplied by potassium iodide (KI), is used as an oxidizing agent giving up an electron to reduce various compounds such as chromic acid (H2CrO4 or H2Cr2O7), potassium dichromate (K2Cr2O7), sodium hypophosphite (NaPO2H2), or copper sulfate (CuSO4). This redox reaction with potassium iodide liberates iodine molecules (I2) in an amount that is proportional to the original amount of the reduced compound. The amount of liberated iodine is then determined by titration with sodium thiosulfate (Na2S2O3) using a starch indicator.
As an example, let’s go through the process of testing the sodium dichromate concentration in a passivation tank used for a QQ-P-35 Type II, an AMS2700 Method 1 or an ASTM A967 Nitric 1 passivation bath. The QQ-P-35, AMS2700 and ASTM A967 specify a range of sodium dichromate concentration between 2.0% and 3.0% by weight. It should be noted that all of these passivation specifications identify sodium dichromate dihydrate as the added salt (Na2Cr2O7-2H2O), so all % by weight calculations assume this salt. The following information is taken from the periodic table and from data on commercially available chemicals. The prevailing interpretation of the passivation specifications assumes 42°Be HNO3 for %v/v mixes. There are other commercially available concentrations of nitric acid; however, if these other concentrations of nitric acid are used for a passivate tank, the %v/v specifications must be adjusted. For example, if you use 36°Be HNO3, which has 22% less nitric acid per unit volume, any %v/v measurement would need to be increased by a factor of 1.28.
Percent by weight is an inconvenient metric, so we will start by converting the 2%-3% by weight sodium dichromate (Na2Cr2O7) to g/L. To be precise, this computation should take the nitric acid concentration into account. The nitric acid concentration is set by QQ-P-35 at 20% to 25% by volume. The weight of 1.0 liter of 42°Be nitric acid is 145/(145-42) x 1000 = 1407.5 g/L (this is the formula to convert °Be to specific gravity and then multiply the specific gravity times the weight of a liter of water which is 1000 grams). If 25% of the solution weighs 1407.5 grams/liter and 75% of the solution is water, which weighs 1000 grams/liter, a liter of solution that is 25% nitric acid by volume weighs a total of 0.25x1407.5 + 0.75x1000 = 1102 grams. A similar series of calculations gives us a minimum solution weight of 1081 grams at 20% nitric acid. So the minimum amount of Na2Cr2O7 required in the solution must be more than 2% of 1102 grams or 22.0 g/L, while the maximum amount must be less than 3% of 1081 grams or 32.4 g/L. The bath specifications are below. If you prefer ounces and gallons, convert g/L (grams per liter) to oz/g (ounces per gallon) by dividing by 7.5. The factor of 7.5 is (28.375 grams/ounce)/(3.7854 liters/gallon). The titration steps and explanation follow the table.
min | mid | max | |
Nitric acid | 20 %v/v | 22.5 %v/v | 25 %v/v |
(42°Be HNO3) | 188 g/L | 212 g/L | 236 g/L |
Sodium | 2 %w/w | 2.5 %w/w | 3 %w/w |
dichromate | 22.0 g/L | 27.2 g/L | 32.4 g/L |
1 Pipette a 10.0 mL sample from bath.
This amount is critical since we will measure the number of molecules in this sample and use the sample size to compute the weight of those molecules (grams) per unit of volume (liter). Any error in the sample size will cause an error in the computation of Na2Cr2O7 concentration.
2 Dilute with 175 mL of DI water.
This measurement is not critical because the total number of molecules of Na2Cr2O7 in the sample is unaffected by the dilution.
3 Add a few grams of ammonium bifluoride (NH4HF2).
The ammonium bifluoride will bind any iron present in the sample. Passivate baths remove iron from the surface of the stainless steel being passivated, so it is impossible to keep the tank completely free of iron. Iron will react with iodine and reduce the accuracy of the analysis. A substantive difference in titration results with and without adding AB is an indirect indication of the need to change your bath. Dissolved iron should be analyzed by AA and kept below 2000 ppm.
4 Slowly add 15 mL of 23 °Be hydrochloric acid
(may be diluted 50:50 or poured
around stopper in 5 mL increments
and introduced down side of flask).
Iodine (I2) will react with water (H2O) as follows: 2 I2 + H2O —> O2 + 4 H+ + 4I-. When the pH is reduced by the HCl to the range of 2-5, this increases the H+ in solution and forces the reaction to the left, thereby ensuring that the iodine remains available.
5 Add ~2 grams of potassium iodide (KI). Replace stopper and wait for ~2 minutes to allow completion.
Keep the flask stoppered during the reaction as iodine is volatile, and perform the titration as quickly as possible. KI is the source of the iodine, and the amount must be sufficient to ensure a complete reaction with the analyte (Na2Cr2O7 in this example). The reaction is: Cr2O7 + 14H + 6I —> 2Cr + 3 I2 + 7 H2O, so there must be 6 moles of KI for every mole of Na2Cr2O7. The maximum amount of Na2Cr2O7 in solution is 32.4 g/L which is 32.4 / 297.997 = .108726 mole/L and the sample size is 10.0 mL, so there could be as much as .00108726 mole of Na2Cr2O7. To fully react with .00108726 mole of Na2Cr2O7, you need .00108726 x 6 = .006524 mole of KI which is .006524 x 166.003 = 1.083 grams of KI. The titration calls for 2 grams of KI which exceeds this minimum requirement. Note that the time for this reaction is reduced by higher concentration of KI (along with lower pH, higher temperature, and agitation). Swirling the flask and increasing the amount of KI are both acceptable ways to increase the rate of reaction. A higher KI concentration will also help to stabilize the iodine in solution.
6 Titrate with 0.5N Na2S2O3 to a straw color.
At this point, the titration is not yet complete: the yellowish, or straw, color of the solution indicates a low iodine concentration, but is not an accurate indicator of the endpoint.
7 Add a few mL of soluble starch indicator. Solution should be blue-black.
The blue-black color of the triiodide ion in the starch is an accurate indicator of iodine presence. However, if the starch were added into an acidic solution with high iodine content, it would form a highly stable complex with the triiodide ion, and the blue-black color could not be easily removed. In a dilute iodine concentration, the starch complex is unstable, so the iodine must be diluted first with the titrant (sodium thiosulphate), then the starch is added. When the starch is added, an intense blue-black color should appear due to the entrance of the triiodide ion into the starch matrix.
8 Continue titration until blue-black color disappears.
Each mL of 0.5M Na2S2O3 is equivalent to 0.5 x 6 moles of Na2Cr2O7. The 6:1 ratio is calculated by comparing the thiosulfate reaction (I2 + 2S2O3 —> S4O6 + 2I) to the original redox reaction to find the ratio of Na2S2O3 to Na2Cr2O7. The redox reaction was Cr2O7 + 14H + 6I —> 2Cr + 3 I2 + 7 H2O, and this shows that each Cr2O7 ion liberates 3 I2 ions. The thiosulfate reaction shows 2 thiosulfate ions equivalent to each I2 ion, so there will be 3 x 2 = 6 moles of sodium thiosulfate reacting with every mole of the original sodium dichromate. So for each mL of titrant (0.5N sodium thiosulfate) there will be (297.997 x 0.5) / 6 = 24.833 grams of Na2Cr2O7-2H2O. Since the sample is 10 mL, there will be 24.833 grams / 10 mL = 2.4833 g/L of Na2Cr2O7-2H2O per mL of titrant.
Other combinations of sample size and reagent normality can be calculated as follows:
a new titration multiplier = 2.4833 g/L x (10 mL)/(new sample mL) x (new reagent normality)/(0.5N)
So with a 1 mL sample and 0.1N Na2S2O3 there is 4.967 g/L (.66 oz/g) of Na2Cr2O7-2H2O per mL of titrant.
Iodometric (back titration of liberated iodine) and iodimetric (direct titration with iodine) are typically used to determine chromic acid concentration in chrome plating baths, bright dip baths, and Alodine/chemfilm/chromate baths. This type of analysis is also used to determine hypophosphite concentration in electroless nickel baths. ChemTrak users are able to import all of these baths and adjust them to specific requirements of tank volume, measurement units, etc.