Determination
Sodium dichromate in nitric acid based passivation baths.
Application
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. We will determine the sodium dichromate concentration in a bath that might be used for a QQ-P-35 Type II, an AMS2700 Method 1 or an ASTM A967 Nitric 1 passivation. The BAC 5625 specification also includes a number of passivation baths containing sodium dichromate, and these bath can also be analyzed using the present method.
The QQ-P-35, AMS2700 and ASTM A967 specify a range of sodium dichromate concentration between 2.0% and 3.0% by weight. Percent by weight is an inconvenient metric, since analysis requires a measurement of specific gravity. Moreover, as iron and other metals dissolve in the bath, use of percent by weight introduces inaccuracy into the analysis. So we will convert the 2%-3% by weight to grams per liter (g/L). If you can convince your auditors that it is easier and more accurate to work with g/L, you will have the correct numbers. If not, just keep using the hydrometer and doing the calculations over and over.
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. In the following table, we assume 42°Be HNO3 to determine g/L equivalents.
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 |
The listed specifications identify sodium dichromate dihydrate as the added salt (Na2Cr2O7-2H2O). 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 shown in the table above.
Boeing uses the more convenient metrics of oz/gal (g/L) so we don't have to convert from %v for acid concentration or deal with %wt and specific gravity. The following table is reproduced from the BAC 5625 specification, Table 9.13, Solution 22A which Boeing claims meets the requirements of AMS-QQ-P-35 and AMS 2700 Method 1. Boeing allows the use of 40-42°Be HNO3 for makeup and replenishment as long as the concentration meets the specification.
min | mid | max | |
Nitric acid | 150 g/L | 206 g/L | 262 g/L |
(as HNO3) | 20 oz/gal | 27.5 oz/gal | 35 oz/gal |
Sodium | 18.75 g/L | 24.37 g/L | 30 g/L |
dichromate | 2.5 oz/gal | 3.25 oz/gal | 4.0 oz/gal |
The BAC 5625 specifications for sodium dichromate are slightly more restrictive than what is allowed by the AMS-QQ-P-35, the ASTM A967, and the AMS2700.
Method
1 Pipette a 10 mL sample into a 100 mL volumetric flask.
Dilute to volume with DI water.
2 Pipette a 25 mL aliquot into a 500 mL iodine flask.
and add 100 mL of DI water.
(This is an accurate way to take a 2.5 mL bath sample
when the sodium dichromate concentration is very low.)
3 Add a few grams of ammonium bifluoride (NH4HF2).
4 Slowly add 15 mL of 23 °Be hydrochloric acid
(or 30 mL of acid diluted 50:50).
Pour around stopper in 5 mL increments
and introduce down side of flask.
5 Add 30 mL of 10% potassium iodide (KI).
Replace stopper and wait for ~2 minutes to allow completion.
6 Titrate with 0.1N Na2S2O3 to a straw color.
7 Add a few mL of soluble starch indicator.
Solution should be blue-black.
8 Continue titration until blue-black color disappears.
Calculation
A mL x Factor = Na2Cr2O7-2H2O
Sample | Titrant | Factor | |
Na2Cr2O7-2H2O (g/L) | 2.5 mL | 0.1N | 1.987 |
Na2Cr2O7-2H2O (oz/gal) | 2.5 mL | 0.1N | 0.265 |
Na2Cr2O7-2H2O (g/L) | 10 mL | 0.5N | 2.483 |
Na2Cr2O7-2H2O (oz/gal) | 10 mL | 0.5N | 0.331 |
A mL x Factor / S.G. = Na2Cr2O7-2H2O
Sample | Titrant | Factor | |
Na2Cr2O7-2H2O (%wt) | 2.5 mL | 0.1N | 0.199 |
Na2Cr2O7-2H2O (%wt) | 10 mL | 0.5N | 0.248 |
Explanation
In this method, iodine, supplied by potassium iodide (KI), is used as an oxidizing agent giving up an electron to reduce sodium dichromate (Na2Cr2O7). 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.
The ammonium bifluoride added in Step 3 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.
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.
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 2.5 mL, so there could be as much as .00022 mole of Na2Cr2O7. To fully react with .00022 mole of Na2Cr2O7, you need .00022 x 6 = .0013 mole of KI which is .0013 x 166.003 = 0.2166 grams of KI. The titration calls for 30 mL of 10% KI which has 3 grams of KI. 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.
At Step 6, 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.
At Step 7, 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.
Each mL of 0.1M Na2S2O3 is equivalent to 0.1 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 (0.1N sodium thiosulfate) there will be (297.997 x 0.1) / 6 = 4.967 grams of Na2Cr2O7-2H2O. Since the sample is 2.5 mL, there will be 4.967 grams / 2.5 mL = 1.987 g/L of Na2Cr2O7-2H2O per mL of titrant.
The factor for other combinations of sample size and reagent normality can be calculated as follows:
a new titration multiplier = 1.987 g/L x (2.5 mL)/(new sample mL) x (new reagent normality)/(0.1N)
So with a 10 mL sample and 0.5N Na2S2O3 there is 2.48 g/L (.33 oz/g) of Na2Cr2O7-2H2O per mL of titrant.
Iodometric (back titration of liberated iodine) and iodimetric (direct titration with iodine) titrations 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.
Accuracy
Using this iodometric (back titration of liberated iodine), with the sampling and dilution method described, should yield an accuracy of ±3% when carefully performed. See cited reference for details.
Reference
Process Control Analytical Procedures
D.R. Graunke and A.H. Petrie
Boeing Aircraft Company
Document D180-17994-1, Release W, 2001
Procedure No.: B-156, Page 210