How to Test for Chloride Ions in Iron Treatment Solutions Using Quantab Test Strips Canadian Conservation Institute (CCI) Notes 4/4

Transcription

1 How to Test for Chloride Ions in Iron Treatment Solutions Using Quantab Test Strips Canadian Conservation Institute (CCI) Notes 4/4

2 by Lyndsie Selwyn Also available in French. Également publié en français. Government of Canada, Canadian Conservation Institute, 2016 ISSN

3 Introduction As an iron object rusts, it becomes covered with iron oxides, which partly protect the iron underneath. This protection is disrupted by chloride ions, which greatly accelerate the corrosion rate. Chloride ions are a serious problem in archaeological iron objects, especially those recovered from a marine environment, and they must be removed to preserve the object. Usually, this removal involves soaking the object in an alkaline bath. A simple test for chloride ions is needed to monitor when the bath should be changed or when the treatment can be stopped. A test with silver nitrate can show the presence of chloride ions in the treatment solution and give rough concentrations over a limited range of chloride ion concentration, as outlined in CCI Note 4/5 How to Test for Chloride Ions in Iron Treatment Solutions Using Silver Nitrate. A more accurate measurement over a wider concentration range can be made with Chloride Quantab Test Strips. These test strips consist of a narrow column impregnated with silver dichromate. After the strip has been placed in the solution to be tested, the liquid enters through a small hole at the bottom of the column and then wicks up the column by capillary action. Chloride ions in the solution react with the silver dichromate, producing a white region of silver chloride on the column. The length of the white region can be converted to a chloride ion concentration using the calibration information supplied with the test strips. The test strips are available in two concentration ranges: 30 to 600 mg/l (30 to 600 parts per million [ppm]) and 300 to 6,000 mg/l (300 to 6,000 ppm). For more information on the reaction involved, see The science behind Quantab test strips. This CCI Note describes the procedure for using Chloride Quantab Test Strips to monitor chloride ion concentrations in treatment solutions. The first step in the procedure involves testing solutions of known chloride ion concentrations to get experience using the test strips and to confirm that the test strips are working properly. Then actual treatment solutions or other solutions of unknown chloride ion concentration can be tested. A laboratory and ventilation are not required for this procedure unless nitric acid is required to adjust the acidity of the solution. If nitric acid is to be used, then consult its Safety Data Sheet (SDS) for health and safety information prior to use. 3

4 Procedure: how to use Quantab test strips Equipment and materials required to test using Quantab strips Solutions with known and unknown chloride ion concentrations o Solutions with known chloride ion concentrations should be purchased or prepared with concentrations in the range of the Quantab strips: 30 to 600 ppm for low range and 300 to 6,000 ppm for high range. A procedure to prepare chloride ion solutions with 30 ppm, 300 ppm and 3,000 ppm is presented in CCI Note 4/5 How to Test for Chloride Ions in Iron Treatment Solutions Using Silver Nitrate. o Solutions with unknown chloride ion concentrations can include a desalination treatment solution or any other solution requiring testing. Test tubes, glass (e.g. 16 mm outer diameter x 125 mm length hold about 20 ml) Test tube rack Chloride Quantab Test Strips, low range and/or high range (consult Suppliers for information on obtaining these strips) Tweezers (to remove the Quantab test strips from the bottle) Pipettes (Pasteur or plastic) or eyedroppers Graduated cylinder (10 ml) Volumetric flask (100 ml) Dilute nitric acid (HNO3) solution, e.g. 5% (v/v) (optional); for preparation instructions, see Preparation of solution Glass storage bottle (125 ml) for the nitric acid solution (optional) ph indicator papers (optional) Filter paper (optional) Funnel (optional) Procedure to test using Quantab strips Exercise caution while using nitric acid and wear personal protective equipment such as gloves, goggles and protective clothing while handling it. Use proper ventilation, especially when working with concentrated nitric acid. For more information on making up chemical solutions, consult Odegaard et al. (2005) and Skoog et al. (2014). Preparation of solution If needed, prepare a 5% (v/v) nitric acid solution Work in a fume hood. 2. Partially fill a 100 ml volumetric flask with distilled water. 3. Measure 5 ml of concentrated nitric acid (68 70% HNO3 by weight) into a 10 ml graduated cylinder.

5 4. Transfer the nitric acid to the volumetric flask (Important: always add concentrated acid to water). 5. Swirl to mix. 6. Add more distilled water to the volumetric flask to fill it to the 100 ml mark. 7. Transfer to a glass bottle for storage and label the bottle. 8. This produces a solution that is approximately 0.8 M HNO3. Testing solutions using Quantabs: 1. If a solution contains suspended particulates, filter it using a funnel and filter paper. This will prevent the opening at the bottom of the strip from being clogged by particulates. 2. Using a pipette or eyedropper, add about 2 ml of a solution with a known chloride ion concentration to a 10 ml graduated cylinder. 3. Transfer the solution from the graduated cylinder to a clean test tube. 4. Insert one Quantab strip into the test tube and let the bottom of the strip sit in the liquid. 5. Wait for the liquid to wick up the column in the strip until the horizontal completion band at the top changes colour from yellow to black. 6. Note the time when the top completion strip turns black. 7. Wait at least 2 minutes (but no longer than 30 minutes) and then remove the Quantab strip from the test tube. 8. Locate the position of the uppermost white tip of the boundary between white and brown in the column, and read the position on the Quantab scale to the nearest division (0.2 units). 9. Refer to the conversion table on the outside of the Quantab container. Convert the number from the Quantab strip scale into a chloride ion concentration in parts per million. The conversion table varies from batch to batch so make sure to use the correct container to convert a reading. 10. Repeat steps 1 to 9 with other solutions of known or unknown chloride ion concentrations. 11. For solutions with unknown chloride ion concentrations, check a separate sample with ph paper. If the ph is above 10, the solution should be neutralized with 5% (v/v) nitric acid, as outlined in Modifying strongly alkaline solutions. 12. If the Quantab strip is to be saved for future reference, remove excess liquid by pressing down along the length of the strip to squeeze out as much liquid as possible, and let it dry. Quantab strips cannot be reused. 5

6 Results of this procedure The two Chloride Quantab Test Strips, available from Hach, are shown in Figure 1. Quantab strips come pre-calibrated, and a conversion chart is provided on the outside of the container (Figure 2). Government of Canada, Canadian Conservation Institute. CCI Figure 1: Chloride Quantab Test Strips, made by Hach, come in two Quantab ranges. The bottle and the strip on the left are examples of the low range, and those on the right are examples of the high range. Government of Canada, Canadian Conservation Institute. CCI Figure 2: Quantab conversion charts on the back of the bottles. The scales are unique for each batch of strips. Figure 3 shows an example of a high range Quantab strip in a test tube with 2 ml of solution at the beginning of the test. 6

7 Government of Canada, Canadian Conservation Institute. CCI Figure 3: High range Quantab strip in a test tube containing 2 ml of solution, at the beginning of the test. Once the column is saturated, a moisture-sensitive horizontal completion band at the top turns from yellow to black and signals the end of the test (Figures 4a and b). Government of Canada, Canadian Conservation Institute. CCI Figure 4a: Detail of a Quantab strip before exposure to a test solution. The horizontal completion band is yellow. Government of Canada, Canadian Conservation Institute. CCI Figure 4b: Detail of a Quantab strip after completion of a test for chloride ions. The completion band has turned black. 7

8 At the completion of the test, the chloride ion concentration can be determined by first reading the tip of the white section, as shown in Figure 5 for low range and Figure 6 for high range. This measurement is used to determine the chloride ion concentration from the calibration chart on the back of the bottle. Although the reading for 15 ppm solution in Figure 5 is below the lowest range in the calibration chart, the strip does indicate that some chloride ions are present in solution. Government of Canada, Canadian Conservation Institute. CCI Figure 5: Three examples of low range Quantab test strips in solutions containing (from left to right) 15 ppm, 35.5 ppm and 100 ppm chloride ions. Government of Canada, Canadian Conservation Institute. CCI Figure 6: Two examples of high range Quantab test strips in solutions containing (from left to right) 1,000 ppm and 3,545 ppm chloride ions. 8

9 Additional information Accuracy of Quantab test strips The manufacturer tests each lot of Quantab strips and certifies an accuracy of ±10% for the chloride ion concentration (Hach 2014). This means that if the reading is 200 ppm chloride ion, the actual concentration could be anywhere between 180 and 220 ppm. Interferences It may be necessary to filter the sample if it contains suspended solids. Such solids may clog the opening at the bottom of the test strip. In this case, the completion band at the top of the column will never turn black. The best filter papers are ashless ones because they contain only trace amounts of chloride ions. Do not use filter paper that contains chloride ions from any bleaching process. To test filter paper for chloride ions, cut up a piece, soak it in a small volume of water in a test tube, and test the solution with a Chloride Quantab Test Strip. Other ions that will interfere with this test and give false results are those that will also form a precipitate with silver nitrate, such as iodides and bromides. Nitrite and nitrate ions have no effect on the test. The reaction between silver dichromate and chloride ions may not work in strong acids and strong bases (Hach 2014). The instructions on the container for the Quantab strips warn not to immerse a strip to the point that the liquid on the outside is above the yellow completion band at the top of the strip. However, with the instructions in this procedure (using a test tube with 2 ml of liquid), this is not a concern. Storage of Quantab test strips Store the Quantab strips in their original container with the lid tightly closed to protect them from light and moisture. Each bottle of Quantab strips contains a small amount of desiccant to help keep the strips dry. Bottles should be stored below 30 C. Use Quantab strips before they expire; the expiry date is listed on the bottle. Expired strips may take longer to react, and the contrast between colours may be less. Modifying strongly alkaline solutions According to the manufacturer (Hach), Chloride Quantab Test Strips should not be used in a solution with a ph above 10. At high ph, silver hydroxide or silver oxide can precipitate, removing silver ions that would otherwise precipitate as silver chloride. The ph of a typical alkaline treatment solution made from 1% (w/v) sodium hydroxide (0.25M) is about Thus, a sample of the treatment solution should be neutralized to 9

10 ph below 10 before it is tested with the test strips. The following instructions outline how to do this. Take a sample of 2 ml of treatment solution. Measure the ph with a ph indicator paper. Add 5% (v/v) nitric acid until the ph is near 7. It should take roughly 0.4 ml of 5% (v/v) nitric acid to neutralize 2 ml of 1% (w/v) sodium hydroxide. If a volume of nitric acid (e.g. 0.4 ml) is added to the initial volume (e.g. 2 ml), then the chloride ion concentration in the initial sample is related to the measured concentration in the diluted sample with the following formula cinitial = cmeasured x (vintial + vadd) / vinitial where cinitial is the chloride ion concentration of the initial sample, cmeasured is the chloride ion concentration of the diluted sample, vinitial is the initial volume of the sample and vadd is the volume of nitric acid added. With the estimate of 0.4 ml of acid needed, then is and so (vintial + vadd) / vinitial ( )/2 = 1.2 cinitial = 1.2 cmeasured For a more accurate value of the sample s initial chloride ion concentration, keep track of the volume of nitric acid added, or measure the final volume of the neutralized solution. Other ways to test for chloride ions There are other ways to measure the chloride ion concentration in aqueous solutions. One simple (but less accurate) way to test for the presence of chloride ions is with silver nitrate, as outlined in CCI Note 4/5 How to Test for Chloride Ions in Iron Treatment Solutions using Silver Nitrate; this note also contains more information on sources of salts in objects and the damage they can cause, and monitoring desalination using conductivity. Semi-quantitative analysis can be done using EM Quant Chloride Test Strips (Odegaard et al. 2011). Another more accurate way is to carry out a titration with silver nitrate (Selwyn 2001). More methods are mentioned and studied by Wang et al. (2008) and Rimmer et al. (2012). 10

11 The science behind Quantab test strips The reaction between the Quantab test strip and chloride ions A Chloride Quantab Test Strip consists of a thin plastic strip with a capillary column impregnated with brown silver dichromate (Ag2Cr2O7) (Hach 2014). When the bottom of the Quantab strip is placed in an aqueous solution, liquid enters through a small hole in the bottom and then wicks up to the top by capillary action. Silver dichromate is slightly soluble in water, whereas silver chloride (AgCl) is insoluble in water. Chloride ions (Cl - ) in the solution react with silver ions (Ag + ) from the silver dichromate, precipitating silver chloride. As silver ions are removed from the solution, more of the silver dichromate dissolves to replace them, until all of the silver dichromate has dissolved and all of the silver ions have precipitated as silver chloride. The colour of the column changes from brown to white where silver chloride is precipitated. The overall reaction is the interchange of chloride ions in solution with dichromate ions (Cr2O7 2- ) from the solid, written as follows: 2Cl - (aqueous) + Ag2Cr2O7 (brown solid) 2AgCl (white solid) + Cr2O7 2- (aqueous) Definition of parts per million (ppm) Chloride ion concentrations are often given in parts per million (ppm). Parts per million is a unit of concentration defined as the weight of the solute (the ion or compound being added) divided by the weight of the solution (after the ion or compound is added) and then multiplied by one million (10 6 ). A common assumption for dilute aqueous solutions is that the added ions do not change the density of the water, so that the solution has the same density as pure water at room temperature (approximately 1 g/ml) (Skoog et al. 2014, p. 72). With this assumption, the definition of ppm simplifies to weight of parts per million volume of solute ( mg ) solution ( L) Note that the solute is the chloride ion. This means that a 1 ppm chloride ion solution contains 1 milligram (mg) chloride ions per litre (L) of solution. Acknowledgements Special thanks to Catherine Machado, Lucy t Hart and Meaghan Whalley, former CCI interns, for their help in developing this Note. 11

12 Suppliers Note: The following information is provided only to assist the reader. Inclusion of a company in this list does not in any way imply endorsement by the Canadian Conservation Institute. Chloride Quantab Test Strips The Quantab strips are made by Hach and distributed by chemical supply companies such as Fisher Scientific. Hach Fisher Scientific Chloride standards Three Orion brand chloride standards (0.1 M, 1,000 ppm, 100 ppm) are made by Thermo Scientific, and fourteen different chloride standards ranging from 1 ppm to 100,000 ppm are made by Ricca Chemical. These are distributed by chemical supply companies such as Fisher Scientific. Thermo Scientific Ricca Chemical Fisher Scientific References Hach, personal communication, 4 December Odegaard, N., S. Carroll and W.S. Zimmt. Material Characterization Tests for Objects of Art and Archaeology, 2nd ed. London, UK: Archetype Publications, Odegaard, N., P. Hill, B. Santarelli and W. Zimmt. Detecting and Identifying Salts During the Desalination Process with Spot Test Papers. WAAC (Western Association for Art Conservation) Newsletter 33 (2011), pp Rimmer, M., D. Watkinson and Q. Wang. The Efficiency of Chloride Extraction from Archaeological Iron Objects using Deoxygenated Alkaline Solutions. Studies in Conservation 57 (2012), pp Selwyn, L. Analysis of the Chloride Ion Concentration in Aqueous Solutions by Potentiometric Titration. CCI Research Report No. 2. Ottawa, ON: CCI, Skoog, D.A., D.M. West, F.J. Holler and S.R. Crouch. Fundamentals of Analytical Chemistry, 9th ed. Belmont, CA: Brooks/Cole,

13 Wang, Q., S. Dove, F. Shearman and M. Smirniou. Evaluation of Methods of Chloride Ion Concentration Determination and Effectiveness of Desalination Treatments using Sodium Hydroxide and Alkaline Sulphite Solutions. The Conservator 31 (2008), pp