What should my electrode be stored in?
When storing a combined electrode the ideal situation is that the sensor is in equilibrium. This applies primarily to the reference part of the electrode where a flow of electrolyte is usually involved. In most cases the best storage medium is the electrolyte contained in the reference system, as this will ensure no electrolyte movement through the junction. In the case of half cells, there are three main types in use. The first is naturally the pH half cell where the best storage medium is a pH 7 buffer. The second type of half cell in common use is the ion sensitive electrode (ISE). For short-term storage most ISEs are stored in dilute (0.001M) solutions of the ion to be measured. This ensures that the electrode is always ready for use. For long-term storage most ISEs are stored dry. The third half cell type is the double junction (or single junction) reference electrode. This electrode should be stored in bridging electrolyte for short-term storage and should be emptied and stored dry for the long term.

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How often do I need to standardize my titrant?
Naturally, this depends on the stability of the titrant and on what measures have been taken to protect the titrant from the typical contaminants that could cause a reduction in concentration. The most common examples of this titrant protection are the storage of light sensitive titrants in dark bottles e.g. iodine solutions, the protection of Karl Fischer titrants from moisture using e.g. molecular sieve or silica gel, and the protection of certain strong bases e.g. sodium hydroxide, from absorption of carbon dioxide.

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How often should I calibrate my electrode?
This depends very much on the samples that are being measured with the electrode, but as a general rule the electrode should be calibrated at least once per day.

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What is temperature compensation and how will it affect my measurements?
When measuring the pH of a solution there are 3 main temperature effects that come into play. The first is that the slope of the pH calibration curve for the electrode as given by the Nernst equation is temperature dependent. Provided the temperature of the buffers is taken into account during calibration, any difference between this temperature and that of the actual samples being measured can be mathematically compensated for. With most modern pH meters and titrators this can be done automatically.

A second effect involves real changes in pH of a sample with temperature. Imagine a weak acidthat only partly dissociates in solution. The higher the temperature of the solution, the greater will be the degree of dissociation of the acid, and therefore the lower the pH will be. This effect is completely sample dependent and cannot be compensated for with any pH meter or titrator.

The third effect relates to the second but involves the calibration with the pH buffers. As pH buffers are often made up of acids and bases, their pH is also temperature dependent. In order that a pH meter or titrator can be calibrated correctly, it is necessary for the instrument to "know" the temperaturebehavior of the buffer. Currently there are numerous manufacturers of pH buffers, some with different compositions to others, and therefore different temperature behavior. Not every pH meter and titrator available has the ability to select the buffer type and therefore some small discrepancies can occur. With most METTLER TOLEDO general titrators, the user has the possibility of selecting from some common buffer types (manufacturers) e.g. DIN, NIST, MERCK and of course METTLER TOLEDO. Within the instruments there are look up tables for each of these buffer types so that the real pH of the calibration buffers is known.

In conclusion, the only way to be absolutely sure that you are measuring the correct pH of your sample is to ensure that your samples and the buffers used to calibrate are at the same temperature.

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Why are my results half or double those expected?
There are two main causes for this phenomenon. The first is that the burette size has been incorrectly defined e.g. titrator has been told that there is a 10mL burette in use when in fact the burette is a 20mL burette. In this case the results will be half of those expected. The new titration Excellence line eliminates this source of error with the automatic burette recognition feature.

The second possibility is a calculation problem with the defined value for z, the equivalent number or valency. Here one should make sure that you are in fact titrating to the correct equivalence point. An example is the titration of sodium carbonate with HCl. If this titration is terminated after the first equivalence point then the equivalent number that should be used for the carbonate is 1 rather than the 2 expected when writing out the chemical reaction. The reason for this is that the reaction is a two step reaction, with the carbonate first reacting to bicarbonate (hydrogen carbonate) and only then continuing to carbon dioxide, sodium chloride and water. If the titration were continued until the gas bubbles are visible (carbon dioxide formation) then z would indeed be 2.

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Why do I get no result or a result of zero when, from the curve, there is a clear jump visible?
There are a few reasons for this although the most common is that too high a threshold value has been set in the method. Print out a table of measured values and check the largest value for the first derivative. The threshold value in the method has to be set lower than this. Generally it is suggested that the threshold be set at approximately 50% of the maximum in the first derivative for steep curves and up to 80% for flatter curves. Remember that the main purpose of the threshold value is to eliminate noise or false equivalence points. Other reasons for no result include specifying the incorrect tendency (direction of the titration curve) and a poor choice of equivalence point range.

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What electrode should I use for non-aqueous titrations?
Generally there are three main electrode problems when performing a non-aqueous titration. The first is the problem of having an aqueous electrolyte with a non-aqueous solvent. Replacing the electrolyte in the electrode easily solves this. The second problem relates to the fact that the sample is non-conductive, resulting in a poor electrical circuit between measuring and reference half-cells or parts of the electrode if combined. This results in a noisy signal, particularly when using a sensor with a standard ceramic junction in the reference. A partial solution to this problem is to use a sensor with a sleeved junction, such as the DG113 electrode. This sensor has LiCl in ethanol as the standard electrolyte and, rather than a ceramic junction, has a polymer sleeve resulting in a larger contact area between working and reference parts and therefore lower noise.

The third problem is not a problem of the electrode itself, but rather the handling of the sensor. In order for a glass (pH) sensor to function correctly, it is necessary that the glass membrane (bulb of electrode) be hydrated. This is achieved by conditioning the electrode in deionized water. During the non-aqueous titration this membrane is gradually dehydrated reducing the response of the electrode. To prevent this or rectify this problem the electrode should be regularly reconditioned by soaking in water.

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Why, when I perform an equivalence point titration using an automatic titrator, do I get a different result as to when I titrate manually using a color indicator?
This discrepancy in results is primarily noticeable when performing acid/base titrations using one of the pH indicators. The first reason for this is that these pH indicators change color over a pH range rather than at a fixed value. The actual point at which the color change occurs is very much sample dependant and may not coincide with the chemical equivalence point. This can result in a small discrepancy in result which is easily nullified by standardizing the titrant using a similar method as is used for samples.

The second reason for this difference is primarily one of the sensitivity of the human eye to color change. While a color change may have already started to occur, the human eye has still not detected any change. This can be demonstrated by using a photometric sensor such as the METTLER TOLEDO DP5 phototrodes. Using one of these sensors there is a clear change in light transmittance long before the human eye detects any color change. In the typical acid/base titration using potentiometric indication with a pH sensor, the sharp change in signal occurs at the first trace of excess acid (or base) and is therefore a more true indication of the end point.

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Why, when I titrate with sodium hydroxide (NaOH), do I sometimes see two equivalence points?
Sodium hydroxide (and potassium hydroxide) has a tendency to absorb carbon dioxide forming sodium carbonate, which also then reacts with any acid in the sample. This results in this double inflection point effect. Even freshly prepared sodium hydroxide solutions often show this effect. The reason for this is that the solid sodium hydroxide (often pellets) also absorbs carbon dioxide, forming a thin layer of carbonate on the surface. The only way to prepare fresh carbonate free sodium hydroxide solutions is to prewash the pellets or use fresh pellets and to use freshly boiled deionized (or distilled) water. Once such a solution has been prepared it should be protected from carbon dioxide by fitting an absorption tube filled with sodium hydroxide on a carrier. Whenever this double inflection point is apparent when titrating with sodium hydroxide (or potassium hydroxide) it is recommended that the solution be discarded and fresh titrant prepared. An alternative to discarding is to boil, allow to cool and filter through a very fine mesh filter paper.

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How do I go about validating a method on my automatic titrator?
When validating a titrator method one needs to check things like accuracy, precision, reproducibility, linearity, systematic errors, robustness, ruggedness and limits of determination. For detailed recommendations on how to go about this validation please refer to our section on Quality Control, Validation or refer to the METTLER TOLEDO applications brochure 16 - Validation of Titration methods.

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When performing a TAN or TBN titration, each sample gets slower and slower and the curves get flatter and flatter until I don't get a result. What could be wrong?
The TAN and TBN titrations are non-aqueous acid/base titrations performed using a glass pH sensor. For a glass pH sensor to work it is necessary that the electrode has a microscopic hydrated 'gel' layer on the bulb of the electrode. This hydration is achieved by soaking the electrode in water for some time. During the course of any non-aqueous acid/base titration the gel layer is slowly dehydrated resulting in a weaker and less stable signal. The loss in stability of the signal can cause an automatic titrator to slow down in an attempt to match the rate of addition to the electrode response. The weaker signal means that the size of any jumps is smaller and, depending on instrument settings, can result in the instrument no longer recognizing the jump. The solution to the problem is to rehydrate the electrode between every sample, and this is in fact specified by the ASTM methods for TAN/TBN determination. This is achieved by immersing the electrode in water for 2 to 3 minutes followed by rinsing with the respective solvent.

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What resolution should my balance have to ensure that I get accurate and precise results?
The answer to this question depends on many things such as expected result and homogeneity of the sample, both of which will determine the optimal sample size, required number of decimal places for the final result, and off course the required accuracy of the final result. As a general rule however, one should have at least 4 significant figures for the sample weight. Below are some recommendations:

Sample size Minimum number of decimal places
1-10g ..................................3
0.1 - 1g ...............................4
0.01 - 0.1g ......................... 5

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Why do I need to adjust the pH or add a buffer to my sample when I do titrations with EDTA or EGTA?
The success or failure of most complex formation titrations, including EDTA and EGTA titrations, depends mainly on the formation of a stable complex and on the color change of an indicator (if indicated by color change). The stability constants of metal EDTA and EGTA complexes are extremely pH dependent as is the stability of the colored indicator complexes that form. For this reason it is important to adjust the pH of most complexometric titrations. The reason that, in addition to a pH adjustment, a pH buffer is also sometimes added stems from the fact that on formation of the metal ligand complex there is usually an accompanied release of protons which would otherwise result in a lowering of pH.

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How can I find out what the software version is of my instrument?
With the increased need for instrument certification and system validation it is necessary to determine the software version of the instrument. The list below gives the procedure for determining this with each of the METTLER TOLEDO titrators.

T50, T70, T90 - view menu Setup-> Global Settings -> System -> Titrator ID
DL70ES, 77 - shown on display on start up and printed in report header.
DL50, 53, 55, 58 - always present on lower right of display.
DL31,38,32,39 - always present on display.

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