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Eliminating Fading Endpoint from Weak Hydroxides

Eliminating Fading Endpoint from Weak Hydroxides Sodium Hydroxide Titrations Weak sodium hydroxide solutions (less than 0.2N) can exhibit "fading endpoint". This is usually due to trace carbonates present in the raw material reagent as well as absorption during the manufacturing process. For most applications, this itself does not normally present practical problems with titre values given tolerances required. In fact, it is common practive to call an endpoint when the color han be held stable for about 30 seconds. But if reqiured, with a little extra care and time, you can eliminate entirely the fading endpoint phenomenon from weak hydroxide solutions by the following steps:

1) Titrate to about 1ml before expected endpoint and note exact mils titrated.

2) Bring contents of flask to gentle boil for 1-2 minutes, cover, and cool.

3) Rinse down walls of flask with previously boiled and cooled DI water, verify that buret reading is at the volume previously dispensed, and continue titration to first light pink endpoint.

Having boiled off the carbonates, your endpoint will now be stable enough for an accurate titration, and non fading for time required, and you will have titrated purely sodium (or potassium) hydroxide not mixed with dissolved trace carbonates.

Tips on Conductivity Measurements

Listed below are some basic tips/facts regarding conductivity measurement in the lab. Most conductivity work is done using potassium chloride, not sodium chloride. Sodium Chloride is preferred in natural salt water/brine sample testing. Many of these points may seem too basic for some, but these tips are meant to help all in the lab work environment

Conductivity solutions of less than 100 u mhos are usually not stable for over 3-6 months with respect to a 1% tolerance which is common by manufacturers. This is primarily due to absorption of atmospheric CO2. This results in an increase of the ions dissolved in the solution, thereby increasing the conductivity value. Of course, the lower the conductivity value, the higher the proportion of the added conductivity contributed by the contaminant CO2. CO2 absorption into conductivity standards higher than 5000 or 10,000 micromhos has a minimal effect under NORMAL laboratory conditions. "Minimal" means the conductivity which absorbed CO2 contributes to the conductivity standard solution will normally be much less than the 1% error tolerance stated on the label. Above a conductivity standard of about 50,000 micromhos the effect is insignificant, and if you get a suspition that something is very wrong with your standard, you should look toward other sources of contamination. Do not store conductivity standard solutions in the refrigerator as this will obviously just attract more CO2 to the solution standard!

Always store platinum probes submerged in clean DI water. You will get quicker, more stable conductivity readings when calibrating. Opt for the lower frequency settings on your meter if given a choice and if in the desired conductivity range. Pre rinse beakers/probes with the conductivity solution after a DI water rinse just before calibration. Choose high density polyethylene for containers, and avoid soft plastics which contain phthalate plasticizers such as DEP or DEHP

Take readings and calibrate at 25C whenever possible. Temperature compensating sensors/thermistors can go bad with little warning on some met!

As an alternative to certified external conductivity standard solutions, you can make your own as a second source using good lab procedures. Material must be dried reagent or better grade, water must be ASTM Type I, vessels must be class A calibration type, and scale recently calibrated. High purity water will absorb CO2 very rapidly to the order of up to 0.5 or 0.6 micromhos in a 4 hour period. If you are making a very low conductivity standard you must, after generating the ultrapure water, dissolve, mix, and package and cap as soon as possible.

Contribution to Scientific Publication

For years Exaxol has been a steady contributor to the most comprehensive publication on high purity compounds "Purification of Laboratory Chemicals" by Wilfred Armarego and Christina Li Lin Chai. Information and data provided by Exaxol includes metal impurities detected by ICP-MS on various purified inorganic compounds, most of which include starting material grade and source of manufacture.

ICP-MS results are listed showing decreasing levels with subsequent recrystallizations providing the reader with an idea of what to expect as typical results for a given grade or source. Also, advice on the purification procedures of several inorganic compounds were provided over the years.

For more information on the book "Purification of Laboratory Chemicals", go to elsevierdirect.

Ion Chromatography-Analytes/Sources

Often, IC standards are described by an element or by an ionic specie which is the source of the element. An example is nitrogen (element) and Ammonium (ionic specie or polyatomic cation). It is easy to confuse concentration values for such a standard unless one is specific about the "title" of the ionic standard. A ammonium standard of 1000ppm (μg/ml) is for the cation NH4+ at 1000ppm, not Nitrogen at 1000ppm. The nitrogen value is much less, of course due to the hydrogens being factored out of the calculation. Where one talks of a nitrogen standard from an ammonium source the value of nitrogen then becomes 1000ppm, and the ammonium is therefore much higher in value, since you must ADD the hydrogens to the ppm weight.

Likewise, when talking of a nitrogen standard one needs to specify the source. Different sources are ammonium, nitrate, nitrite. Ammonia as a source is actually not accurate as in aqueous solution ammonia ionized to NH4 with water. An "ammonia" standard in aqueous form is not accurate/does not exist. Yes, you can report the value as ammonia if you factor out the extra hydrogen from ammonium, but the actual ion chromatographic results are due to ammonium adsorption/resolution.

A phosphate-phosphorous standard of 1000ppm is P at 1000, not PO4 at 1000. Also, a phosphorous, source phosphate is also P at 1000. A phosphate 1000ppm standard has a 1000ppm value of the PO4, not P. The P will be much less from factoring out the oxygens. This is basic chemistry but it continues to be a source of confusion and mistakes for ordering the correct standard from a standards supplier. We at Exaxol are aware of this and are ready to offer clarification at the time of order minimizing errors down the line. Exaxol has gravimetric and conversion factors for all Ion Chromatography standards at hand and welcome your questions should they arise.

Blending elements for ICP, ICP-MS, ICP-OES standards

Blending stock standards for your own custom in house internal standard takes a little time but is very economical provided you already have the stock standards available. Pre rinse all glassware/plasticware with dilute nitric or hydrochloric, depending on matrix. Rinse 3X with high purity DI water. Volumetric pipettes (class A calibrated) are a good tool for this. Make calculations and have a second check done before proceeding. depending on final volume of stock solution required, back calculate for the balance of matrix solution (eg 2% nitric, etc), and add first to the flask enough matrix so that addition of the stock will not go over flask volume. This also will prevent concentrated stock from possible precipitation with other elements (likely with higher concentrations such as 50,000 ug/ml, and even 10,000 ug/ml). Stock solutions in HF must be made in plastic based flasks. Plastic pipettes are also a must. Generally, keep total concentration of blended elements below a maximum of 30,000ppm (ug/ml). Below 20,000 is even preferred. QS to volume with appropriate matrix.

pH Buffer Calibration Verification

pH Buffer Calibration work requires calibration standards that are dependable and accurate to the specifications expected/listed. Providing your equipment is maintained well, measuring pH should not be a chore.

Be sure you go through the meter checkout procedure for your meter according to manufacturer's directions. Also, perform a check on your electrode. To our surprise, a prominant brand of (high quality)electrodes did not include the electrode check out procedure in their owner's manual. But with a quick phone call, their tech person explained it to us. Electrode checks involve taking millivolt readings for the electrode when in buffers 7 and 4, then subtracting the difference, and checking that that figure falls within their parameters. Failure of the electrode will result in erratic readings, and drift.

Three point calibrations give the most accurate results. For less demanding work, 2 point is useful and quicker.

Buffers sold in the lab market, both by Exaxol and our respected competitors are traceable to NIST Standard Reference Material. Exaxol, in 2003, did some internal testing and found that of 3 brands obtained, not one was within the 0.1 pH unit tolerance guaranteed by the manufacturer, and total variation between the 3 was a staggering 0.5 units.

NIST Traceable means that the manufacturer is using these materials provided by NIST, drying the appropriate ones (some should not be dried), weighing them on a semi micro balance, and dissolving the salts in high purity, carbonate free water to volume. These prepared standards are then used to calibrate the meter, before measuring/testing the values of the buffers manufactured in house. This is repeated again for extra accuracy.

Exaxol performs these steps in preparing high quality standards for pH measurement. Our buffers are cataloged as B0470 for your convenience.

Titrating Carbonates with Alternative Indicator

The titration of carbonates (typically sodium carbonate) involves a weak base vs a strong acid (Hydrochloric 0.1N) and color change results with simple methyl orange are quite subjective, as the color change is very gradual. A good alternative is mixed methyl orange-xylene cyanole FF. This will give a sharper color change within a narrower pH scale , the color changing from grey to violet with a minimal addition of titrant hydrochloric acid. Exaxol has this indicator available, catalog #M5056, for your convenience.