Instruments
4 articles
Calibrating Fluids for pH Meters
Calibrating pH meters A pH meter can only be accurate if it has been calibrated. This needs to be done daily if highly accurate results are required and the environment varies, or weekly if conditions are less demanding. To achieve good uniformity over the entire range, at least two calibration points are required. Very few meters provide for an optional third point. If your measurements are always near pH 7 (or the value of the other two standards) calibration using a single point may be sufficient. However, most people use two points and usually the two lower pH solutions are chosen, e.g. pH 4 and pH 7. If accuracy in the high pH range is most important then pH 10 calibration buffer should be preferred as the second point. pH calibration solutions are buffers and their pH is not affected by a small amount of distilled water, which has virtually no buffering capacity. A small volume of solution (usually pH 7 first) should be used for calibration. Our calibrating solutions are supplied in a twin top bottle - squeeze a little into the built-in reservoir. The electrode is rinsed with distilled water (wash bottles are handy) before inserting the tip into the calibration solution and adjusting the meter to read the solution's pH value (see meter's instructions). Again rinse the electrode tip before measuring or adjusting for a second pH solution. After the calibration process, readings of test solutions may be performed. A brief distilled water rinse between readings is essential. Storage of pH electrodes and electrolytes Electrodes (pH probes) contain an electrolyte solution, which frequently is a KOH solution. This may need to be topped up after a year or more of service. Electrodes are best kept moist or wet during storage periods. Tap water is more suitable than calibration solutions or distilled water. Dedicated electrode storage solution is available - see APE12 on page A1 for storage. That page also features electrolyte solutions. Cleaning solutions Many materials measured for pH are totally soluble and cleaning is not required - but others are a problem and foul up the electrode. Well known examples are blood products, wines (particularly reds) and milk products. Appropriate cleaning fluids should be used briefly after each use, prior to storing the probe in storage solution. Occasionally an overnight treatment in cleaning solution may be required. Some materials contaminate badly and special, more resistant electrodes are available. Alternatively, electrodes may be regarded as expendable and replaced after a time. pH Pen or pH stick These are an economical combination meter that is appropriate if greater accuracy and special features are not needed. This includes measurement of badly contaminated solutions, since these meters are cheaper than just replacing an electrode on other meters. The above notes apply to these simple meters. The cap may be used to hold a little storage or other solutions to maintain the pH probe in good operating condition.
Calibration Weight Classes
Class E1 These weights are the highest accuracy class, and are intended to be used for traceability between national mass standards and OIML class E2 weights and lower. The maximum permissible error from nominal value at 1kg is ±0.5mg. Used as primary laboratory reference standard. Class E2 Suitable for use for traceability of OIML class F1 weights and lower; also with OIML accuracy class I weighing instruments. The maximum permissible error from nominal value at 1 kg is ±1.6mg. Used as a high precision standard for calibration of weights and special precision analytical instruments. Class F1 Suitable for use for traceability of OIML class F2 weights and lower; also with OIML accuracy class I weighing instruments. The maximum permissible error from nominal value at 1kg is ±5mg. For the calibration of high accuracy class 2 balances. Class F2 Suitable for use for traceability of OIML class M1 weights and lower; also with OIML accuracy class II weighing instruments. Intended for use in high value commercial transactions such as gold and precious stones. The maximum permissible error from nominal value at 1kg is ±16mg. Used as working standards for precision and analytical work, build-in weights, external weights to calibrate moderate precision balances. Class M1 Suitable for use for traceability of OIML class M2 weights and lower; also with OIML accuracy class II weighing instruments. The maximum permissible error from nominal value at 1kg is ±50mg. Class M2 Suitable for use for traceability of OIML class M3 weights; also with OIML accuracy class III weighing instruments. Intended for use in normal commercial transactions where goods are sold by weight. The maximum permissible error from nominal value at 1kg is ±160mg. Used for accuracy class III applications, industrial scales, dial scales, trip balances, platform scales Class M3 Suitable for use with OIML accuracy class III or IIII weighing instruments. The maximum permissible error from nominal value at 1kg is ±500mg. May be used for accuracy class III L and IV.
Custom Refractometer Scales
Acetic Acid Acetone Ammonia Ammonium Chloride Ammonia Sulphate Barium Chloride Baumé Boric Acid Calcium Chloride Cesium Chloride Citric Acid Cobaltous Chloride Creatinine Cupric Sulphate Dextran EDTA Ethanol Ethylene Glycol Ferric Chloride Formic Acid Fructose Glucose Glycerol Hydrochloric Acid Inulin KMW (Klosterneuburger Mostwaage) Lactic Acid Lactose Lead Nitrate Lithium Chloride Magnesium Chloride Magnesium Sulphate Maltose Manganous Sulphate Mannitol Methanol Nickel Sulphate Nitric Acid nD (Refractive Index) OE (Oechsle) Phosphoric Acid Potassium Bicarbonate Potassium Biphthalate Potassium Bromide Potassium Carbonate Potassium Chloride Potassium Chromate Potassium Dichromate Potassium Ferricyanide Potassium Ferrocyanide Potassium Hydroxide Potassium Iodide Potassium Nitrate Potassium Oxalate Potassium Phosphate, Dihydrogen Potassium Phosphate, Monohydrogen Potassium Sulphate Potassium Thiocyanate (Wide-Range) Procaine Hydrochloride (Wide-Range) Propylene Glycol Sea Water Serum or Plasma, Human Serum or Plasma, Rabbit or Guinea Pig Silver Nitrate Sodium Acetate Sodium Bicarbonate Sodium Bromide Sodium Carbonate Sodium Chloride Sodium Diatrizoate (Wide-Range) Sodium Dichromate Sodium Ferrocyanide Sodium Hydroxide Sodium Molybdate Sodium Nitrate Sodium Phosphate, Dihydrogen Sodium Phosphate, Monohydrogen Sodium Phosphate, Tribasic Sodium Sulphate Sodium Tartrate Sodium Thiosulphate Sodium Tungstate Strontium Chloride Sucrose (Wide-Range) Sulphuric Acid THAM Trichloroacetic Acid Urea Urine Solids, Cat Urine Solids, Guinea Pig Urine Solids, Human Urine Solids, Rabbit Vol AP (Probable Alcohol) Zinc Sulphate
Weighing Technique
The long obsolete beam/apothecary's balance was sensitive and accurate, but painfully slow and complicated to operate; digital balances are powerful but they have a few peculiarities which need to be understood by users for best accuracy. Digital scales need to be calibrated to allow for the earth's magnetic field and, when moved, for minor changes in level. Like other analytical instruments they are calibrated, and the recommended accurate standard weight is used to electronically draw a line from zero through the recommended weight to the scale's maximum. From this it is obvious that the longer the range of the scales the less accurate the scales will be furthest from the calibration weight; this is only a minor effect and it only affects critical, very low mass determinations when performed near zero. It should also be obvious that like any other analytical instrument, greatest accuracy is near the calibration standard used. Linearity is the deviation from a straight line, and that deviation is likely largest near zero and the maximum range of the scales. The implication is that scales with a high maximum range, however desirable that feature is, are a little compromised when minimum weight accuracy is needed. As an example, scales with a maximum range to 3000g and a readability of 0.01g, may only become accurate at around 0.2g; whereas scales with a range to only 600g may become similarly accurate at about 0.10g. When weighing very small quantities it makes more sense to have a 1g weight (or greater) on the weighing pan. Note also that using an arbitrary weight on the pan, and taring this, does not change the basic problem with linearity near zero. In summary, minimum weight is a somewhat arbitrary figure, because the calibration may have been with a 200g weight, then scales have plotted an electric current sloping down to zero and up to the scales' maximum weight. Further away from the calibration weight the linearity will not be as good, and it is poorest near zero and near the scales' maximum. Some scales give a minimum weight recommendation, but this is arbitrary as is a percentage reliability factor. The practical solution for weighing very small mass (10 to 20x the minimum reading possible ‐ as a guide) is not to start at zero, but weigh either with the standard calibration weight or with a 1 or 10g weight on the pan, then subtract that to obtain the sample's mass. If small calibration weights are available it would be useful to establish a reliable minimum weight for particular scales. See also Calibration Weights Classes