18.44 Field analysis set ph soil/isfet

Transcription

1 18.44 Field analysis set ph soil/isfet operating instructions Contents About these operating instructions Introduction Description Technical specifications Preparing the equipment for use General Adjustment of the temperature probe Adjustment of the ph-isfet electrode Adjustment of the multimeter + ISFET ph electrode Use Taking measurements in liquids Taking measurements in semi-solid substances Applications Problems and solutions Maintenance...8 All it takes for environmental research P.O. Box 4, 6987 ZG Giesbeek, the Netherlands April 2011 T F E I info@eijkelkamp.com M E

2 About these operating instructions! When this mark (as shown on the left) is followed by text, it means that the text contains an important instruction. When this mark (as shown on the left) is followed by text, it means that the text contains an important warning, pointing out risk of injury to the user or damage to the instrument. Text Text written in italics is the actual text that will appear on the display. To use the equipment to best advantage: Read through all the manuals carefully before starting! 1 Introduction In compliance with international standards ph measurements are carried out electrochemically with the combined use of a ph electrode and a ph meter with temperature compensation. The Soil ph/isfet field analysis kit contains a conventional ph meter and a special ph electrode. What makes it special is that the electrode does not have a ph sensitive glass membrane but an acid sensitive synthetic/ metal semiconductor, visible at the tip as a shiny small area (see picture). As with a classical ph electrode, the ISFET electrode has a porous shaft containing an electrolyte that via a diaphragm (shown white on the second picture) is in contact with the material in which the measurements will be made. The signal from the tip is converted in the head of the electrode into a signal that is identical to the signal from a classical ph electrode. Due to this glassless construction, the robust ISFET electrode is more suitable for measurements out of doors or in the soil. The degree of acidity or ph of the soil is of great importance with regard to plant or crop growth. The ph value of soil can be influenced through the addition of fertilizer containing lime. To make the soil able to absorb fertiliser, acid ground must first be neutralised with lime or calcareous fertiliser. However, before treating the soil with lime or calcareous fertiliser, the ph value of the soil first needs to be determined. The aim of this is to establish how much lime needs to be added. As a general rule, the ph value of clay soils needs to be higher than that of sandy soils. Other important reasons for determining the ph of the soil or groundwater are: to determine the living conditions for micro-organisms or to determine whether soil contamination has been caused by the discharge of acids or alkalines. Determining the ph of soils is traditionally done in laboratories in accordance with international and Dutch standards. It involves the collection of samples that are then mixed, dried, sieved and extracted with (possibly salt) water. There isn t always enough time, nor is it always desirable or necessary to follow this complicated method. This Soil ph /ISFET field analysis kit makes it possible to determine the ph soil simply and rapidly by taking direct measurements in the field. Of course, the result is in a number of cases not equal to the ph that would be determined in a laboratory. The analysis kit is not only suitable for soil measurements but also for measuring in liquids and other semi-solid substances. 2

3 2 Description The Soil ph/isfet field analysis kit has the following components: The multimeter, art. no.: This multimeter is a universal measuring instrument that suitable for measuring parameters such as ph, mv, EC and temperature. The meter works extremely well in combination with the ISFET soil ph electrode. The combination is capable of measuring ph values between 0 ph and 14 ph. An additional temperature probe connected to the multimeter ensures that the ph value is temperature compensated as is usual when a glass ph electrode is used. It is also possible to use a type meter (also measures dissolved oxygen) or another brand of multimeter. The main condition is that the ph electrode can be connected to it using a BNC plug. The ambient temperature around the ph electrode must also be passed on to the meter. Whether manually; via the separate temperature probe; via the temperature information from an EGV/T probe that is also connected and placed in the soil. This probe must be close to and at the same depth as the ph electrode in the measuring material. ISFET Soil ph Electrode, art. no.: This ph electrode combines an ISFET sensor, temperature compensator and a reference system all in one body. In the top part of the electrode the voltage and temperature that have been measured are converted to a signal in exactly the same way as though it was a conventional ph electrode. In this way the ISFET electrode emulates the old-fashioned vulnerable glass electrode with the result that the electrode can be connected to all conventional ph meters with the use of a BNC connector. This intelligent ph electrode takes care of rapid, accurate and reproducible measuring values. The refillable reference system in the electrode is provided with a porous PTFE diaphragm that has an extra large surface area so that the chances of blockage are minimised. The electrode can withstand high temperatures (up to 105 C). The electrode can also withstand many types of aggressive chemicals. Stainless steel temperature probe for connecting to the EGV input, art. no.: The temperature probe is of the thermistor type. It is used during the adjustment procedure and while measuring in water, soil, etc. If no temperature sensor is available, the temperature must be adjusted manually or an EGV sensor must be connected. This contains an integrated temperature sensor. Take account of the slower temperature acclimatisation. Gouge, art. no.: The gouge can be used to drill a hole in the soil which allows the ph electrode to be placed into the soil together with the temperature probe for measuring the ph. Thumb spatula, art. no.: The thumb spatula can be used for scraping the dirt off the auger. Buffer solutions ph ph 4.00 The buffer solutions are for calibrating the meter/electrode combination or, when the deviation is too great, for making adjustments. Maintenance kit for the ph electrode This small kit consists of a toothbrush for brushing clean the bottom end of the ISFET electrode. Also included is an injection syringe and top-up solution to refill the KCL/Silver reference system in the ISFET electrode. 3

4 3 Technical specifications ph electrode ph : ph 0 to ph 14 Temperature : -5 C to +105 C Electrode accuracy : 0.03 ph Resolution : 0.01 ph Seal : IP 67 Calibration : at ph 7.00 Battery life : 700 hours Refillable : with KCL / KCL silver gel Tip : conical, suitable for aggressive chemicals Measuring depth : 80 mm Electrode diameter : 10 mm Electrode length : 200 mm Temperature probe Sensor element Measuring range Measuring depth Probe diameter : Thermistor : -10 C to +110 C : 130 mm : 3 mm Multimeter ph : ph 0 to ph 14 mv : ± 2000 mv Resolution : ph, 0.1 mv, 0.1 C Inputs : BNC for ph electrode : 8-pin connector for EGV and/or temperature; Ambient Temp. : C Relative humidity : 0 to 90 % Power supply : 4 x alkaline batteries, type AA Seal : IP 67 Dimensions : 185 x 95 x 58.5 mm (without rubber cover) 191 x 101 x 61 mm (with rubber cover) 4 Preparing the equipment for use Definition of terms used: Calibration is the determining of the amount by which the equipment deviates from the calibration standard. For example: The electrode is held in a liquid with known and buffered ph and the deviation is noted. Adjustment is the correction of any deviation identified during the calibration. Normally the calibration and adjustment are part of the same action. This is commonly referred to as calibration to a standard. Calibration to a standard is the official adjustment made by a person authorised to do so by the NMI (Netherlands Institute for Metrology & Technology). Note: The Anglo-Saxon term calibration actually stands for all of the three checks mentioned above. 4.1 General Adjustment of this equipment differs slightly from the familiar method used to adjust a ph meter with a traditional glass electrode. In the first place, the emulation of the ph/isfet electrode needs to be checked. This is done purely using 7.00 ph buffer solution. Once this has been done, the next step is the customary adjustment at ph 7.00 and 4.00 (or buffers with other values). 4

5 4.2 Adjustment of the temperature probe The temperature probe does not normally require adjustment. However it is important to check regularly whether the temperature indication on the meter is responding properly to electrode heating (check for a broken lead). If a deficiency is clearly established, it is recommended that the electrode (or better still, the electrode/meter combination) be sent back to the manufacturer. A correct adjustment for temperature requires a fairly difficult set-up or a special standard calibration thermometer. Before sending a meter to be repaired because the temperature no longer responds, check whether in the Set up>system menu the temperature input is set to ph/cond. This means that the meter collects the temperature data from the temperature electrode that is connected to the EGV input. Do not (when using meter 18.52) use oxygen electrodes to measure the temperature, because the membrane will be immediately damaged in the soil. 4.3 Adjustment of the ph-isfet electrode Before measuring can commence, the zero voltage of the ph ISFET electrode needs to be adapted to the meter. This is done as follows: Check whether there are batteries in the meter by switching it on. Connect the ph-isfet electrode to the meter (ph/mv input terminal). If you want to check the status of the batteries of the ISFET electrode use the MODE button to set the meter in the mv measuring mode. The figure that is displayed must be divided by 10. The result is the battery load percentage of the electrode. The picture shows an electrode with a battery charge of 938.4/10, rounded off 94% Remove the protective covering. The O-ring and collar should be pushed up out of the way. Rinse the tip of the electrode with clean water or demineralised water. Connect a Temperature probe to the meter (EGV input). If a temperature probe is not available, the temperature of the adjustment liquid should first be set (refer to the general manual of the meter or 18.52). If required, use the temperature sensor that is built into the EGV electrode, if you have it. Place both sensors in ph = 7.00 liquid (always use buffered liquid, never distilled water). Switch the meter on and press the MODE button until the ph measuring function has been activated. Strange values will now start to appear and possibly even the error message UNDER (see picture). This is because the electrode also needs to be activated! The ph electrode is activated by pressing on the red dot at the top. The meter will show a ph-value based on the last calibration. If the reading on the display is stable the emulation of the ISFET electrode then needs to be adjusted. Put the electrode in the buffer solution with ph 7.0 and press the red dot switch at the top of the electrode for 5 seconds. The value only changes when you release the switch after 5 seconds. The electrode will adjust itself in the buffer solution to ph The ISFET electrode is now ready to be used with a classical ph meter with a BNC input. Once this has been done, the meter to which the ISFET electrode is connected must be adjusted as a normal ph meter. To do this, follow the short or comprehensive manual supplied with the meter for ph adjustment. It should be adjusted to ph 7.00 and ph It is also possible to adjust the meter using buffers of other standard values. Do not forget that the electrode must be ON (red LED on) during the adjustment. 5

6 4.4 Adjustment of the multimeter + ISFET ph electrode After the ph electrode itself has been set, the multimeter can be adjusted to the ph electrode. This procedure is described in detail in the instruction manual provided with the multimeter (a brief description of the adjustment is also provided in the short guide). If you intend to use another type of ph meter, refer to the instruction manual provided with that meter! After the adjustment the ph electrode is rinsed with distilled water and is then ready for use. Between regular measurements you can naturally test the ISFET ph electrode with separate temperature probe by immersing it in the buffers to establish deviations, and therefore the need to readjust, as often as you wish. Deviation of up to a few per cent should be regarded as normal. 5 Use Use only buffered solutions for calibration and adjustment. Tap water and distilled water are not suitable for this. 5.1 Taking measurements in liquids Remove the protective covering of the ph electrode. Visually check whether there is sufficient electrolyte in the electrode, especially after a long period of not being used. Clean the tip of the electrode with distilled water. If the electrode has dried out and has not been stored in a KCl solution, check if there is sufficient KCl solution in the electrode. If not, replace the content of the electrode with new 3 M KCl solution (supplied). If the electrode has dried out, while there is more than sufficient electrolyte present, it can be placed in the KCl solution for 10 to 15 minutes to soak. Turn on the ph-isfet electrode and check its the battery voltage. When the electrode is not placed in measuring material the meter will show various readings. This can also be the case when the electrode is in a measuring material that is too dry. If this is the case, moisten the measuring material, for instance soil, with a few drops of distilled water. When the electrode stays in the material to be measured, the electrode will stay in active mode. After 30 minutes the electrode will not switch off automatically. Only when the electrode is not in the material to be measured it will switch off automatically after 30 minutes. When the electrode, in the active mode, is replaced in the protective cover with KCl liquid, it is possible that the electrode stays in the active mode, which leads to the unintended use of energy. Therefore, do not fill the protective cover with 3 Mol KCl as you would probably do with a classical electrode. Before starting a series of important measurements, it is normal to calibrate and if required adjust a meter. (See Section 4) The ISO/NEN standard describes ph measurements in water. 6

7 5.2 Taking measurements in semi-solid substances When the material for measuring (soil, for instance) is too hard for the ph electrode, a small shallow hole needs to be made using a gouge. A thumb spatula can be used to scrape the gouge clean. The ph electrode can now be put into the soil at the same time as the temperature probe (the temperature probe preferably in another spot; but at the same depth). Press the tip into the bottom of the drilled hole. Handle the ph electrode with care and lower, do not drop, it into the soil. Then use a finger to push the electrode into the soil somewhat to make good contact with the soil. If the soil is too dry or too hard to achieve proper contact with the tip of the electrode, a few drops of distilled water can be added. The use of a few drops of a KCl 1.0 Mol or CaCl Mol solution is another possibility. Whether you use a salt solution and which one, depends on what the purpose of your measurements is (horticulture, agriculture, forestry, etc.) and what degree of accuracy is required Wait for the value to stabilise in the display window ( STABLE ) and take a reading of the ph value. Once measuring is completed, the ph electrode can be rinsed with distilled water or, if necessary, cleaned with the toothbrush provided. ISO 10390:2005 describes the official international method of measuring the ph of soils (NEN 5750 for the Netherlands). The direct field method as described above may differ in some respects from the ISO and NEN methods. However, the laboratory method takes much more time and is more expensive. In addition, the measuring results cannot be used immediately. 6 Applications The ph ISFET electrode is suitable for direct measurements in all liquids (including many aggressive ones) and solid substances. When it concerns scratching and resistant solid materials a small hole should be drilled first. The ISFET electrode can be applied directly into soft materials like dough, cheese, mud, peaty soil, soft loose sand and other such soft materials. In the case of drier and/or harder materials a hole should always be pre-drilled and the material moistened deeper down. This is normally achieved with distilled water (horticulture) but the use of waterwith 1 Mol KCl (traditional agriculture) or water with 0.01 Mol CaCl 2 (modern agriculture) to facilitate extraction is also possible. The electrode is then inserted a small distance into the material with light pressure or twisting. It is important to wait for a chemical equilibrium. This may take some minutes and in some cases even hours (in the case of soils with a high lime content). Measurements should be taken in several places to obtain an average. Please note: This field method differs from the standard laboratory method! The measurement results will therefore be (slightly) different!! If a specific chemical substance needs to be measured which you suspect may pose a risk to the materials in the electrode, the advice of the manufacturer should be sought. Do not take measurements in water on which oil is floating. 7

8 7 Problems and solutions Replacing the batteries Measure the capacity of the battery in the electrode with the meter in the mv measuring mode. Once the battery capacity has fallen to a level that leaves no more than 25 hours of measuring, the electrode will send a message to the meter. When the electrode is then switched on it will twice give an alternating message: in mv mode: 400 mv to 0 mv to -400 mv and in ph mode: ph~0 to ph~7 to ph~14. Unscrew the black handgrip anticlockwise and pull it back very carefully over the lead until the battery is exposed. Remove the battery. It can only be removed from one side. Carefully click a new battery into place and once more slide the handgrip downwards. Take care that the O-ring does not move out of place (otherwise, use some Vaseline or silicone oil/ grease) and screw the handgrip with a clockwise motion back on till secure. Do not use tools or too much force to tighten the handgrip as this could damage the electronics. Hand-tight is sufficient. Messages on the display of the meter When the meter is switched on but not the electrode a number of messages can appear on the meter display depending on what mode it is in. In the mv mode: display shows a high value in mv, e.g mv (this figure divided by 10 indicates the remaining battery capacity). In ph mode: ph signal out of range message of another error message. Solution: switch the electrode on When the electrode is on, but is not immersed in liquid (is dry), the electrode will pass this information on to the meter. The display will show a continuously changing signal. In mv mode, 0 mv to -400 mv and in ph mode, ph~7 to ph~14. This can also be the case when the electrode is in soil that is too dry. In this case, first moisten the soil somewhat. If problems arise that cannot be solved by consulting the instructions, the manufacturer should be contacted. 8 Maintenance Keep all the items clean and dry during and after use. A ph meter is an extremely sensitive current meter. It is not possible to take measurements in condensing conditions; therefore allow the meter and the electrodes to first acclimatise to the ambient temperature. In rainy conditions, also prevent the BNC plugs becoming wet. If required, place a strong freezer bag over the meter and the plugs. Once a carrying case has become wet it must be allowed to dry completely before being closed. Keep the liquids stored away from the meter as much as possible to minimise the effect of leakage from the buffer and maintenance liquids as the salt is corrosive, though the liquids are otherwise harmless. The status of the ph electrode and multimeter batteries should be checked regularly. The ph electrode contains a ½ AA 3.6 Volt lithium battery. Therefore, ensure they are ordered sufficiently in advance. The great advantage of lithium batteries is that they can be stored for an extremely long time without any appreciable loss of quality. Nothing in this publication may be reproduced and/or made public by means of print, photocopy, microfilm or any other means without previous written permission from Eijkelkamp Agrisearch Equipment. Technical data can be amended without prior notification. Eijkelkamp Agrisearch Equipment is not responsible for (personal damage due to (improper) use of the product. Eijkelkamp Agrisearch Equipment is interested in your reactions and remarks about its products and operating instructions. 8