fix: add pH page.

Fixes #202

docs/advanced-guides/nmr-repositories/analysis.md

@@ -1,5 +1,5 @@
 ---
-sidebar_position: 10
+sidebar_position: 15
 id: data analysis
 title: NMR Repositories Overview
 ---

docs/advanced-guides/nmr-repositories/common-issues-encountered-in-nmr-repositories.md

@@ -1,5 +1,5 @@
 ---
-sidebar_position: 12
+sidebar_position: 18
 id: common issues encountered in nmr repositories
 title: Common Issues Encountered in NMR Repositories
 ---

docs/advanced-guides/nmr-repositories/organism.md

@@ -1,5 +1,5 @@
 ---
-sidebar_position: 9
+sidebar_position: 10
 title: Organism
 ---
 

docs/advanced-guides/nmr-repositories/ph.md

@@ -0,0 +1,48 @@
+---
+sidebar_position: 9
+title: pH
+---
+# Sample pH
+[Notebook link](https://github.com/NFDI4Chem/repo-scripts/blob/main/notebooks/ph.ipynb)
+
+## Support by Ontologies
+The [definition of the pH](https://terminology.nfdi4chem.de/ts/search?q=ph) is easily found in ontologies
+https://terminology.nfdi4chem.de/ts/search?q=ph, the difiniton of [Sample pH](https://terminology.nfdi4chem.de/ts/ontologies/nmrcv/terms?iri=http%3A%2F%2FnmrML.org%2FnmrCV%23NMR%3A1000019) too. However, when users provide this value, it is not clear whether it really means the pH of the sample, the solvent, the solvent with the buffer, or another value. Due to the possible confusion, users tend to provide a detailed explanation within the solvent field as a free text, which unfortunately affects the machine-readability of both the solvent and the pH. You can find examples in the [solvent](/docs/advanced-guides/nmr-repositories/solvent.md) page.
+
+## Data Sanitisation and Missing Values
+It was possible to get the pH details only from [MetaboLights](https://www.ebi.ac.uk/metabolights/).
+
+<table>
+  <tr>
+    <th></th>
+    <th>Field Type</th>
+    <th>Field Name</th>
+    <th>Values Readability</th>
+    <th>Unit</th>
+    <th>Missing</th>
+  </tr>
+  <tr>
+    <td><b>MTBLS</b></td>
+    <td>dedicated</td>
+    <td>Sample pH</td>
+    <td>free text</td>
+    <td>none</td>
+    <td>The field is not provided; or the value is provided as N/A or other similar expressions; or the study "assays" value is "null"; or the value cannot be converted into a float.</td>
+  </tr>
+</table>
+
+## Results
+
+### pH
+It was possiblt to obtain the pH only from MTBLS, even though most of the studies didn't provide this metadata. Most samples' pHs were leaning towards the neutral 7, while a small percentage was around 4.
+
+<div style={{textAlign: 'center'}}>
+<img src="/img/analysis/ph-all.png" width="500"/>
+<figcaption>A rough estimate of the percentages of all studies in MTBLS repository based on the sample pH</figcaption>
+</div>
+
+Here one can see the number os studies providing the pH of the sample and its value.
+<div style={{textAlign: 'center'}}>
+<img src="/img/analysis/ph-h.png" width="1000"/>
+<figcaption>The number of studies in MTBLS based on the sample pH</figcaption>
+</div>